Astronomical Companion

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THE ASTRONOMICAL COMPANION SECOND EDITION

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The illustration on the front cover represents a sphere of space, our stellar neighborhood. At the center is our home, the solar system, though all that is visible on this scale is the light of the Sun. Scattered around are the very nearest stars.

My way of showing their three-dimensional positions is to fix them on stalks, which stand or hang from a plane. The plane is that of Earth’s equator. So the stars above it are what we call “north” of us; those below are in the southern half of our sky.

The plane is emphasized by a grid of lines, like wires. This serves a second purpose, indicating scale: the lines are 3 light-years apart. The radius for this sphere is 12 light-years (or 3.7 parsecs, the unit called the parsec being about 3.26 light-years).

It is impossible to show the stars themselves, let alone planets around them, or even the orbits of the planets: all are far too small. Instead, the stars are represented by glows of light, proportional in size to the stars’ “magnitude,” that is, brightness. The Sun’s real width is about 1/7,000,000 of a light-year; the Earth’s, 1/740,000,000; even the diameter of Earth’s orbit is only 1/31,600 of a light-year, and of Neptune’s orbit (the outermost major planet) 1/1,000. So the Sun and what we think of as its vast surroundings are buried microscopically in the center of its glow.

Several of the stars are double or even triple (some can be “split” by a skilled observer with a telescope, others have been discovered in more indirect scientific ways).

In most cases we cannot show the separation between the stars in any literal way: it may be less than the Sun-Earth distance or hundreds of times more yet still too small for our scale. Sometimes the partner stars are similar in size and color (like 61 Cygni). Sirius and Procyon—the brightest star in our sky and the one that famously rises into view before it—are brilliant stars which have been found to have tiny white-dwarf companions. I’ve artificially shown them, but there’s one case where a companion is so far from its primary that it is a whole 1.4 millimeters away in our picture. The nearest system to us, Alpha Centauri (so far south that it can be seen only from our tropics or southern hemisphere), is a beautiful mixture: a Sun-like star, a smaller orange one, and a specimen of those red dwarfs which seem to be the commonest kind of star. And that red dwarf, in a huge slow orbit 15,000 Sun-Earth distances (0.24 of a light-year) from the other two, is called Proxima Centauri because, at only 4.2 light-years, it is and will be for several centuries our nearest star of all.

For more about these stars, see the NEAREST STARS section of the book.

The main illustrations running in a series through the book are on the same principle.

They originated from my wish to make a drawing of the nearest stars, which necessitated a model. In this model the stars are beads, of six sizes and five colors, on lengths of bicycle-spoke planted in a square black wooden base. I used the model to make a painting (though it was not easy for the eye to keep the stars and stalks lined up); but then, wanting to make a balancing picture for the BRIGHTEST STARS—a larger volume in which longer stalks would have to stand from a more remote base—I realized that we are dealing not with a cube of space but with a sphere, so the reference plane should be not a square at the bottom but a disk through the center. To calculate the stars’ positions relative to this disk, student friends told me about sines and cosines and showed me how to write my first computer program—the seed of what became an enormous tree.

The majority of the illustrations are of the same type as that on the front cover. They show a sphere of space centered on our own position, though we rocket away from it to look on the sphere from outside. The pictured sphere is like one of those transparent plastic celestial globes—except that we can fill it with solid contents and vary the markings on the spherical surface.

North is always up, and we keep looking from the same direction, a celestial map position called 17h 15m, +22°, in the constellation Hercules, near the star Delta Herculis or Sarin. (Sarin is actually two stars in line. They are explained in the BRIGHTEST STARS picture.) We are looking past Earth toward the opposite point in the sky, 5h 15m —22°, in the constellation Lepus on the south side of Orion. Orionward orientation seems natural, at any rate for those who learned first the stars of northern winter.

It would be possible to show the sphere from any direction, such as straight down from the north. But keeping to one viewpoint keeps things clear. I think it will give you a rock-steady conception of the whereabouts of everything around us in space, from the small scales out to the large.

In these standard pictures we are always looking from the same relative distance: not infinity (which would make the sphere seem less real) but 3 sphere-radii from the center. The distance from your eye to the nearest bulge of the sphere is the same as the distance from there to the far surface. This determines the foreshortening. Things on or inside the near part of the sphere look larger. This varying scale is made clear by the grid-lines, spreading apart as they come toward you.

Some of the pictures, illustrating geometrical systems, are abstract, with no particular distances. Others form a long series with multiplying distances. You are making the straight voyage out past Sarin, accelerating toward infinity, and looking back at the place you came from.

The four systems you have to keep thinking about in astronomy are those of the equator, horizon, ecliptic, and Milky Way. So after introducing them I keep all four at least minimally present throughout the series. Each has its turn as the most relevant, its plane being the foundation of the model (if we were building a model): the grid stretches across it and the stalks are attached perpendicularly to it.

It may seem that in the pictures of vast outer regions the planes relating to our puny Earth have no business. But any plane extends to infinity. More important, the presence of the planes ties together the levels of astronomy, from the observer with trees around him, to the quasars. It is good to keep them mentally related, because even if looking out to the quasars we are looking through our galaxy, and through our solar system, and from our own sloping and spinning foothold.

The horizon is different from the others in that it is only a horizon, an example. It

is the horizon for latitude 40° north (where a lot of people live) and at a certain time: 0h sidereal time, which occurs for instance about midnight in September, 10 p.m. in October, 8 p.m. in November, 6 p.m. in December, midday in March, 6 a.m. in June . . .

The objects above the horizon plane are those you could see then. Mentally rotate the horizon a quarter turn to the left for the same night six hours later, or the same time of night a quarter of a year earlier. Mentally tip the horizon more steeply, for a more southerly latitude.

Look at a picture so that the horizon is horizontal; or look at it from, say, the galactic point of view. The picture is round: it has (like the universe) no one top of bottom.

Extensions (some of which I have exploited elsewhere than in this book): —The “stars” shown with stalks to indicate their positions in space can be successive positions of a planet or other moving body. (See the solar-system pictures.)

—Stalks, besides those perpendicular to a plane, can be drawn to the center or surface of the sphere, so as to show how we see stars projected on the sky. Two or more kinds of stalk can be used at the same time. (See the final “universe” picture.)

—Distances can be made proportional to the logarithms of themselves, so that objects near and far appear in the same solid picture. (See the “Logarithmic universe.”)

—Instead of a grid, the plane and distances on it can be indicated by concentric circles (as in the logarithmic picture, where indeed a grid would be impossible).

—Instead of the whole sphere, a cone or wedge of it can be selected (such as a constellation) and shown at a larger scale (as on the back cover of the first edition).

—The center of the sphere can be the Earth (as in the “Moon’s orbit” and “Logarithmic universe” pictures) or a point on its surface (as in the “Horizon” picture) or the Sun (as in the “Earth’s orbit” and most succeeding pictures) or can be shifted to some other point, such as another star.

—The viewpoint can move in closer toward the surface of the sphere (as in the “universe” picture), increasing the contrast between things near and far.

—The viewpoint can lie in the skin of the sphere or move inside it. The bubble, thus pricked, does not burst, as I at first feared! The whole sphere could still be shown, but would not look like one: it would be bounded by a large circle representing one point (the point where your eye is probing through, or the point directly behind you).

—What happens when the viewpoint moves all the way down to the center of the sphere? Instantly the picture becomes a flat map. This is what has happened in the “Star names” maps: they result from the same logic as the solid pictures, with the “standoff,” as I call it, reduced to zero. They could have been drawn as one map: the other half of the sky would become a ring added around the outside, ending at a large circle which represents the point directly behind the viewer.

In such maps we still have a direction of view: the center of the map. This does not have to lie on the equator: it can be at the north or south pole (resulting in polar maps of the usual kind) or at any other point in the sky (see the two small maps in the PRECESSION section, with their centers at the ecliptic poles). Of this kind are the maps designed for plotting meteors, each with its center at a meteor-shower’s radiant.

—The method of projection in all the solid pictures is to find, for every object and every point along a geometric form, what its angular distance and direction from the center would be as seen from the viewpoint, and to convert these into distances and directions on the paper. (It is a process performed by the visual instinct in one leap and by trigonometry in a terrifying number of steps.) When applied to a flat map, this results in the type of projection called “equidistant azimuthal.” (Properties of it are that all great circles through the center appear as straight lines, all others as curves; all directions and distances from the center are true; but, far from the center, shapes become stretched sideways.) This seems to me the most generally useful projection for astronomical maps, since it can give maps of any part of the sky—polar, equatorial, or the “meteor-radiant” kind; and, though looking different, they harmonize because resulting from the same logic. However, by changing the equations at a late stage one can get other projections: equal-area azimuthal (which removes stretching-distortion, at the cost of falsifying angular distances); stereographic (which removes distortions, but can reach to only a hemisphere of the sky); orthographic (as viewed from infinity, can also reach only to 90°); gnomonic as on a photographic plate (which cannot be extended to a radius of 90° or more because the points would fall infinitely far away); rectangular, as in the usual equatorial maps (in which degrees or right-ascension and declination all have the same scale, so that the poles become lines, and which is not really a “projection” at all).

And there are other projections not based on centers.

For the earlier Astronomical Companion, I drew all the illustrations by hand. (They were accurate enough, in that I had made a computer calculate many of them, but the plots produced by devices of the time were not of reproduction quality, so I traced over them.) Freehand, with pens and a few other tools, was flexible, allowing emphases and afterthoughts; and people told me they liked it, because it gave a feeling of personal communication, as with sketches made on a blackboard or during a conversation.

But some were less clear than neat plotting could make them. And the book had been created back in the time of paste-up and photographic plates; everything needed to be digitized. And there have been decades of new information, discoveries, star distances; every large diagram needed to be revised. Having in the meanwhile developed thousands of ways of making the computer do the work, I couldn’t face the hand labor over again. The dilemma long prevented me from getting on with a new edition. My solution is to keep the little drawings that are like gestures in the margins, and program the charts and the sphere pictures. (Changing, incidentally, from parsecs—loved only by the mathematically-minded—to light-years.) It’s taken six months longer than I expected to rediscover how on earth I managed to make some of these plots. And I can’t understand how I managed to learn so much during that one year when I made the Companion as an offshoot from the Calendar.

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THE ASTRONOMICAL COMPANION Copyright © 2010 by Guy Ottewell. Printed in the United States of America. All rights reserved. Parts may be reproduced with prior permission and with acknowledgment.

ISBN 0-93456-60-7 First edition, October 1979 Reprinted with revisions and corrections, January 1981, July 1983; reprinted July 1984, February 1985, February 1986, November 1987, December 1988, December 1989, May 1991, August 1992, July 1993, November 1994, December 1995, December 1997, August 2000, October 2001, November 2002, February 2008 Second edition, October 2010

Preface This is a book to look in for explanations, for reference, or for mere enjoyment. I think it is for those who already enjoy either astronomy itself or the idea of it.

The book may be used to guide or supplement a short non-mathematical course in astronomy. Sections such as POSITION and EVOLUTION would be required reading, while others such as NAMES and CALENDARS can be left to catch the eye of the interested student.

The book also functions as a companion volume to my annual Astronomical Calendar in similar format. The Companion was born from the non-year-dependent supplementary material that had begun to swell the bulk of the Calendar.

Douglas Roosa, my helper in several issues of the Astronomical Calendar, made at my specification the physical model of the nearest stars (referred to in the note on the front cover illustration), and did the drawing (now superseded) for the Hertzsprung-Russell diagram.

Few changes were made in most of the hurried reprintings. In the 2000 printing I introduced computer-programmed versions of the first few sphere-pictures.

Some other publications by Guy Ottewell from the Universal Workshop:

American Indian Map, and Navajo Map

Astronomical Calendar 2011

The large map is colored by language-families, also shown in a table

Events throughout the year; many charts; same page-size as the Companion

The Arithmetic of Voting

Albedo to Zodiac Glossary of astronomical names and terms, with pronunciation, origin, and meaning

To Know the Stars Children’s introduction to astronomy

The Thousand-Yard Model, or, The Earth as a Peppercorn Instructions for a walk making vivid the scale of the solar system

The Under-Standing of Eclipses The geometry, history, and beauty of eclipses

Berenice’s Hair Novel: what really happened to the stolen tress that became a constellation?

The Troy Town Tale Novel: the whole legend of Troy, the “collective dream of the Western world”

Portrait of a Million Poster conveying the concept of a million, with selected million-facts

A fairer system, getting rid of the “voter’s dilemma”

The Spiral Library An amusingly miraculous solution for libraries running out of space

Plurry: a musical instrument whose structure leads to an understanding of scales, keys, and modes

Stripe Latin: a grammar game Ten-Minute History of the World; and, Queen Guinevere’s Rules Think Like a Mother: a photo book of human rights Turkey, A Very Short History The Winged Velocipede; or, how to f ly overseas with your bicycle Pembrokeshire prints Language (poems) And prints of paintings

Universal Workshop P.O. Box 102, Raynham, MA 02767-0102, U.S.A. 800-533-5083 (toll-free) 508-802-5660 fax 508-967-2702 customerservice@UniversalWorkshop.com author: guy@universalworkshop.com

www.UniversalWorkshop.com

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Contents sections

large illustrations

______________________________________________________________________________ 4 6 7 8 10 12 14 17 18 19 21 21 22 23 24 27 30 31 32 34 36 37 38 40 42 44 46 48 50 53 56 57 58

72 74

12 light-years (very nearest stars) OVERVIEW OF ASTRONOMY Logarithmic Universe POSITION Equatorial system FOUR PLANES Four planes HORIZON SYSTEM Horizon system ORIENTING TO THE SKY Orienting to the sky CONSTELLATIONS Constellations NAMES Star names, Ophiuchus half of the sky Star names, Orion half of the sky DESIGNATIONS EARTH’S ORBIT Earth’s orbit ZODIAC SOLAR SYSTEM PLANETS Solar system, inner BODE’S LAW Solar system, outer TIME TIME-UNITS JULIAN DATES SEASONS Seasons PRECESSION Precession CALENDARS CHRISTMAS MOON’S ORBIT Moon’s orbit PHASES Phases MOONLIGHT EARTHLIGHT MOON’S ATTITUDE MOON AS SIGNPOST ECLIPSE SEASONS SAROS Pattern of eclipses ASTEROIDS Asteroids, inner Asteroids, outer COMETS Comets METEORS Meteor-shower orbits DISTANCE 1.6 light-years (Oort cloud) NEAREST STARS 16 light-years (nearest stars) BRIGHTEST STARS 100 light-years (bright stars) SPECTRAL TYPES Hertzsprung-Russell diagram HERTZSPRUNG-RUSSELL DIAGRAM EVOLUTION DOUBLE STARS VARIABLE STARS OUTRUSH 600 light-years (clusters) 1,600 light-years (Orion) 3,300 light-years (Deneb, Gould Belt) 10,000 light-years (spiral arms) 30,000 light-years (edge of Milky Way) 80,000 light-years (all of Milky Way) 300,000 light-years (satellite galaxies) 3 million light-years (Local Group) 30 million light-years (nearby groups) 150 million light-years (Virgo Cloud) 1,500 million light-years (superclusters) 13,700 million light-years (quasars, universe) SPACE EXPLORATION a chronology by Alastair McBeath The four solar-system-escaping spacecraft INDEX Hertzsprung-Russell aurora

front cover 5 6 7 8 10 12 14 15 17 20 21

24 28 33 35

39 40 41 42 45 46 48 50 52

58 59 60 61 62 63 64 65 66 67 68 69 72 back cover

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The Astronomical Companion

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“Constellation” (Latin “together-star-ness,” from stella, “star”) meant originally a group of prominent stars apparently near each other in the sky, so that they could be perceived as a geometrical form or a picture. Since individual stars appear as featureless points, their mutual arrangement was the only means of recognizing them Some constellations, or parts of them, happen to be real groupings in space: thus most of the stars in Coma Berenices belong to a certain star-cluster; 5 of the 6 stars in the “face” of Taurus belong to another; 6 of the 7 bright stars in Orion to another; and 5 of the 7 bright stars in Ursa Major to another. But, mostly, the stars we perceive as a constellation merely lie along roughly the same line of sight from us, at greatly different distances. If we could travel to other viewpoints the constellations would dissolve.

Each culture has had its own way of dividing and pictorializing the sky. Our system is basically that which originated in ancient Mesopotamia and was elaborated by the Greeks.

Till recently, the traditional pictures were in real use and were included in star atlases. The way of identifying a particular star was “in the crook of the Ram’s right foreleg” and the like. The most prominent star in a constellation was its lucida. The dim or disregarded stars between constellations were amorphôtoi or informes, “unformed.”

Later, so tha]t any star could be named and catalogued, every part of the sky had to be assigned to a constellation; therefore boundaries had to be drawn. Thus the constellations became essentially directions from us in space, or, rather, wedges of space outward from us.

The boundaries as at first drawn were informal curves. As finally fixed, they became straight lines, which, however, turn a crazy

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NAMES The oldest star-names still used in our culture are three that have come all the way from the pre-scientific Greeks: Sirius, Arcturus, and Canopus. The words seem to be respectively a plain adjective, a dubious reference to position, and the name of a minor legendary personage—but there is something insecure about the explanations of all three.

At least as old is the idea of a certain star’s wide-ruling character; so it was called in the different languages “king” (Basileus, Rex), “royal” (Basilica, Regia), even, strangely, “little king” (Basiliscus, Regulus), which, stranger yet, means also the cockatrice, the lizard that poisons you with its eye. The form used by Copernicus is that which happens to have survived other earlier and later ones.

The Greek astronomers ending with Ptolemy, and the Greek (Hesiod, Aratus) and Roman (Ovid, Manilius) poets, recorded names that presumably were in use before their time. Meanwhile other cultures had their own networks of star-names. For instance the Arabs of the jâhiliyya (the “ignorance,” as they later called their preIslamic stage) regarded a certain pair of stars as Kalb ar-Râ‘i, “dog of the shepherd”; and two other chains of stars as biers with mourn-

ers bearing them, so that the star leading one chain was Qâ’id Banât an-Na‘sh al-Kubrâ, “chief of the daughters of the greater bier.”

Civilization passed to the Islamic world, and its scholars preserved the Greek works in Arabic and added encyclopaedias of their own.

They accepted Ptolemy’s constellations, and thus redescribed the two adjacent stars in phrases (ra’s al-jâthi and ra’s al-h≥awwâ’) that mean “head of Hercules” and “head of Ophiuchus.” In Arabic there are no capital letters; by inventing them we have forced ourselves to decide whether everything is a name or not. When Europe reawoke, and translated the Arabic works and the Arabic translations of Greek works, the star-labels were not translated but transliterated, and thus took the appearance of names: Rasalgethi, Rasalhague. (A few of the native Arab names survived rejection by the later city-dwelling scholars: though for us the biers are Bears, the star at the end of the bear’s tail, having been at the front of one of the biers, receives the two apparently unconnected names Alkaid and Benetnash.) The bulk of our names are neither Greek in form nor Arab in origin, but are fragments of Arabic descriptions of parts of Greek constellations.

Copied by writers who did not understand them, they were corrupted and re-corrupted: ra’s al-h≥awwâ’, reasonably accurate as Rasalhague, appeared as Rasalauge, Ras Alhagus, Rasalhagh, Ras al-Hangue, Azalange, Alangue, Ras al Hayro . . . Most of these weeds withered, but we are left with variant names, variant

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Arcturusa

Kornephoros

Sualocin

us

gin g Se

b

Markab

C

r b Nusakan Izaruolcherrima P e a Gemma or Alphecca

Maasym d Sarin

a

C or

CORONAS BOREALI

l

VULPECU

s ali re lis o na B stra ul x Au l A la u Al

ps m Alkaluro

Albireo

SUS

lgeni b

b

HERCULES

b

g Sulafat

PEGA

Ag

Gienah

adalb

r

Nakka

e

Gre at S q ua re of Pe gasu s

e

S NE CI A C ATI N VE ara b Ch

a

a

S

or

n Be

li aro

Vega

CYGNU

a Alk

h

Kuma m n Arrakis b Rastaban

on yst Ke

b Sc hea t m S

Mat ar

ash

etn

id

LYRA

h

a

s

i

th

io

Al ar

or Alc Miz z

Secundu

x

lfafa

n

ba

u a Th

e

or Nodus

DRACO

RTA

b

N

MI

AR RS U AJO M

g

ah

b

EU

d

Ku rh

ok b K g SA rkad UR OR

M Bi g D i eg ppe rez r

bCaph

h d Achair dir She

PH

x

er

ab

CE Ald

Dubhe

d

Lit Yildun tle D i pp

g rrai E

g

The Astronomical Companion

Gliauzar

d

ih RuchbaTs h

14

spellings, variant pronunciations, and it is a matter of taste which to choose.

This has been the main stream of our star-name inheritance.

Without feeling free to abuse the tradition, we do not need to regard it as sacred, certainly not as official; there is no clear demarcation around it; it is a lake with wide channels to the lakes of other cultures. Whatever part the populace played in generating the first names, litterateurs early took a controlling position, and they have changed and added. They did so unintentionally by corrupting, and intentionally by borrowing and inventing. Even now, someone who wants to offer a list of the “names” of stars in, say, Fornax will ransack a book and give some Chinese syllables—or some Sumerian ones. (The Sumerian civilization may have been ancestral to all others, but it was utterly lost and only recently dug up, so there can be no pretence here of traditional flow.) As for invention, the most delightful story is that of Piazzi (discoverer of the first asteroid) who in his observatory had an assistant and eventual successor called Niccolo Cacciatore (“Nick Hunter”); Latinizing these names as Nicolaus Venator and reversing them as Sualocin and Rotanev, Piazzi in his Palermo Catalogue of 1814 smoothly attached them to two stars of the little Dolphin constellation. The secret eluded even his friends; authors solemnly puzzled out the etymology of the names with ludicrous results; and the old process of corruption set in

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The Astronomical Companion Ròtanev (b Del): Lat. Venator, “hunter,” reversed.

Rúkbah (d Cas): Ar. ar-rukbah, “the knee.”

Rúkbat (a Sgr), same.

Sàbik (J Oph): Ar. as-sâbiq, “the preceding.”

Sadáchbia (g Aqr): Ar. sa‘d al-akhbi’ah, “lucky [star] of the tents”—plural of khibâ’ as in Alchiba. The root s-‘-d carries the meaning of “happiness, fortune” and appears in the names Saudi, Sadat, Assad (as‘ad).

Sadalbàri (m Peg): Ar. sa‘d al-bâri‘, “lucky [star] of the excellent one.”

Sadalmélik (a Aqr): Ar. sa‘d al-malik, “lucky [star] of the king.”

Sadalsùud (b Aqr): Ar. sa‘d as-su‘ûd, “luck of lucks”—very lucky.

Sádr (g Cyg): Ar. as≥-s≥adr, “the breast.” Cf. Shedir.

Sàiph (k Ori): Ar. as-saif, “the sword.”

Sàrin (d Her), given in some catalogues; I don’t know origin.

Schèat (b Peg): Ar. as-sa‘q, “the leg.”

Scùtulum (i Car): Lat., “little shield,” diminutive of scutum; see Aspidiske.

Ségin (e Cas) and Segìnus (g Boo): I don’t think anybody knows the origin.

Shàula (l Sco): Ar. ash-shaulah, “the raised [tail].”

Shédir or Schédar (a Cas): Ar. as≥-s≥adr, “the breast.” Cf. Sadr.

Shélyak or Sheliak (b Lyr): Per. shalyâq, “tortoise” (from whose shell the lyre was made). Cf. Sulafat.

Shératan (b Ari): Ar. ash-sharat≥ain, “the two signs” (Pisces and Aries).

Sírius (a CMa): Gk. seirios, “scorching.”

Sírrah (a And): see Alpheratz.

Skat (d Aqr): Ar. as-sa‘q, “the leg,” like Scheat, and rather similarly corrupted.

Spìca (a Vir): Lat., “ear of wheat” held in the hand of the maiden.

Stérope (21 Tau): Gk., one of the 7 Pleiades sisters.

Suálocin (a Del): Lat. Nicolaus reversed; see Rotanev.

Suheil: the Arabic word sahl means “smooth” and a “plain” (as in the Sahel across Africa south of the Sahara); the diminutive suhail is a boy’s name and the name for Canopus; and it is applied to many other stars low on the southern horizon— Alsuhail is only one of them—in combination with the words wazn and muhlifain.

Sùlafat or Sulaphat (g Lyr): Ar. as-sulah≥fah, “the tortoise”; Cf. Shelyak.

Sy"rma (i Vir): Gk., “train.”

Tània Austràlis and Boreàlis (m and l UMa) and Tàlitha A. and B. (k and i UMa): Ar. ath-thâniyah and ath-thâlithah, “the second” and “third,” with Lat. words for “south” and “north.” See Alula; and Kaus. iUMa is also called Dnoces. The root th-l-th, “three,” is also in Mothallah.

Tárazed (g Aql): Per. shâhin-e-tarâzad, “plundering falcon,” applied to the constellation; see Alshain, which preserves the “falcon” part, with Arabic al- prefixed. Actually I do not know tarâzad; the -zad part means “striking.”

Tarf (b Cnc): Ar. at≥-t≥araf, “the tip.” Contrast Alterf.

Taýgeta (19 Tau): Gk., one of the 7 Pleiades sisters.

Tégmine (m Cnc): ablative case (for some reason) of Lat. tegmen, “cover.”

Téjat (m Gem): I don’t know origin. Actually this star is Tejat Posterior; Tejat Prior is the one that also has the name of Propus.

17

Thùban (a Dra): Ar. ath-thu’bân, “the snake.” Cf. Rastaban, Eltanin.

Tsih or Cih (g Cas): Chinese, said to mean “whip.” I can’t vouch for it. Also called Navi.

Tyl (e Dra), given in some catalogues; origin undisclosed.

Unukalhai (a Ser): Ar. ‘unuq al-h≥ayyah, “neck of the snake.”

Véga (a Lyr): Ar. al-wâqi‘, “the stooping” eagle. Nothing to do with Spanish vega, “meadow.” See Altair.

Vindemìatrix (e Vir): Lat., “vine-harvestress.”

Wásat (d Gem): Ar. wasat≥ as-samâ’, “middle of the sky,” i.e. the ecliptic, to which it is near.

Wazn (b Col): Ar. al-wazn, “the weight.”

Wézen (d CMa): same.

Yed Prior and Posterior (d and e Oph): Ar. al-yad, “the hand,” and Lat. Cf. Tejat. A more westerly star “precedes” an easterly one because it moves before it across the sky, reaching the meridian first.

Yíldun (d UMi): apparently Turkish yÈldÈz, “star.” Cf. Kokab.

Zàniah (e Vir): Ar. az-zâwiyah, “the corner.” Cf. Zavijava.

Zàurac (g Eri): Ar. az-zauraq, “the boat.”

Zavíjava (b Vir): Ar. zâwiyat al-‘awwâ’, “corner of the barker.” Cf. Zaniah and Auva. Also called Alaraph.

Zósma (d Leo): Gk. zôsma, “enzonement, loincloth.”

Zùbenelgenùbi and Zùbeneshamàli (or -sch-) (a and b Lib): Ar. azzubân al-janubi and ash-shamâli, “the southern claw” and “the northern claw” of the Scorpion (to which they once belonged).

See Acubens.

1460? -1528 Al-Birjandi thought-experiments like Galileo's 1403 -1474 Ali Qushji science free from religion 1401 -1464 Nicholas of Cusa anticipated Copernicus, Galileo, Kepler 1394 -1449 Ulugh Beg great observatory at Samarkand 1323? -1382 Nicole Oresme the Earth rotates 1304 -1375 Ibn ash-Shatir brought Ptolemy closer to reality Some medieval astronomers.

1221 -1284 Alfonso X of Castile the Alfonsine Tables 1201 -1274 Nasir ad-Din Tusi got the Mongols to build his observatory 1195? -1256? John of Sacrobosco "Tractatus de Sphaera", textbook for 4 centuries 1149 -1209 Fakhr ad-Din ar-Razi suggested many worlds, many universes 1114 -1185 Bhaskara II mathematical astronomer 1048 -1131 Omar Khayyam observatory; calendar more accurate than ours 1031 -1095 Shen Kuo eclipses, improved instruments 1029 -1087 Az-Zarqali (Arzachel) astronomical instruments and tables 988? -1061? Ali ibn Ridwan described the supernova of 1006 965 -1040? Ibn al-Haytham (Alhazen) all-round scientist 946? -1003 Gerbert d'Aurillac (Pope Sylvester II) brought Muslim astronomy to Europe 940? -1000 Abu Mahmud Khujandimural Khujandi mural sextant, obliquity of Earth 903 - 986 Abd ar-Rahman as-Sufi (Azophi) first noted Andromeda and Large "Magellanic" Cloud 803 - 873 Muhammad ibn Musa ibn Shakir one of 3 astronomical "Sons of Musa" 800? - 870? al-Farghani (Alfraganus) measured the Earth, translated Ptolemy 787 - 886 Abu Ma`shar al-Balkhi (Albumasar) preserved Aristotelian theory for Europe 598 - 668 Brahmagupta invented zero 476 - 550? Aryabhata founder of Indian astronomy

DESIGNATIONS Referring to stars as “Mirfak (the one in Hercules, not the one in Perseus)” or “the second of the ten lashes of the whip in the hand of the Charioteer” remained practicable only as long as one commanded plenty of time and not many stars. There have to be briefer and more extensible designations. But many different systems have been started, no one of them good for all purposes.

Bayer letters, principally Greek letters, were mainly introduced by Johann Bayer on the maps in his Uranometria of 1603.

But the idea dates to earlier; Bayer’s letters were not generally used till the next century; he applied them only in the 48 ancient constellations, not in the 12 new ones that he invented, and only after his time were letters assigned in these and other new constellations.

In some constellations all 24 Greek letters are used; in Lynx and Vulpecula only a, and in Leo Minor only b and o! One Greek-letter sequence serves the three constellations into which Argo was broken—Carina, Puppis, and Vela—so the latter two also have no a.

If there is an outstanding lucida star it is usually a, but the order of the other letters only very roughly corresponds with their brightness: it is more like an order of importance in the constellation figure.

Thus in Cygnus Bayer first lettered the principal 2nd- and 3rd-magnitude stars that form the cross shape, then worked with decreasing systematicness around the 4th- and 5th-magnitude stars till the letters were used up. Orion and Gemini each have two outstanding stars, and in each case Bayer awarded a to the one that is in fact less bright—Betelgeuse perhaps because it is conspicuous by its redness and because its rival Rigel is lower in the sky and more liable to dimming by the atmosphere; Castor probably because he is customarily named before his twin Pollux. The Big Dipper stars are lettered west-to-east along the Dipper. In west-to-east order both Cassiopeia and Corona Borealis happen to be b a g d e. The most alpha beta gamma delta epsilon zeta eta theta iota kappa lambda

A B G D E Z H Q I K L M

a b g d e z h q i k l m

nu xi omicron pi rho sigma tau upsilon phi chi psi

N X O P R S T U F C Y W

n x o p r s t u f c y w

perverse order is in Sagittarius, where a and b are minor stars off in a southeastern corner, while the brightest star at the center of the figure is no higher than s.

In designating a star, the letter is followed by the constellation name in the genitive—which is the reason why astronomers have to learn Latin genitives if they learn no other grammar! Thus Aldebaran is Alpha Tauri, “Alpha of Taurus,” which we can write that way, or as a Tauri, or a Tau. Only the lower-case forms are used as Bayer letters (we show the capitals in case you want to know). The vowels eta and omega were (in ancient Greek) longer and with lower tongue-positions than epsilon and omicron.

Bayer gave alternative Greek letters to a few stars that belong to two constellation figures: b Tau (El Nath) = g Aur, a And = d Peg, s Lib = g Sco. Now only the first in each pair is official.

A letter may be spread to more than one star by means of subscript numerals (sometimes written superscript). Thus a1 and a2 Capricorni and a1 and a2 Librae are wide double stars (in each of which a2 is the brighter); but o1 and o2 Eridani are a degree apart and at very different distances. p1-6 Orionis and t1-9 Eridani are loose chains of stars; and 19 stars spread over an area of Auriga (the “Lashes” of the Charioteer’s whip) are all called y.

Where the 24 Greek letters ran out, Bayer resorted to lower-case roman letters (such as s Carinae) and finally to upper-case ones (G Scorpii).

Flamsteed numbers (1, 2 . . . ) were actually instroduced by Halley and Newton in a 1712 prepublication of Flamsteed’s catalogue unauthorized by Flamsteed himself, did not appear in Flamsteed’s own version (published posthumously in 1725), and gained currency with Lalande’s 1783 version of Flamsteed’s catalogue. The numbers are assigned more systematically than Bayer letters, through each constellation in order of right ascension (west to east); they include all stars down to about magnitude 6. For stars that also have Bayer letters those tend to be preferred. The numbers reach as high as 141 Tauri. By Lalande’s time all 88 of our constellations were in existence, so he might have given us a uniform system; but he carried it down only to declination —35°. So Eridanus, Puppis, Centaurus, and Scorpius are only partly numbered, and all the more southerly constellations not at all.

Then came many more kinds of designation, in many of which the first few symbols refer to somebody’s catalogue, and the remaining numbers are sequential or else are a brief form of a map position (thus 1653+40) means right ascension 16h53m, declination +40°).

Examples of designations from star catalogues: Grb 1830: Groombridge’s catalogue of circumpolar stars, 1838.

BD 2579: Bonner Durchmusterung, by Argelander, 1859.

DM —52°12220: Cordoba Durchmusterung (of southern stars), 1892.

HD 245770: the Henry Draper Catalog, actually by Annie Jump Cannon and E.C. Pickering, 1918-1924.

GC 5605: Boss’s General Catalogue, 1936.

ZC 3539: Zodiacal Catalogue, by Robertson, 1940.

SAO 122731: Smithsonian Astrophysical Observatory catalog, 1966.

LSI +61°303: Luminous Stars in the Northern Milky Way, I, 1959.

GJ 1061: Catalogue of Nearby Stars by Gliese and Jahreiss, 1991.

GSC 9537 380: the Guide Star Catalog, 1989 and versions to 2009.

HIP 118218 and TYC 9537 380 1: Hipparcos Catalogue and even larger Tycho Catalogue of stars measured by the Hipparcos satellite in 1989-93.

Double stars: ADS 17020: Aitken’s catalogue.

S 3001: published by Friedrich Struve.

OS 507: published by Otto Struve.

Variable stars (see the section on them): R Sagittarii, AQ Sagittarii, V356 Sagittarii, etc.

Clusters, nebulae, and galaxies: M44, the famous Messier list of objects that might be mistaken for comets, compiled by Charles Messier and published 1771-86.

H IV 37: Sir William Herschel’s nebula no. 37, of class IV (planetary nebulae).

NGC 2632: Dreyer’s New General Catalogue, 1888 (based on Sir John Herschel’s).

IC 1954: Index Catalogue, one of the supplements to the NGC. III Zw 2: Zwicky’s third list of galaxies.

Most rapidly proliferating now are the objects of the new branches of astronomy: Circinus Z-1: first X-ray source in the constellation Circinus.

SMC X-2: second X-ray source in the Small Magellanic Cloud.

ESO 113-IG45: European Southern Observatory southern sky survey in blue light, field 113, interacting galaxy 45.

3C 231: Third Cambridge Catalogue of Radio Sources.

PSR 1913+15: a pulsar.

MXB 1730-335: Massachusetts Institute of Technology Z-ray burst sources.

OAO 1653-40: objects discovered by the Orbiting Astrophysical Observatory (the Copernicus satellite).

One object may bear several different designations in its aspects as double star and variable star, as nova and nova remnant, as optical and non-optical source.

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The Astronomical Companion

September December

June Sun

March

c r rcti nce -- -Ca rn rico p a ---C

---A

A view with the ecliptic plane level, and from a lower angle (ecliptic latitude 7.5°, longitude 190°).

Sun enlarged 5 times, Earth 2000 times.

After the March equinox, the Sun moves to ecliptic longitude 1°, 2°, 3° . . . Since there are 360° in the circle and 365¼ days in the year, it gains a little under a degree a day. It moves through the rest of Pisces to Aries . . . Always on the ecliptic, it moves north of the Earth’s equatorial plane; its declination changes from 0° to +1°, +2°, more slowly than the gain in longitude. (The Earth’s equatorial plane, moving with the Earth, is dropping south of the Sun.) The Sun’s rising-point moves north along the horizon. It moves slowest if you live on the equator, faster farther north, till at the pole it moves at infinite speed—that is, it instantly leaves the horizon.

Everywhere at sunrise shadows point south at least slightly (since the Sun is rising north of east). In the north hemisphere, this lasts only till the Sun has slid up to where it is east from you. Still the only place for which the Sun rises vertically is the equator, but it does not rise toward the zenith (being now on a “small circle” of the sphere). For a place just north of the equator, the Sun, though climbing at a slight slope, reaches the zenith at noon.

The Sun sets a little north of west. Like any star in the north celestial hemisphere, it has spent slightly more than 12 hours in the sky for places in the north hemisphere of the Earth—days are getting longer than nights—and slightly less for the south.

The north hemisphere of the Earth is starting to tilt toward the Sun, and will remain so till the other equinox. As seen from the Sun, the north pole has crept inward of the limb: it is nearer than the center of the Earth is, and remains permanently in view till the other equinox; the other pole has disappeared till the other equinox. The terminator, since it does not pass through the poles, slants across the map. The people for whom the Sun is rising at the same time as for you are those north and slightly west of you, and south and slightly east. (And the opposite at sunset.) The lines of latitude as seen from the Sun begin to appear curved. Those in the south hemisphere are shorter, and some at the south end disappear; some around the north pole come completely in view as narrow ellipses. But of the equator always half remains in view. The “top” of the Earth remains a point on the Arctic Circle (though appearing closer to the pole); but the trailing and leading points of the Earth move inward from the two tropics toward the equator, while the points where the two tropics touch the ecliptic plane move around toward the subsolar point (Cancer) and the antisolar point (Capricorn).

The rate at which the Sun’s rising- and setting-points creep northward along the horizon is fastest at the equinox; toward the June solstice it becomes imperceptible. The same happens with the northward movement of the latitude at which the Sun is overhead; and the lengthening and raising of the Sun’s trajectory in the sky and the lengthening of the daylight; and the tilting of the north hemisphere toward the Sun and the shortening of the distance of the north pole from the Sun; and the northward gain in the Sun’s declination.

At the June solstice, the Sun reaches geocentric longitude 90°, on the Taurus-Gemini border (the Earth being at 270° heliocentrically). The Sun is at its northernmost declination of 23½° in the map of the sky. (The equatorial plane, centered in the Earth, is dipping farthest south of the Sun.) The Sun’s rising- and setting-points are farthest north around the horizon. For places on the equator this is just 23½° north of the east and west points.

For places farther north it increases; till at 66½° N. (the Arctic Circle) it is 90°: that is, the rising- and setting-points are the same, the north point; the Sun on this one day just touches the horizon and does not set. For the Antarctic Circle the Sun comes up from below and touches the horizon and goes down again. For the north pole, the Sun’s rightward circle around the sky (parallel to the horizon as always) is highest—23½°—above the horizon.

The Sun seems to pass overhead at noon many days in a row for people in the Sahara and Mexico and other places along the Tropic of Cancer.

(This may be a reason why they, and the Australian and Kalahari and Atacama deserts on the other tropic, are more scorched than the equator, where the Sun is overhead twice a year but moves away more quickly.)

Outside the two tropics, the Sun is never overhead (nor is it ever overhead for anyone except at noon). Shadows (north of the Tropic of Cancer), pointing north at noon as always, are at their shortest. Students, like ancient men, can mark the tip of a pole’s shadow and thus determine the time of noon; mark it each noon and thus determine the date of solstice.

Days are longest for the north hemisphere, nights for the south, and they are still essentially equal at the equator. The north hemisphere is most tilted toward the Sun (and the south away) by the maximum angle of 23½°.

As seen from the Sun, the north pole is on the north-south meridian of the Earth; it is at its minimum distance from the Sun (as compared with the center of the Earth). The “top” of the Earth, still on the Arctic Circle, is now also on the central meridian, all the Arctic Circle being in view from the Sun; the “bottom” of the Earth is on the Antarctic Circle which, but for this one point, is out of view on the night side; the bow and trailing points of the Earth are both on the equator; the subsolar point, center of the disk as seen from the Sun, is on the Tropic of Cancer which here dips to touch the ecliptic plane, while the other tropic rises to touch that plane at the antisolar point on the night side.

All these geometrical rhythms can be filled in by analogy for the other three quadrants of the year.

There are some modifications for more fastidious accuracy: —Atmospheric refraction curves the path of light by more than half a degree (34´) at the horizon. So the Sun appears on the horizon when geometrically it is this much below. This lengthens every day by about 2¼ minutes at either end for places on the equator, where the Sun is rising straight up; by longer times for places at higher latitudes. For the poles it means that on each equinox day the Sun appears not exactly on the hori-

zon, but more than half a degree above; so the polar “day” is not exactly half the year, from equinox to equinox, but 1.445 days longer at each end.

—The Sun is not a point but a disk with angular width of about half a degree (32´). It is so bright that daylight begins as soon as the merest point of it shows, without waiting for its center. So day begins at the equator about another minute earlier (the time it takes for the center to climb vertically half the Sun’s width), more at other latitudes where the Sun climbs slantingly. At the north pole the upper limb of the Sun creeps into view .68 of a day before the center does. So the combined effect is that the Sun comes into view here 2.125 days before the March equinox and stays in view for as long past the September equinox; at the south pole the converse, so the two polar “days” overlap each other by 4¼ days. Seen from a “real” point of view: the Sun is huge, and there is a time when light from the northern part of it is shining on (and a little past) the north pole of little Earth while light from the south of it is shining on our south pole.

—The equinoxes and solstices are strictly instants, not days. So it is not really true that at the March equinox, for example, the Sun for all places rises at the exact east point. It does so only for those places (along a north-south line of longitude) where the instant of sunrise coincides with the instant of equinox. For the next band of places, the Sun as it rises is already a fraction of a degree farther along the ecliptic and a hair farther north along the horizon. And the band of places for which it was setting at the instant of equinox is on the opposite side of the globe.

—The Earth’s orbit is elliptical. So for example the north pole is nearest to the Sun at the June solstice only as compared with the center of the Earth, a difference of a mere 6,000 kilometers. The whole Earth is 2,500,000 kilometers nearer in January and farther in July.

—Also because of the ellipticity of its orbit, the Earth’s speed along the orbit varies, while it rotates at a constant rate; this causes a variation in the length of the day, so that it is not quite true that, for example, day and night remain of equal length all year at the equator.

We might expect our changing distance from the Sun to be the reason why we get hotter and colder. Since perihelion brings us 2,500,000 kilometers nearer than average in January and aphelion as much farther out in July (whereas the axial tilt only ever makes a 400th of this difference), we might think that the whole Earth would have summer in January and winter in July; or at least that northern winter and southern summer would be warmer, and northern summer and southern winter colder—in other words, for the north hemisphere to have moderate seasons and the south extreme ones. But it is the other way around! The extreme temperatures are recorded in the continents of the north hemisphere. This is because of the greater amount of ocean in the south. Water gains and loses heat more slowly than land. (The exceptions which indeed prove the rule are the caps of the two hemispheres, water for the north and land for the south: Antarctica has more extreme temperatures than the Arctic Ocean.) So not only is Earth’s climate kept steady by the comparative smallness of its orbit’s eccentricity, but it is kept even steadier because the oceans work in opposition to the eccentricity.

For some other planets, which have more eccentric orbits and do not have oceans at all, this is a season-causing factor more equal with rotational tilt. We can see it on Mars. The north hemisphere of Mars leans Sunward on the more distant side of its orbit (around aphelion). So on this side of the orbit the north has a not very warm summer, with the north polar icecap not shrinking much, while the south has a terrible winter, its icecap spreading greatly. On the other side of the orbit, the north has a relatively mild winter, with its icecap not spreading much, while the south has (for Mars) a hot summer, in which its icecap may disappear altogether.

What most greatly affects our temperature is not the changing distance from the Sun, nor the changing length of daytime: it is the angle at which sunlight strikes. Looked at from where we are, this is the altitude of the Sun. If the Sun is at or below the horizon, If the Sun is at or below the horizon, you are getting no heat from it. As it climbs, the amount of heat you receive from it rapidly goes up; the growth slows as the Sun reaches its highest. This suggests a sine curve, and in fact the amount of radiation hitting the atmosphere is proportional to the sine of the Sun’s altitude.

Then add the effect of the atmosphere, which reduces the radiation received, and is, say, 1000 kilometers thick. As the Sun’s altitude lowers, its light is coming more slantingly through the atmosphere, till at altitude 0 it is traversing about 3700 kilometers of it. Thus the atmosphere exaggerates the difference caused by altitude. But it also scatters sunlight and thus brings it to us by routes other than the direct; so that for instance the amount is not zero when the Sun’s altitude is zero—we are receiving some glow after and even before dawn. Then add, of course, the clouds in the atmosphere and other variations of weather.

In general, though, the most radiation per unit area is reaching Earth at the subsolar point, the center of the disk as seen from the Sun, and it decreases from there to the limb, where the Sun appears on the horizon.

Some places, in the tropics, move from the limb to the center; others traverse shorter arcs across the Sunlit face and do not reach the center. As the subsolar point moves to its northernmost in June, all places north of the Tropic of Cancer have their longest times in the sunlight and come their

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MOON’S ORBIT The Earth is a planet, revolving around the Sun in a year, and the Moon is a satellite, revolving around the Earth in about a month— this picture is now familiar to almost everyone, and is approximately true, but can be misleading.

Suppose you are on some comet high above the solar system.

You might see the third planet, far out from the Sun, as a tiny speck of light, or you might be able to resolve it into two specks, one 4 or 5 magnitudes fainter than the other. Together they pursue their vast yearly cycle.

As they go, you may notice them gradually changing places.

Sometimes the smaller speck is in the lead, then it slows slightly, thus falling back to the inside position, and then to a position in the rear. Then it speeds up again—slightly—till it overtakes on the outside, and continues forging slowly ahead till it is again in the lead.

The familiar picture tends to make us imagine that the Moon’s course through space must describe loops or perhaps cycloid cusps: But, in fact, it does not even include convex waves: It is everywhere concave toward the Sun! This statement amazes people, but from the true-scale diagram it becomes easy to accept. The distance the Moon oscillates in and out is only ¼ of 1 percent of the radius of the huge circle. And it performs this tiny oscillation only about 12 times in going around the circle. In the large diagram, about 1/12 of the circle is drawn, connecting the positions of the Earth. If you draw a curve connecting the positions of the Moon, it is almost the same circle. The curvature just decreases slightly around the time of new Moon.

A truer picture still is that both Moon and Earth revolve around the barycenter (center of mass) of the Earth-Moon system; and it is this barycenter that revolves in a smooth orbit around the Sun. The Earth has 81.3 times as much mass as the Moon. So their barycenter is 1/81.3 of the distance from the center of the Earth to the center of the Moon. This proves to be inside the Earth, 4728 km from the center and 1650 km (1025 miles) below the surface. There is no one particle that is the barycenter: the Earth rotates, so the barycenter keeps traveling (at an average speed of 1196 km/hr) through the Earth’s mantle, staying always below the Moon. If you happen to be in a tropical country, then at some time in the month the Moon will pass over your head, and at that moment you can tell yourself that the barycenter of the Earth-Moon system is a thousand miles under your feet, gliding through the rock at the speed of a fast jet plane.

The Earth, then, sways around the barycenter, but slowly, with a period of about a month: the main mass of the Earth keeps on the opposite side to the Moon, but has a much shorter circle to travel.

When astronomers, standing on the Earth’s surface, measure the apparent position of some other planet, they are not measuring from a smoothly traveling station, so they find that, for instance, Mars when at its nearest appears displaced up to 17″ ahead of where it should be (when Earth is swinging behind the barycenter) or 17″ behind (when Earth is ahead of the barycenter). Indeed this provided the experimental, as opposed to theoretical, way of calculating where the barycenter is, and of determining the relative mass of Earth and Moon.

Having first become clear about the true almost-circular orbits of barycenter, Earth, and Moon around the Sun, let’s revert to the more familiar picture in which we imagine the Earth (or barycenter) standing still and the Moon therefore in a far smaller almost-circular orbit around it. This, after all, is convenient in thinking about the relations of these two little planets.

Periods of the Moon in its orbit around the Earth are also called “months.”

The Moon’s mass (1/81.3 of the Earth’s) and average distance from the Earth determine its averaqge period around the Earth, 27.32166 days. This is the sidereal month; that is, in relation to the stars, to space in general. If at a certain instant the Moon is crossing the line in space that leads from us to the star Omega Piscium, then a sidereal month later it will again be in that same direction.

However, the map position of Omega Piscium will not be quite the same: because of the Earth’s precession, the star will have moved a small distance (about 3.76”) eastward; or, more truly, the map framework will have moved westward. Suppose, at the first pass, the star was on the R.A. 0 line: then, the next timearound, the Moon will reach this position about 7 seconds of time before it reaches the star. So this tropical month, or period in relation to our sky-map coordinates, is 7 seconds shorter, or 27.32158 days.

Suppose also that on the first pass the Sun was in the same direction as Omega Piscium; in other words, this was the moment of new Moon. A sidereal month later, when the Moon comes round to the direction of Omega Piscium, the Earth-Moon system has moved 26.929 of its curved orbit around the Sun (360 × sidereal month/sidereal year); so the direction to the Sun is now that much different. So the Moon has to travel on considerably farther before again reaching the Sunward line. So this synodic (or synodical) month is 29.53059 days. It is the period from new Moon to new Moon, during which the Moon goes through its cycle of “phases” or apparent shapes to us, caused by the angle at which we see the Sunlit side of it.

This cycle is most conspicuous to mankind, so the calendar month originated from it.

The synodic month is a sliding measure of time; one can also say that two full Moons, or first quarters, or any other phases, are a synodic month apart. A word for the anchored unit of time, from a certain new Moon to the next new Moon, is lunation. Lunations are numbered in a series begun by E. W. Brown from 1923 Jan. 16; thus lunation 953 began with the new Moon of 2000 Jan. 6.

Eccentricity. The Moon’s orbit around the Earth is not quite a circle, but an ellipse, with an eccentricity of .054900, the Earth (or,

strictly, the barycenter) being at one focus of the ellipse. In other words, the Moon’s distance from the Earth varies by about 5.49% more or less than its average distance. This is not greatly eccentric: about 3 times more so than the Earth’s (or, strictly, the barycenter’s) orbit around the Sun, and almost exactly the same as Saturn’s.

(The orbit of the Earth’s center around the barycenter must, then, be a tiny ellipse of the same shape. Thus the depth of the barycenter beneath Earth’s surface varies between about 1900 and 1400 km. When the Moon is at perigee, the barycenter is deepest below Earth’s surface.)

At perigee the Moon looks larger, and brighter, and appears to move faster through the sky, both because it is nearer and because it actually is moving faster in its orbit. From these observations Hipparchus discovered its varying distance from Earth, though he thought it could be accounted for by a circular orbit with the Earth placed eccentrically. And this did fit well enough for many centuries, till the observations became more exact.

The ellipticity varies. Roughly, at the times when the ellipse has its axis pointing toward the Sun, it is stretched to a longer shape; when it is broadside to the Sun, it is pulled more toward the circular.

Thus if the Moon is at perigee at the time of new or full Moon, its position is nearer in—at its closest to the Earth, in fact—and if perigee coincides with first or last quarter, it is farther out. These varying distances of course affect the apparent size of the Moon.

Nor does the ellipse remain oriented the same way in space: its axis precesses, forward. This, too, is due mainly to the pull of the Sun. It takes 3236.2 days (about 8 years and 10 months) for the perigee to precess all the way around. Each time the Moon comes around to where the perigee was before, the perigee has moved about 3° onward, so the Moon has to travel on for another quarter of a day to catch up with it. So the period from perigee to perigee is this much longer than the sidereal month; it is 27.55455 days, and is called the anomalistic month.

Inclination. Satellites, at leat those rather close to their planets as the Moon is, generally revolve in their planets’ equatorial planes.

Look at Jupiter’s flock, obediently circling their master’s middle as if on leading-strings from his belt. The Moon does not do this. It asserts its independence by ignoring the Earth’s equator and swimming along in the same plane around the Sun as the Earth does, the ecliptic plane.

This is not fully accurate. The ecliptic plane is, most truly, the plane in which the barycenter of the Earth-Moon system moves, with Earth and Moon revolving around it. The chances are against the Earth and Moon themselves revolving in exactly the same plane.

In fact they revolve in a plane inclined to it by 5°8´43″ (5.145°).

Thus they rise and fall slightly through the ecliptic plane. The Moon can get up to 37,000 km north or south of the plane (11 times its own diameter), the Earth’s center only 456 km (1/28 of the Earth’s diameter). So the Earth’s body is always floating in the ecliptic plane, even if a little high or low; whereas the Moon does not stay in a plane at all, but rides a gentle roller-coaster around the Sun. (The Earth bobs like a floating ball with the surface (the ecliptic) always around its waist; the Moon, like a dolphin, curving over and under.)

The inclination or tilt (just like the eccentricity or shape) of the Moon’s orbit varies in amount, and precesses in direction; again, the main cause is the pull of the Sun; and, again, the speed of the precession is remarkbly rapid. However, the direction of the precession is backward (retrograde, westward).

The variation is ±9´ (0.15°), with a period of 173 days (just under half a year). Thus the tilt to the ecliptic is at a maximum of 5.295°, and then 3 months later it is at a minimum of 4.995°.

The direction of tilt regresses around the orbit at a rate of 0.053° a day, so that it goes all the way around in 18.6 years (6793.5 days).

This is called the nutation-period. If it is hard to visualize a “plane’s direction of tilt precessing,” recall the sight of a dropped plate, spinning not quite flat to the floor (before settling flat).

At a certain point the Moon is at its ascending node, where it cuts upward through the ecliptic plane. It goes around its orbit, cutting downward again half way around, and after a sidereal month it would reach the same point again. But meanwhile the ascending node has moved 1.44° backward to meet it. It therefore reaches the

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Orbits of the major showers (Cardboard models!)

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Three winter showers. All have “direct” motion (counterclockwise as seen from the north, like the planets). But the Quadrantid and Ursid orbital planes are tilted steeply to the ecliptic plane, the Geminids much less so. The three descend into the part of our orbit that we traverse in December and January. The ascending node of the Quadrantids (where they pass northward through the ecliptic plane) is just inside the orbit of Jupiter. The Geminid orbit, unlike the other two, passes very close to the Sun; the orbit, smaller than any comet’s, was puzzling until it was realized that the parent body is an Apollo asteroid, 3200 Phaethon. The line along the middle of each orbit is its “major axis,” also called “line of apsides” because it connects the perihelion and aphelion points. All these lines intersect at the Sun.

The Lyrid and Perseid orbits are far the largest for any of the major showers, and happen to be almost identical in size and shape. They extend out to 57 a.u. from the Sun (as against 35 for Halley’s Comet and 49 for Pluto); only about ¼ of each of them appears in our picture. Their planes are almost perpendicular to each other. Most of the Lyrid orbit is north of the ecliptic plane, most of the Perseid orbit south. Yet they are so arranged that in both cases the meteors descend steeply across our orbit, at points we pass in April and August. The Lyrids have the very steep inclination of 79°; the Perseids are almost as steep, at 67° (technically 113°, since they are moving in retrograde direction around the Sun). The Perseids were the dominant steady stream of the 20th century. The Lyrids were observed in China in 687 B.C.; showed 90 per hour in 1982. The Draconids are an example of a youthful swarm still moving around their orbit in irregular huge clumps. Notice that the Lyrid ascending node is just outside the orbit of Saturn, while the Draconid comet similarly was captured by Jupiter.

Two July showers. The lines of nodes for the Capricornid and Delta Aquarid orbits almost coincide.

But the nodes are at opposite ends of the line: the Capricornids descend where the Delta Aquarids ascend.

To be exact, we meet the Delta Aquarids slightly earlier, at our orbital longitude 305° (measured from the equinox point, 첛), and the Caricornids at 307°; that is why the date usually given for the Capricornid maximum is July 28 and for the Delta Aquarids, July 30. Actually, the showers spread over many days, since the meteor streams are really broad bundles of orbits of which the one we show is the central one.

Both Capricornids and Delta Aquarids move directly (counterclockwise) around the Sun, and have shallow inclinations (7° and 27°), so that they appear to us rather slow (especially the Capricornids) and come from near the ecliptic (the Capricornids just north of it, the Delta Aquarids just south). The orbit of the Delta Aquarids is the very eccentric (elongated), and has its perihelion point the nearest to the Sun (only 0.069 a.u. or 10 million km). It looks as though we ought to see meteors from another part of the Capricornid stream about January 28 (appearing to come from Pisces or Cetus).

A comet and its meteors. Halley’s Comet itself misses the Earth by many million kilometers: its ascending node is well outside our orbit, its descending node well inside. But the particles that separate from it, mainly at its perihelion passages, follow various diverging orbits (which, barring other influences, will reconverge at the point of separation, though at different times according to the lengths of their new orbits). Thus the particles collectively form a huge bundle of orbits in space, vastly wider than the comet. Within this great “bundle,” those “threads” that happen to intersect the Earth’s orbit are seen by us as meteor streams. Since the Halley orbit is fairly flat to the ecliptic plane (18°), it passes relatively near to our orbit twice, and two of the other threads in the bundle are able to strike us, one on the way in (at the October part of our orbit) and the other on the way out (at the May part). In looking at this model, realize that (1) the particles that encounter us in May as Eta Aquarid meteors are not really the “same” as the Orionids in the sense that they could have hit the Earth in October: they would have passed a few million kilometers above. (2) Other “threads” of the bundle must surround the comet’s orbit in all directions, so that if we were not on the Earth but riding ten million kilometers above we would still see Halley-derived meteors in May and October.

Three showers of November. The Andromedids are catching up with the Earth and hence appear to us as slow meteors; the Leonids are meeting us head-on and so appear extremely swift. The inclination of the Andromedids is only 13°; that of the Leonids, 164°—which is to say, they have retrograde motion, and are inclined 16° to the ecliptic plane the other way. A 16° triangle of cardboard is visible supporting the Leonid plane in the model. The Andromedid and Leonid planes intersect rather close to the Earth’s orbit. Adding the Taurid orbit would make a situation too tangled to picture this way (it is inclined only 5°, and would be mostly hidden by the other two planes). Mentally slide it in along the dashed line till its focus (the dot) is at the Sun. Unlike the other two, the Taurids are coming up from just under the ecliptic plane; that is, we see them at their ascending node. They then pass only 0.375 a.u. from the Sun. They are following (approximately) the shortest cometary orbit, that of Comet 2P Encke.

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DOUBLE STARS

AND OTHER MULTIPLE STARS

A star which, on examination, proves to be two or more stars is a “double star.” This term is in familiar use though the subject includes triple and other multiple stars. A star system is any natural group of one, two, or more stars.

Till 1803 (when William Herschel showed otherwise) it was assumed that doubles are merely optical: stars that happen to be in the same line of sight and have no real connection, one (probably the fainter) being farther away. There are some such: Sarin (d Herculis) is two stars whose paths happen to be crossing). But we now know that at least 60% are true binaries. This term, too, is unsatisfactory (seeming to refer to no more than two stars); so is physically connected (suggesting that there is some kind of bridge between them). What is meant is that the two or more stars are gravitationally bound to each other, and travel together through space, orbiting around their common center of mass or barycenter.

(There is a third conceivable case between the opticals and the

binaries: stars that are actually passing very near each other, and may even be bent into temporary hyperbolic orbits around their common barycenter, but are destined to separate forever, unless, very improbably, they collide.)

Some companion stars such as Proxima Centauri, 36 Ophiuchi C, Van Biesbroeck’s, and Alcor, though they appear close to their primaries, are really hundreds of solar-system widths away, so that it is not quite sure that they are physically connected.

Over 100,000 visual binaries are known. If we add non-visual binaries, and future discoveries, it seems that half the stars in the universe may belong to binary systems. If we take the nearest stars to us as a typical sample, it seems that an even higher proportion may have faint companions, undetectable if farther away. If we take the very nearest stars as typical, it seems that a further proportion may have unseen companions, like Jupiter, slightly too small to have become luminous as stars. If we take our own star as typ-

ical, it may be that even single stars are non-simple in the sense of having systems of smaller companions, grading from planets all the way down to dust. And a distant small companion of the Sun may yet be discovered against the starry background.

Many members of double systems are themselves closer doubles; the large multiples grade into small clusters; a cluster may have doubles within it, or a multiple system as its core. This must have to do with the way stars form, as centers of contraction in nebulae that are irregular in size and density.

On maps, double stars are often distinguished by a line through the symbols.

Binaries break down into the following types according to how they are detected or their physical connectedness is shown (and, roughly, this typology also arranges them in order of discovery and of increasing closeness to each other):

Visual doubles (either optical or binary) are those that can be resolved into separate images. First, there are the naked-eye ones, presumably known of old: Mizar and Alcor (z and 80 Ursae Majoris, 12´ of arc apart); a Capricorni (6´); to keen eyes, a Librae (4´) and e Lyrae (3´). Others, such as q1 and 2 Tauri in the Hyades and q1 and 2 in Orion’s sword, are considered too far apart to count as doubles.

The first telescopic multiples were among Galileo’s discoveries in 1617: Mizar itself; and a triple, q2 inside the Orion Nebula, later found to be quadruple and called the Trapezium; it is now known to have at least 7 other members and is perhaps the heart of a cluster.

For some visual doubles the evidence that they are binary is only that they are at a common distance, or show common radial velocity (speed toward or away from us, deduced from the blueward or redward shifting of lines in their spectra), or common proper motion (they travel parallel to each other over the years). Thus the two stars of Albireo, being at least 100 times as far apart as the Sun and Neptune, must be orbiting each other so slowly that it could be thousands of years before we detect even which direction they are taking. These pairs are also known as “relfixes” (“relatively fixed,” where the word “relative” is pleasantly ambiguous).

Others are close enough to each other that their orbital motion around each other is detectable.

These include most of the close doubles that challenge the telescopic observer.

Non-visual types: If refined measurements of the proper motion of a star show it moving from year to year in a slightly wavy line, it is probably, as it travels, revolving around the barycenter of itself and a companion too faint to see. Sirius was the classic case of such an astrometric binary from 1844 till 1862 when its companion was found visually.

There may also be irregularities in the orbital curves of two stars in a visual pair, meaning that one of them has a closer companion, perhaps planet-sized.

New doubles in the zodiacal band of the sky are continually being discovered by amateurs from stepwise occultations by the Moon: a star’s light is seen (or photoelectrically measured) to be cut off gradually or in two or more quick steps instead of one.

The spectrum of a star’s light can reveal its duplicity in two ways: First, the lines in the spectrum may shift to and fro, indicating that the star’s radial velocity is rhythmically altering. It must be revolving swiftly around its barycenter with an unseen companion, and the orbit must be roughly edge-on to us. Or the lines may keep dividing in two and then coming together again: the observed light is from both stars and when one is approaching the other is receding. (One component of, again, Mizar was the first example discovered.) The term for this type is spectroscopic binary, but sometimes it loosely covers also the second type: spectrum binary, in which the spectrum is composite, and must contain light from different types of stars.

If two stars happen to revolve in a plane edge-on to us (it must be almost exactly so, or the stars must be close together), they periodically eclipse each other, wholly or partially, as can be seen from sharp or sloping drops in their combined light-curve. These eclipsing binaries therefore are treated as a class of both double and variable stars.

Some of these latter types are thought to be so close that they tidally distort each other or are actually touching (contact binaries), and matter is flowing between them. An example is b Lyrae. In many close systems one star is a nova or other variable, its instability being the effect on it of the other. Or one may be a black hole or other strange body, detectable through its effect on the light of the visible star.

The brighter star of a pair is called the primary. The other may be called secondary, companion, or comes (Latin for “companion”; plural comites).

Their apparent relative position is given by two figures: angular separation (usually in seconds), and position angle of the secondary (measured counterclockwise from the north).

Since the stars are orbiting, both these quantities keep changing. If the orbit is counterclockwise (position angle increasing) it is called direct; if clockwise (position angle decreasing), it is a retrograde orbit. This follows the use of these terms for the solar system, where almost everything moves counterclockwise (as seen from the north). The change in position is rapid for stars physically close together, with short orbital periods, so that printed information a couple of decades old cannot be relied on. Orbit diagrams are more satisfactory in that they show, and explain, the situation for a span of many years.

The true situation is that each star of a pair moves in its own ellipse, around a barycenter which is at one focus of both ellipses. The more massive star has the smaller orbit. More truly still, each orbit slowly precesses, and thus is a rosette rather than an ellipse.

For simplicity we ignore not only precession but the orbital movement of one of the stars (the brighter—which is not always the more massive). We use a frame of reference in which it is held still, and draw the other star’s relative orbit around it (which will be larger than the true ellipse around the barycenter).

The line of apsides is also the major axis of the ellipse. The star moves fastest at periastron and slowest at apastron.

This orbit may be tilted any way to our line of sight, so the apparent orbit usually looks quite different. It may be rounder or more elongated; the primary star appears to be no longer at the focus; the line of apsides is foreshortened and no longer runs along the major axis. Minimum angular separation no longer coincides with periastron nor maximum with apastron; there are often (not always) two minima and two maxima. The only thing always in the same place is the center of the ellipse (half way along both major axis and line of apsides); and the proportion between periastron and apastron distances remains the same.

The line of nodes, running through the primary star, divides the part of the orbit farther from us than the primary star (dashed) from the part nearer (solid). In practice, we do not know which is which unless radial-velocity studies of the star have been made.

Binary stars are important for science because determining their orbits, from many observations, enables measurement of their masses and hence much else about them.

And double stars make good telescopic targets for city-dwellers, because sky brightness does not wash them out as it does nebulous objects or planetary details. Also, the most systematic way to train your eye and test your telescope’s resolving-power is to see which doubles it will “split.” Start with wide pairs, move to closer ones till you find the limit. Just above the limit, you see a definite waist or minimum of light between the stars; just below, you see one elongated image. From such observations W.R. Dawes found the Dawes limit, an empirical formula for the closest double a telescope should split. It is 4.56 seconds divided by the aperture in inches. Thus, a 9-inch telescope 0.51″ 5-inch telescope 0.91″ 1-inch telescope 4.56″ 10 should split 0.46″ 6 should split 0.76″ 2 should split 2.28″ 12.5 2 stars of 0.36″ 7 2 stars of 0.65″ 3 2 stars of 1.52″ 16 separation 0.29″ 8 separation 0.57″ 4 separation 1.14″

To find the combined magnitude of two stars, the formula is: —log(2.512 to the power of —mag1 + 2.512 to the power of —mag2) / —0.4 2.512 is the factor between magnitudes and is the 5th root of 100; 0.4 is the logarithm of it. An easier method (suggested by Kenneth Rose of Bryn Athyn, Pennsylvania), giving a good enough answer in most cases, is: if difference between mags. is this or more: 0 .1 .3 .5 .8 1.1 1.5 2.1 3.4 then subtract this from smaller mag. (brighter star): .8 .7 .6 .5 .4 .3 .2 .1 .0

However: (1) Dawes worked in the 19th century and used refracting telescopes; modern reflectors should do slightly better. (2) Dawes was a seasoned observer; it may take you a while to catch up with him! (3) Blue stars are easier to separate than red ones. (4) The brighter the star, and the larger the telescope, the more the image spreads by irradiation (on eye retina or film); strictly, Dawes’s formula applies to a pair of stars of magnitude 4.5 seen in a 3-inch telescope 7.1 seen in a 10-inch telescope 6 6 7.6 12.5 6.6 8 8.2 16 —brighter stars being more difficult, and fainter ones easier (so long as they can be seen at all). (5) The greater the magnitude contrast, the more difficult: thus d Cygni is difficult though not very close; and Sirius and Procyon are very difficult, the glare of the primaries drowning the faint companions.

To learn that stars have color may come as a slight surprise. They tend to appear just white, by contrast with surrounding darkness. Fainter ones, especially, seem without color because fainter light stimulates only the rods of the retina, which do not discriminate color. However, if two of these pale spots of light happen to be close together, the slight tints in each become more noticeable. Thus double stars offer the strongest sensations of color in the night sky (other than auroras, the eclipsed Moon, some fireballs, and scintillation of stars low in the atmosphere; the spectacular colors that photographs, or printing, can bring out in nebulae are far below the limit of vision in any telescope).

But “color” is not a simple objective measurement: it is a judgment made in the brain, and is affected not only by the mixture of wavelengths in light falling on the cones of the retina, but also by light that fell just before (fatigue), by light falling on neighboring cones (contrast), and by expectations, even by language. Thus the contrast between two stars may not merely heighten the color in each, but may seem to push them apart to strange places in the spectrum. Of two yellows, the more redward may seem amber or purple, the more blueward turquoise or gray . . . For reasons like this, double stars cause not only relatively strong color-impressions but an endless variety of human responses and descriptions.

So if you take up the pleasures of double-star observing you will be led to curiosity about the “real” color of each star. And this is bound up with the questions of its distance, size, and physical nature. The answer is best summed up by the short descriptive formula such as K5 III, where III means that it is a giant, and K5, the spectral class, means that it probably has a certain surface temperature that makes it glow orange. Thus spectral type gives an idea of the color the star would display if we were near to it.

Still, the only star we have seen in such a way is our G2 Sun; when we travel far enough toward other stars to see their surfaces we may be in for some surprises!

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Sphere radius 13,700,000,000 light-years, or about 4,200 megaparsecs.

Radius of inner sphere 1,500,000,000 light-years.

Grid lines on equatorial plane 5.000,000,0000 light-years apart.

can only become a larger and larger fraction of that distance, approaching it as a limit. Also, the number of light-years of distance is the number of years the light has been traveling to reach us; if the distance could become greater than the universe is old, we would be seeing objects before the beginning of the universe of which they are part.

The redshift, however, can increase indefinitely. No matter how distant they get and how close to the speed of light away from us, the light of the quasars still reaches us with the usual speed of light. But we cannot observe it: longer wavelength is the same as lower energy, and it has been shifted off the end of the spectrum, where it has no energy.

The quasars pile up, as it seems to us, just inside the limiting distance which they cannot reach. It would seem from our point of view—or, put another way, if space were not curved—that they must be ever closer together just inside that skin. On the contrary, they are drawing apart from each other at the same increasing rate as from us. From the point of view of each one of them (or whatever has by now evolved out of them) they are the centers; the limiting distance is equally far away all around them, and we are among the objects vanishing toward it.

This Hubble distance, as it is called, is found by dividing the speed of light c (299,792 kilometers per second) by the Hubble constant (or parameter) of recession H (in kilometers per second per megaparsec). This is a number that matters: the size, age, and fate of the universe depend on it. Edwin Hubble estimated it as 550.

That is, if H is 550, then for each megaparsec of distance from us a galaxy cluster or quasar is receding from us 550 kilometers per second faster. If H is smaller the Hubble distance is larger. And the estimates for H gradually came down. Around 1970 there was dispute (with public debates) between those who favored values

around 100 and around 50. In 1979 a likely value seemed to be 55.

Recent studies, using various methods, have found values between 77 and 70, still with uncertainties of up to 15%. So it is with uncertainty that we now pick 71, giving a Hubble distance of about 4,200 megaparsecs, or 13,700 million light-years.

Only figuratively is it the “edge of the universe.” The universe, though finite, has no edge (just as, two-dimensionally, the surface of a balloon is finite but has no edge). It is perhaps the “distance where galaxies would be if they could be moving away from us at the speed of light,” but, as they cannot, it is a distance slightly greater than any actual distance can be—until the universe grows older.

Objects at the largest possible distances from us in opposite directions are twice as far from each other, right? No. They are close to each other, in the sense that they were—at the time when the light now reaching us started from them—near the beginning of the universe, which was then small. In this sense the outermost sphere, with seemingly the vastest area of all, is a point.

But the Hubble distance is not really the radius of the universe at present: the expanding universe is more than 3 times larger.

Objects whose light left them, on its way to us, nearly 13.7 billion years ago have been carried away from us all that time by the universal expansion, and are now at the “co-moving distance” of perhaps 47 billion light-years.

And now it has been found (from the otherwise unexplained dimness of distant supernovas) that the speed of expansion, instead of gradually slowing, must, after half the universe’s present age, have gone over to acceleration. And this in turn requires something else that, being as yet unobserved, is called “dark”: dark energy. Of the whole content of the universe, 73 percent is dark energy, 23 percent dark matter, 4 percent the ordinary matter and energy that we know.

The Big Bang 13.7 billion years ago created space, which expands with the universe; there is no space outside it, and that is why though finite in size the universe has no edge. Nor does it have a center, though what our picture shows as its edge is in another sense its center, since it is where, for us, its beginning is located.

This is unpicturable: a space which has no center; every point in it seems to be the center, surrounded by a different sphere of points that are the most remote. It is unpicturable by our brains because these evolved to survive in a certain small environment.

Since we cannot in any way see as far as this distance—nor could we be situated outside it, since there is no outside—it is paradoxical to make a picture in which we are outside it and can see into it, all the way to the Earth and as far again on the other side. So I have suggested the strangeness by the distorted projection that arises when I bring my eye close to the skin of our universal bubble and peer in.

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2004 Jan Jan Jan Mar Jun Jun Aug Aug Aug Sep Nov

2: 4: 25: 2: 21: 30: 3: 9: 31: 8: 8:

Dec 21:

The Stardust craft passed within 140 miles of comet Wild 2, collecting samples for Earth return.

A U.S. rover named Spirit landed on Mars.

Another U.S. rover, Opportunity, landed on Mars.

Rosetta was launched by Europe. It is slated to orbit a comet in 2014.

For the first time a private space vehicle, SpaceShipOne, reached space.

Cassini, launched in 1997, went into orbit around Saturn, first craft ever to do so.

Messenger was launched on a seven-year journey to the planet Mercury.

The first deployment of solar sails in space was achieved by Japan.

The 63rd and last Atlas II rocket was launched.

The Genesis craft, loaded with solar wind particles, crashed on reentry.

A new Soyuz 2-1a rocket, the first major upgrade since the 1960s, had its maiden launch.

A Delta IV heavy rocket, twice as powerful as a standard Delta IV, made its maiden flight.

2005 Jan Feb Jul Jul Sep Oct Oct Oct

14: 12: 4: 26: 12: 11: 12: 19:

2006 Jan Jan Mar Apr

15: After seven years in space, Stardust landed on Earth with samples from a comet.

19: New Horizons was launched. It will reach the dwarf planet Pluto in 2015.

10: Mars Reconnaissance Orbiter went into Martian orbit.

11: Europe’s Venus Express craft went into orbit around Venus.

Europe’s Huygens probe landed on Saturn’s moon Titan.

The heavy-lift version of Ariane 5 made its first successful launch.

Deep Impact made a successful collision with comet Tempel 1.

America’s manned spaceflight program resumed after more than two years.

A Japanese spacecraft, Hayabusa, orbited asteroid 25143 Itokawa.

Cosmonaut Krikalev set a record for cumulative days in space: 803.

China sent its second manned mission into orbit.

The 368th and last Titan rocket was launched. The first flew in 1959.

Aug 15:

Voyager 1 became the first craft to reach a distance of 100 AU from the Sun.

2007 Jan 11: A Chinese weapon shot down a satellite, creating 35,000 pieces of junk.

May 14: China entered the commercial space arena with a launch for Nigeria.

Sep 27: Dawn was launched to study the asteroids 4 Vesta (in 2011) and 1 Ceres (in 2015).

Oct 29: A Chinese satellite orbited the Moon.

2008 Jan 14: Feb 11: Mar 9: May 25: Jun 4: Jul 26: Sep 25: Sep 26: Sep 28: Nov 8:

The Ulysses craft, launched in 1990, flew over the north pole of the Sun. The European-built Columbus module was attached to the ISS. Europe launched its first space freighter, the ATV, to resupply the ISS. A U.S. probe named Phoenix landed on Mars.

The Japanese module Kibo was attached to the ISS. The first Soyuz 2-1b rocket was launched.

China launched its first 3-person crew into orbit.

China performed its first spacewalk, a feat first performed by Russia in 1965.

Falcon 1 became the first privately-built liquid-fueled rocket to reach orbit.

A craft launched by India went into lunar orbit.

2009 Feb 10: Mar 6: May 14: May 18: Jun 23: Jul 1: Jul 17:

The first collision of two intact satellites occurred as Cosmos 2251 hit Iridium 33.

The Kepler telescope was launched to search for Earth-like planets in deep space.

Europe launched two space observatories, Herschel and Planck.

Shuttle astronauts repaired the Hubble space telescope for the last time.

The U.S. craft Lunar Recon Orbiter went into orbit around the Moon.

The heaviest comsat ever built (15,233 pounds) was launched on an Ariane 5.

The ISS crew complement reached 13 people, the most ever in space at one time.

2010 Feb 1: The U.S. canceled plans to return astronauts to the Moon.

Apr 22: The first robotic spaceplane, the X-37B, was launched by the U.S. Air Force.

Jul 10: Rosetta made a fly-by of the asteroid 21 Lutetia, the largest asteroid yet seen close up.

INDEX 2012 scare: 30 3C 273 quasar: 68 47 Tucanae: 61 61 Cygni: 49; 58 A.A.V.S.O.: see American Association of Variable Star Observers a.u.: see astronomical unit absolute magnitude: 50-51; 53 absorption lines: 53 Acamar: 27 acceleration of expansion: 69 accretion disks: 68 Achelis, Elizabeth: 31 Achernar: 27 active galaxies: 68 Adhara: 50-51 Adonis (asteroid): 40 age of Moon: 35 age of universe: 69 Akkadians: 30 albedo: 36 Alcor: 56 Alexander the Great: 30 Algol: 11; 57 alien beings: 49 Alkaid: 11; 14; 58 All Fools’ Day: 26 All Saints’ Day: 26 Alpha Centauri: inside front cover; 48-49; 50-51 alphabet: 29 Altair: 49 altazimuth: 8-9 altitude: 11 Aludra (Eta Canis Majoris): 60 American Association of Variable Star Observers: 57 Amor asteroids: 41 amorphôtoi: see unformed Andromeda Galaxy: 65; 67 Andromedid meteors: 43; 44 angular measurement: 46 angular momentum: 19 Annunciation: 26; 31 anomalistic month: 32; 38-39 anomalistic month and year: 23 antapex of the Sun’s way: 58 Antarctic Circle: 24 ff. ante meridiem: 22 antihelion source: 44 apex of the Sun’s way: 58 aphelion: 18; 25 Apollo asteroids: 41; 45 apparent magnitude: 50-51 apparent time: 22 apparent vs. real: 4; 18 apparitions: 43 Aquarius, Age of: 29 Arabic: 9; 14; 16 Aratus: 14 Arctic Circle: 24 ff.

Arcturus: 63 Argelander, Friedrich: 57 Argo: 17; 62 Aries: 19; 29 Arietid meteors: 43 Aristotle: 42; 43 ascending node: see nodes asssociations of stars: 59 Assyrian calendar: 30 asterisms: 12 asteroid designations: 43

asteroids: 4; 19; 40-41 Astraea (asteroid): 40 astrology: 19; 29; 30 astrometric binary stars: 56 Astronomical Almanac, The: 22; 41; 50 Astronomical Calendar, The: inside front cover; 22; 30; 31; 37; 50 astronomical unit: 18-19; 47 Aten asteroids: 41 atomic second: 23 Attila: 43 attractor: 67 Augustus: 31 autumn: 26 azimuth: 7 Babylonia: 29; 30; 38 Backlund, Oskar: 43 bar of Milky Way galaxy: 62 Barnard, Edward Emerson: 49 Barnard’s Star: 49 barred spiral galaxies: 62; 65 barycenter: 35 barycenter of Earth-Moon system: 32 barycenter of star system: 56 Bayer letters: 17; 57 Bayer, Johann : 12 Bayeux Tapestry: 43 bearing: 9 Beehive Cluster: see Praesepe Beltane: 26 Benetnash: see Alkaid Bessel, Friedrich: 49 Besselian year: 23 Betelgeuse: 10; 22; 51; 57; 58; 60 Betulia (asteroid): 41 Biela’s Comet: 43; 44 Big Bang: 69 Big Dipper: 11; 12; 22; 58 binary stars: 56 birds, migration: 29 Birkat Ha-H≥ammah: 26 black hole: 54; 67; 68 blackbody radiation: 53 Bode, Johann: 21 Bode’s Law: 21; 40 Bok globules: 53 Bolton, John: 68 Bopp, Thomas: 43 Brahe, Tycho: 42 brightest stars: 50-51 Brigid, Saint: 26 Brocchi’s Cluster: 12 brown dwarfs: 49 bulge, central, of galaxy: 62; 63 Bunton, George: 31 Burnham, Robert, Jr.: 50 Burnham’s Celestial Handbook: 50 Cacciatore, Niccolo: 14-15 calendar month: 32 calendars: 30-31 Caligula: 31 Candlemas: 26 Canis Major: 59 Cannon, Annie Jump: 53 Canopus: 11; 50-51 Capella: 27 Caph (Beta Cassiopeiae): 22 carbon cycle: 54 Castor: 50-51 Catalogue of Nearby Stars: 49 Cat’s Eye Nebula: 19

celestial equator: see equator celestial sphere: 4; 19 celestial sphere, area of: 12 Cepheid stars: 54; 57; 64 Ceplecha, Zdenek: 44 Ceres: 40 Chaffee, Roger: 15 Chandrasekhar limit: 54 Chi Orionis: 61 Chicago asteroids: 41 China: 42 Chinese calendar: 30 Christian calendar: 31 Christmas: 26; 31 Cidenas (Kidinnu): 29 Ciotti, Joseph: 31 Circlet of Pisces: 12 circumpolar: 11; 12 civil time: 22 Clavius, Christopher: 31 Clement of Alexandria: 31 clock time: 22 clusters of galaxies: 67; 68; 68; 69 clusters of stars: 56; 58; 60; 63 Coathanger: 12 cocoon nebula: 53 colliding galaxies: 65 Collinder 399: 12 color index: 53 color of stars: 56 color-luminosity graph: 53 Coma Berenices: 12 Coma Cluster of stars: 58 coma of comets: 42-43 comets: 4; 19; 31; 42-43; 44; 46 comets, objects mistaken for: 59 co-moving distance: 69 companion stars: 56 conjunction, triple of Jupiter and Saturn: 31 constellations: 12-13; 19 contact binaries: 56 coronagraph: 43 Costsworth, Moses: 31 crescent Moon: 34 Cronia festival: 31 cross-quarter days: 26 Crux-Scutum Arm: 62 culminating: 9; 12; 22 Curtis, Heber: 65 dark energy: 69 dark matter: 68; 69 Dawes limit: 56 Dawes, William Rutter: 56 day, kinds of: 23 day, length of: 25 daylight-saving time: 22 day-night cycle: 30 decans: 19 declination: 6; 19; 27-29 deep-sky objects: 17 degenerate matter: 53 Deimos: 40 Delphinus: 29 Delporte, Eugène: 12 Delta-T: 22 Deneb: 50-51; 58; 59; 60 Denebola: 22 density waves: 62 descending node: see nodes designations of asteroids: 40 designations of comets: 43

designations of stars and deep-sky objects: 17 designations of variable stars: 57 Dio Cassius: 31 Diocletian: 31 Dionysius Exiguus: 31 Diphda (Beta Ceti): 59 disk of Milky Way galaxy: 60; 63 distance: 46-47 distance, ultimate: 69 distances between stars vs. galaxies: 65 diurnal motion: 10 diurnal parallax: 47 Domitian: 31 Double Cluster in Perseus: 61 double stars: 50; 56 Draco: 24 draconic month: 33 Dreyer, J.L.E.: 59 Dubhe: 58 dust clouds: 62 dwarf galaxies: 64 dwarf planets: 40 dwarf stars: 53 Dynamical Time: 22 early-type stars: 53 Earth: 20 earthlight: 36 Earth’s orbit: 18-19 earthshine: 35; 36 Easter: 26 eccentricity of Moon’s orbit: 32 eccentricity of orbit: 18 eclipse year: 30; 38-39 eclipses: 35; 38-39; 43 eclipses of Moon: 36 eclipsing binaries: 56; 57 ecliptic: 4; 7; 18-19; 32-33; 63 ecliptic poles: 19 Edgeworth, Kenneth: 19 Edwards Perpetual Calendar: 31 effective temperature: 53 Egyptian calendar: 30 Einstein, Albert: 53 ellipse, elliptical orbit: 18 ellipsoid, triaxial: 37 ellipsoidal stars: 57 elliptical galaxies: 65; 67 elongation: 34 Emiliani, Cesare: 31 emission lines: 53; 68 Encke, Johann Franz: 40 Encke’s Comet: 43 Ephemeris Time: 22 Ephorus: 43 epoch: 29 equation of time: 22 equator: 4; 9; 27 equatorial system: 6; 7; 19 equinox points: 6 equinoxes: 19; 24 ff.; 27-29 Equitable Calendar: 31 era: 30 Eros (asteroid)F: 41 escape velocity: 54 Eta Aquarid meteors: 43 evolution of stars: 53=55 expansion of space: 67; 69 extra-solar planets: 49 facing south: 10 figured area of sky: 27

fireballs: 44 First Point of Aries: 19; 29 First Point of Libra: 29 first-magnitude stars: 15; 50-51 Fish, Age of the: 29 fixed objects: 4 Flamsteed numbers: 17 Flamsteed, John: 17 flare stars: 57 Fleming, Williamina: 53 Flying Wedge: 12 Fomalhaut: 27 Franklin, Benjamin: 22 full Moon: 19 galactic equator: 27 galactic plane: 59; 66 galactic system: 7; 60 galactic year: 62 galaxies: 60; 66; 67; 68; 69 galaxy clusters: 67 galaxy groups: 67 genitive: 12; 17 giant stars: 49; 53 gibbous Moon: 34 Giotto: 43 Gliese, Wilhelm: 49 globular clusters: 61; 63; 64; 65; 67 goddesses: 40 gods, Greek and Roman: 21 Gould, Benjamin Apthorp: 12 Gould’s Belt: 60 Great Attractor: 67 great circle: 10 Great Square of Pegasus: 22 Greek: 12; 14; 16; 19; 20; 34; 40 Greek calendars: 30-31 Greek letters: 17; 57 Greeks: 12; 38 Greenwich: 22 Gregorian calendar: 23; 26; 31 Grissom, Virgil “Ivan”: 15 Groundhog Day: 26 groups of galaxies: 67; 68 guest-stars: 42 Guy Fawkes Day: 26 Hale, Alan: 43 Hale-Bopp, comet: 43 half-wave of eclipses: 38-39 half-year: 38-39 Halley, Edmond: 17; 43 Halley’s Comet: 42-43; 45 Hallowe’en: 26 halo of galaxy: 63 handspan: 37 Hardy, Thomas: 26 Harvard classification: 53 Heaven’s Gate cult: 43 hegira: see hijra Helin, Eleanor: 41 Hencke, Karl: 40 Henderson, Thomas: 49 Henry Draper Catalogue: 53 Herbig-Haro objects: 53 Hercules: 29 Herod I: 31 Herschel, John: 36 Herschel, William: 56; 58 Hertzsprung gap: 54 Hertzsprung, Ejnar: 53 Hertzsprung-Russell diagram: 49; 5255; 57 Hesiod: 14

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The Astronomical Companion Hevelius (Hoevelke), Johannes : 12 Hidalgo (asteroid): 41 hijra: 30 Hilaria festival: 26 Hilda asteroids: 41 Hill, Betty and Barney: 49 Hipparchus: 29; 32; 33 Hipparcos: 17; 47; 49; 50-51; 53 Hirayama families: 41 horizon system: inside front cover; 7; 8-9 horns of the Moon: 34 Horsehead Nebula: 53 hour of right ascension: 6 hour of time: 23 houses, zodiacal: 19 Hubble constant of recession: 66; 69 Hubble distance: 47; 69 Hubble, Edwin: 65; 69 Hudson, George: 22 Huli festival: 26 Hungaria asteroids: 41 Hyades: 58 Hyakutake, comet: 43 hyperbolic orbit: 43; 56 I.A.U.: see International Astronomical Union I.C.: see Index Catalogue Ibex: 19 ice ages: 63 Ides of March: 26; 31 Iliad: 41 illuminated fraction: 34-35 Imholc festival: 26 In Defense of Variety: 29 inclination of meteor orbits: 45 inclination of Moon’s orbit: 32; 38 inclination of the ecliptic: see obliquity; 24 ff.

Index Catalogue: 59 Indian calendar: 30 informes: see unformed Innes, Robert: 49 insolation: 26 intercalation: 30 intergalactic medium: 68 International Astronomical Union: 12; 22; 43 International Fixed Calendar: 31 International Meteor Organization: 44 interstellar medium: 60 invariant plane of the solar system: 19 Iranian calendar: 30 irregular galaxies: 65 jâhiliyya (“time of ignorance”): 14 Jahreiss, Hartmut: 49 Janus (god): 31 Jerusalem: 31; 43 Jesus: 31 jets: 67; 68 Jewish calendar: 30 Josephus, Flavius: 31 Julian calendar: 23; 31 Julian Dates: 23 Julius Caesar: 31 Juno: 40 Jupiter: 20; 40; 41; 42-43 Jupiter and the invariant plane: 19 Jupiter-family comets: 43 Jupiter-masses: 49 Kant, Immanuel: 65 Kepler, Johannes: 31 Kepler. Johannes: 43 Kidinnu (Cidenas): 29 kiloparsec: 47 Kirkwood gaps: 41 Kleszcz, Evarist: 31 Kokab: 27 Kreutz, Heinrich: 43 Kuiper Belt: 19 Kuiper, Gerard: 19 Labor Day: 26 Lacaille: 12 Lady Day: 26 Lagoon Nebula: 27; 60 Lagrangian points: 41 Lalande, Jérome: 17 Lammas: 26 Lashes of Auriga’s whip: 17 late-type stars: 53 Latin: 12; 16; 17 latitude: 6; 29 latitude, ecliptic: 19 latitude, terrestrial: 10; 24 ff. leap week: 31 leap year: 23 leap-days and years: 31 Leonardo da Vinci: 36 Levy, David: 43 libration: 37 light curves: 57 light, speed of: 47; 68; 69 light-time: 47 light-year: 47; 69 limb of Sun: 25

line of apsides of Moon’s orbit: 38 line of nodes: 38 Little Dipper: 12 Local Bubble: 60 Local Group of galaxies: 65; 66; 67 local time: 22 logarithmic universe: 5 Long Count: 30 longitude: 6; 29 longitude of the Moon: 34 longitude, ecliptic: 19 longitude, terrestrial: 24 ff. lucida: 12 Lugnasad: 26 luminosity of stars: 53 luna incognita: 37 lunar mansions: 19 lunation: 23; 32; 38 Lyttleton: 42 M31: 65 M33: 65 M35: 27; 60 M35 cluster: 62 M87: 67; 68 Machholz, Don: 43 Maffei 1 and 2 galaxies: 66 Maffei, Paolo: 66 Magellan, Ferdinand: 64 Magellanic Cloud, Large: 19 Magellanic Clouds: 64 magnitude: 56; 57 magnitude of Moon: 36 magnitude, apparent and absolute: 50-51 main belt: 40 main sequence: 53; 59 major planets: 19 map symbols for double stars: 56 map symbols for variable stars: 57 March equinox: 29 maria on the Moon: 36 Marius (Mayr), Simon: 65 Mars: 20; 40 Mars, seasons on: 25 Marsden, Brian: 43 Martinmas: 26 mass of stars: 53 Maury, Antonia: 53 May Eve: 26 Maya calendar: 30 Mazapil meteorite: 43 mean Sun: 22 Méchain, Pierre: 43 megaparsec: 47 Mercury: 20 merging galaxies: 65 meridian: 9; 22 meridians of longitude: 22 Mesopotamia: 12 Mesopotamian calendar: 30 Messier numbers: 59 Messier objects: 67 Messier, Charles: 43; 59 meteorites: 40; 44 meteoroids: 44 meteors: 4; 19; 43; 44-45 Metonic cycle: 23; 31 metric calendar: 31 micrometeorites: 44 Milky Way: 4; 7; 27; 59; 65; 67 Milky Way, center: 62 Milky Way, disk: 60 Milky Way, rotation: 62 Milton, John: 44 minor planets: 19; 40 Mintaka: 10; 27 minute of right ascension: 6 minute of time: 23 Mira: 51; 57 Molineux, Emerie : 12 month: 30 month, kinds of: 23; 32 month, sidereal: 32 Moon: 4; 32-37 Moon, declination: 33 Moon, interfering with meteor observation: 44 Moon, shape: 37 moonlight: 36 moons: 19 motion of star star: 58 motion, diurnal: 10 Muh≥ammad: 30 multipe stars: 56 Muslim calendar: 30 N.G.C.: see New General Catalogue nadir: 9 names: see also designations names of stars: 14-17; 51 near-Earth objects (N.E.O.s): 41 nearest stars: 48-49 nebulae: 59; 59; 62; 63; 65 nebulae, pre-stellar: 53 Neptune: 21; 42; 43 neutron star: 54 New General Catalogue: 59 New Year’s Eve: 26 Newton, Isaac: 17; 33; 43 NGC2158: 27

NGC2158 cluster: 62 Niyazov, Saparmurad: 31 nodes of Moon’s orbit: 32-33; 38 nodes, ascending and descending: 19 nodical month: 23; 33; 38-39 nomenclature: see designations non-gravitational effect: 43 noon: 22 Norma Arm: 62 Norma Cluster of galaxies: 67 Northern Cross: 12 nova: 31; 57 Nubecula Major and Minor: 64 nuclear fusion: 49; 53; 68 nucleogenesis: 54 nucleus of comets: 43 nutation: 32 obliquity: 18; 27 occultations: 36; 56; 68 Olympiad: 31 Omar, caliph: 30 Omega Centauri: 11; 61 Omega Piscium: 6; 22; 32 Oort Cloud: 42; 46 Ophiuchus: 19 Öpik, Ernst: 44 optical double stars: 56 optical doubles: 50 orbits of double stars: 56 orienting: 10-11 Orion: 12; 59 Orion Arm: 60; 61 Orion association: 60 Orionid meteors: 43 Outer Arm: 62 overview of astronomy: 4 Ovid: 14 Palisa, Johann: 40 Palitzsch, Johann Georg: 43 Pallas: 40 parabolic orbit: 42-43 parallax: 47; 49 parsec: 47 Paschal Moon: 26 Patrick, Saint: 26 Pazmino’s Cluster: 61 peculiar galaxies: 65 perihelion: 18; 25 periodic vs. long-period comets: 43 Perseus Arm: 61 Perseus Cluster of stars: 58 Perseus Double Cluster: 61 Persian: 16 Phaethon (asteroid): 45 phase angle: 34 phases of Moon: 34-35; 37; 38 phases of Moon and Earth: 36 Phobos: 40 Piazzi, Giuseppe: 14; 40; 49 Pickering, William Henry: 53 Pinwheel Galaxy: 66 Pioneer spacecraft: 70-71 Pisces: 31 Plancius, Petrus : 12 planes: inside front cover; 9 planetoids: 40 planets: 19; 20 planets, extra-solar: 49 planets, viewing: 27 Pleiades: 58; 60 Pluto: 19; 21 Polaris: 6; 11; 27; 59; 60 poles: 9 Pons, Jean-Louis: 43 Pope, Alexander: 18 population I and II: 54; 63; 65 post meridiem: 22 Praesepe or Beehive Cluster: 58 precession: 19; 27-29; 32 precession of double-star orbits: 56 precession of Moon’s axis: 37 precession of Moon’s orbit: 32 precessional amounts: 29 prime vertical: 9; 11 projections: inside front cover; 19 pronunciation of constellation names: 12 pronunciation of star names: 16 proper motion: 12; 49; 56 Propus (Eta Geminorum): 27 proton-proton cycle: 53 proto-stars: 53 Provençal (“Quan li jor sont lonc en mai”): 29 Proxima Centauri: inside front cover; 49; 56 Ptolemy: 12; 14 pulsar: 54 pyramids: 27 Quadrantid meteors: 43 quarter-days: 26 quasars: 40; 68; 69 rabbits: 10 radial velocity: 56 radian: 12; 46 radiants: 44

radio source Sagittarius A*: 62 radio sources: 67; 68 Rasalgethi: 14 Rasalhague: 14 real vs. apparent: 4; 18 Realm of Galaxies: 66 recession: 66; 67; 68 red dwarfs: 49; 54 red giants: 53; 57 reddened light on Moon: 36 redshift: 68; 69 reformed calendars: 31 refraction: 25; 36 regression of Moon’s orbit: 38 Regulus: 14; 50-51 Reinmuth, Karl: 40 relativistic speed: 68 Research Consortium on Nearby Stars: 49 rift in Milky Way: 27; 62 right ascension: 6; 19; 22; 27-29 rings: 19 rising and setting: 10; 24 ff.; 35 Roman calendar: 31 Rose, Kenneth: 56 Rotanev (Beta Delphini): 14-15 rotation of Moon: 37 royal stars: 19 Royer, Augustin : 12 Russell, Henry Norris: 53 Sagittarius: 62; 63 Sagittarius A*: 62 Sagittarius Arm: 61 Saint John’s Eve: 26 Saint Lucy’s Day: 26 Samhain (festival): 26 San Juan Capistrano: 26 São Paulo: 26 Sarin (Delta Herculis): inside front cover; 50; 56 saros: 38-39 satellite galaxies: 64; 65 satellites, natural: 19 Saturn: 21 Saturnalia: 26; 31 Scaliger, Joseph: 23 Scancalendar, The: 31 scattered disk: 19 Schiaparelli, Giovanni: 44 Schmidt, Maarten: 68 Scorpius-Centaurus association: 60 Sculptor Group of galaxies: 66 season, eclipse: 38-39 seasons: 4; 24 ff. seasons, words for: 26 Secchi, Angelo: 53 second of right ascension: 6 second of time: 23 semimajor axis: 18 Seneca,Lucius Annaeus: 42 setting: see rising Shapley, Harlow: 63; 65 Shetland Islands: 31 Shoemaker, Eugene: 41 Shoemaker-Levy 9, comet: 43 shooting stars: 44 Sickle: 12 sidereal time: 6; 9; 22; 27 sidereal year: 29 Sigma Octantis: 6; 27 signs of the zodiac: 19; 29 Silvester’s Day: 26 Sinus Medii: 37 Sirius: 50-51; 58 Sky Catalogue 2000: 50 small circle: 10 Solar and Heliospheric Observatory (SOHO): 43 solar system: 4; 19; 68 solar time: 22 Solar Walk: 18 solstices: 24 ff.

Sombrero Galaxy: 67 Sosigenes: 31 south: 10 Southern Cross: 12; 27 space: 69 space exploration: 70-71 Spanish: 12 spectral type: 53-54; 56 spectroscopic binaries: 56 sphere pictures: inside front cover spiral arms: 60; 61; 62 spiral galaxies: 65; 66; 67 spring: 26 standard time: 22 star: 53 stars, nearest: inside front cover; 4849 Sualocin (Alpha Delphini): 14-15 subsolar point: 24-25 Sumerians: 14; 30 summer: 26 summer time: 22; 26 Summer Triangle: 12 Sun, altitude: 25 Sun, angular width: 25 Sun, motion: 58; 62 Sun, position in galaxy: 59; 62 Sun, variation: 57

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sundial time: 22 Sun-grazing comets: 43 Sun’s quit: 58 superclusters of galaxies: 67; 68 supergiant stars: 53 supernova: 31; 54; 57; 67; 69 surface of Milky Way disk: 60 swallows: 26 symbols for the zodiac: 19 synodic month: 23; 32; 38-39 systems of stars: 56 syzygy: 34; 38 T Tauri stars: 53 Table Mountain: 36 Tara: 26 Taurid meteors: 43 Taurus: 12 Taurus and the Alphabet: 29 Teapot: 12 Teegarden, Bonnard: 49 terminator: 24 ff. terminator of Moon: 36 Thales: 27 Theodor, Pieter : 12 Thuban (Alpha Draconis): 27 time: 22-23 time signals: 22 time zones: 22-23 time-units: 30 Titius, Johann: 21 transit: 9 transneptunians: 19; 43 Triangulum Galaxy: 65 triaxial ellipsoid: 37 Trifid Nebula: 27; 60 Trojan asteroids: 41 tropical year: 23; 29 tropics of Cancer and Capricorn: 24 ff. tughra (Turkish monogram): 47 Tunguska event: 43 Turkish: 27 Türkmenistan: 31 Tycho catalogue: 49 U.T.: see Universal Time Ulysses spacecraft: 43 Under-Standing of Eclipses, The: 38 unformed stars: 12 Unicorn: 19 Universal Time: 22 universe, age of: 69 universe, “edge” of: 69 Uranus: 21; 40 Ursa Major: 12 Ursa Major galaxy cluster: 66 Ursa Major Group: 58 Van de Kamp, Peter: 49 variable stars: 50; 53; 57 veering and backing: 9 Vega: 11; 27; 58 Venus: 20 vernal: 26 vernal equinox: see equinoxes; 24 ff.

Vesta: 40 Veterans’ Day: 26 Virgo Cluster of galaxies: 67 Virgo Supercluster: 67; 68 visual double stars: 56 Vogel: 53 voids among galaxies: 68 Voyager spacecraft: 70-71 walls of galaxies: 68 waning Moon: 34 watches of day and night: 30 waxing Moon: 34 weeks: 30 Wells, H.G.: 36 Whipple, Fred: 42 Whirlpool Galaxy: 66 white dwarfs: 53; 54 White, Edward H., II: 15 Willett, William: 22 winter: 26 Wolf, Max: 40 Working List of Meteor Streams: 44 World Calendar: 31 Wright, Thomas: 65 X-ray bursters: 54 year: 4; 18-19 year, kinds of: 23 Yildun: 27 Yule: 26; 31 zenith: 9; 24 ff.; 29 zenithal hourly rate: 44 zero-age main sequence: 53 Zeta Reticuli: 49 Zeta Tauri: 27 zodiac: 19 zodiacal constellations: 12; 29 zone of avoidance: 66 zones, time: 22 Zulu Time: 22

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Hertzsprung-Russell Aurora. This picture (first used on the cover of Astronomical Calendar 2007) pretends that the stars have miraculously f litted to positions in your sky such that the intrinsically brighter ones are higher, and the hotter ones are to the left. It is thus a colored version of the Hertzsprung-Russell diagram (see page 52 of this book), the famous graph that is a tool for understanding the nature and evolution of the of stars.

The upright dimension is absolute magnitude (the Moon happens to mark level 0); the horizontal dimension is spectral class, from hot blue O at the left through B, A, F, G, K, to red M at the right.