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Astronomical Calendar 2013

Dec 25 C /2 0 1

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The planets are exaggerated 500 times in size, but the Sun is at scale. Lines on the ecliptic plane are 1 astronomical unit apart. IS

Vega

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STOP PRESS—Comet C/2012 S1 ISON

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On 2012 Sep. 21, just in time to complicate several parts of this intricate book when it was almost ready for printing, appeared a comet that could, in November and December 2013, become a daylight-brilliant immense-tailed The comet at and after periheSun-Grazer, to rival the few great ones of the past four centuries. lion. Scale 0.5 cm to 1 degree. It was discovered by Vitali Nevskiy and Artyom Novichonok, using a 16-inch The horizons move left (east) reflecting telescope belonging to ISON (the International Scientific Optical each day as the Sun does. The Gemma Network) in the observatory at Kislovodsk at the northern foot M ofaathe tail could be less or more strongsymCaucasus range in Russia. (ISON is, for an acronymic name, less distasteful than ly curved, and could be longer. 19 in PANSTARRS, but how a Comet Nevskiy would have delighted music-loversSarand Russian patriots.) It was in Cancer close to Gemini. More observations, and pre-discovery images found by others, enabled its orbit to be calculated. It was dimmer than magnitude 18, but more than 6 a.u. from the Sun—farther than Jupiter. When in January we look outward to it at opposition in Gemini, it will still Kornephoros be beyond Jupiter. We lose sight of it behind the Sun in July. In August and September, brightening (we hope) past magnitude 11, it may appear in amateur 17 telescopes, though low in the pre-dawn sky. At the end of September it crosses above the orbit of Mars; Mars happens to be there, so that on Oct. 1 they are only 0.07 a.u. apart. In October and November, now out as far as 50° into HERCULES our morning sky, Comet ISON races through Leo and Virgo at a rate increasing from 1° to 6° a day; its brightness increasing, if the forecasts hold, past any star, its tail trailing west. It crosses Earth’s orbit inward on Nov. 1; appears 2° north of Regulus on Oct. 16; dips on Nov. 9 into the short fraction of its orbit that is thi Rasalge hi south of the ecliptic; brushes only 0.38° north of Spica on the night of Nov. 15 17/18 (9 p.m. EST, 2 UT). Bright in the pre-dawn sky, it could begin to stay visCujam ible into the day, a decreasing distance west of the Sun. On Nov. 25, entering Rasalhague Libra and perhaps reaching the magnitude of Sirius, it bustles past (1.2° south of) one of its chief competitors this year, the slower and far dimmer Comet Encke. It catches the Sun in the northern panhandle of Scorpius, and at perihelion, late on Nov. 28, comet whips around star at a distance of only 0.012 a.u. from 13 the center (1,166,000 kilometers or 0.68 of a Sun-radius from the surface), hence at a speed of 190 km per second. Using what formulas we can for magnitude, we have it reaching —12.6, the brightness of the full Moon! So it just may be distinguishable (with care—masking the Sun out) like a lighted match at the Sun’s edge. For New Zealand dawn observers the growing tail may wash far out over Centaurus. At the dawn of Nov. 29 for North America, Comet 11 Celba i the Sun. Its tidal ordeal ISON should rise moments before and to the leftlraof may have wrenched it into fragments—which would add to the brightness. The tail will bend back down toward and perhaps under the horizon. Caution always: predicting the appearance of comets is guesswork, because SERPENS they are lumps of grit and ice which as they warm expel gas and dust—into the (CAPUT) sunlight-reflecting coma and tail—at times and quantities depending on their 9 structures and how they are spinning. So far it does not seem that ISON was in a misleading state of outburst when discovered, or has a composition like that which caused Kohoutek C/1973 E1 to be a relative disappointment. After the dramatic perihelion, we drive like a train toward ISON’s bridge, so see it racing straight north, each day more north and west of the Sun; at first only 7 low in the post-sunrise and pre-sunset skies, then rising rapidly earlier before 4 midnight. The tail slopes north over us so that we see its width as well as length; c e having had time to sprout outward but not to straighten, it gradually angles itself LIBR A D to get ahead, perhaps sweeping over the summer Milky Way. The Moon is full N º in Taurus on Dec. 17, at last quarter in Virgo on Dec. 25. From Dec. 27 the 0 comet is above our horizon all day and night, though lowest at 10 p.m. On Dec. 5 4 26 it crosses out over our orbit, but now we are there and it is almost vertically n o north of us (in the ecliptic sense), in Draco, at its nearest, 0.43 a.u., perhaps OPHIUCHUS z i down to magnitude 4. By the end of the year it is still climbing at more than 3° r a day, and will be north of us in the equatorial sense, passing within 4° of the o h north celestial pole on 2014 Jan. 7. still perhaps discernible to the naked eye. Dec 3 t See Astronomical Companion, COMETS, for a section on the Sun-Grazers. Of e s the eleven listed there, allSERP butENS one belong to the Kreutz group, chasing each n (CAU u other along one orbit, pieces ofDA) a proto-comet that broke up. Our comet’s orbit s is similar to that of the exception, C/1680 V1. Though 333 years and 20 days apart, they may constitute a second family; they have dropped from a region of Dec 1 the Oort Cloud just north of the direction of Castor, whereas the Kreutzers come from the direction of Sirius. All of the eleven except the first, C/1668 E1, grazed the Sun more closely than C/2012 S1 will. But it could be large. Sabikk The Kreutz sungrazer C/1843 D1, the Great March Comet, was brighter SCORPIUS than the full Moon. C/1882 R1, the Great September Comet, may have been even brighter. Brightest since then was Ikeya-Seki, C/1965 S1—overwhelmingly beautiful, a diagonal plume across the blue sky. Attach no superstition to its Jabbah “S1,” which merely means that it too was the first comet discovery in the secGraffias ond half of a September. (The system, imposed only in 1995, is Jan AB, Feb CD, Mar EF, Apr GH, May JK, Jun LM, Jul NO, Aug PQ, Sep RS, Oct TU, Nov VW, Sun Nov 29 Dec XY.) Ikeya-Seki was discovered only 33 days before perihelion—a sudden Nov 29 Nov 27 Dec 4 Dec 9 visitor compared with the 14 months warning that sophisticated instruDschubba Dec indeed 29 Dec 14 Dec 24 C/2012 S1. Dec 19 not bode bloody ments have given us for ISON It will revolution, as Indonesians thought the 1965 comet did; nor will a magic spacecraft follow it, as a suicidal cult believed of Hale-Bopp C/1995 O1. Nor is it certain to be as bright as the full Moon and bisect the sky with its tail. But it may. —Guy

y ilk

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Kaus Borealis

ARIUS

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Antares

Zubeneshamali