ASTRONOMY

THE UNIVERSE FROM THE CENTER Th e exploration of the concept of time

By Dr. George Spagna Special to Ashland-Hanover Local

Last month I threatened a rant about the silly notion of year-round daylight time to me. Before I get there, I’d like to take a few months talking about time, timekeeping and calendars. So this column will be about the nature of time and measuring it in years, days and hours. Next month we’ll get down to minutes and seconds and hopefully you’ll already see why I think daylight time is silly.

What is time? Surprisingly, it’s hard to define – enough so that many books and treatises have tried. St. Augustine wrote in his “Confessions” that he knew perfectly well what time is until he was asked and at which point he had no idea. Isaac Newton postulated that time, whatever it is, is absolute and passes at the same rate everywhere in the Universe. He also postulated absolute universal, three-dimensional space. That’s the realm of classical physics and it works quite well for everyday mechanics.

In the 20th century, Einstein’s Special and General Theories of Relativity merged space and time into a fourdimensional manifold which is neither universal nor absolute. Motion and gravity affect both the rate at which time passes and the measurement of distances, with the speed of light the arbiter of both “length contraction” and “time dilation.” But we live neither at substantial fractions of light speed nor intense gravity, so let’s just work with regular perceptions of time.

Earth and sky present us with discernible cycles of time: sunrise, sunset, swiftly flow the years. Humans evolved on a planet which rotates on its axis daily and revolves around the sun yearly. Because of the tilt of our axis and our elliptical path around the sun, days measured relative to the sun are not uniform in duration. During our northern hemisphere winter, we’re closer to the sun and moving a bit faster in orbit, which makes noon to noon a bit longer than during our summertime when further from the sun. The tilt of the axis makes the time from sunrise to sunset vary as well, but in the opposite direction. Times of daylight are shorter in the winter and longer in the summer. To accommodate these variations, we’ve come up with a “mean solar day” of 24 hours for civil time keeping. Astronomers measure something called a “mean sidereal, relative to the stars, day,” which is about four minutes shorter.

And, of course, as we orbit the sun the constellations change with the seasons, so which stars you use to measure the sidereal day will also change.

Hours haven’t always been uniform either. Ancient Mediterranean civilizations, probably starting with Sumerian or Babylonian practice, divided the daylight time into 12 “hours” no matter the season, so a summer hour was longer than a winter hour. Night time was measured in “watches” – typically four watches from sunset to sunrise. The notion of a single length hour and a 24-hour day are a later invention. This then led to defining the hour as 1/24th of the “mean tropical day” for the year 1900.

As noted above, our species evolved on this rotating planet. We’re not naturally nocturnal

like owls, so until the invention of artificial lighting, starting with control of fire, our normal circadian rhythms were tied to sunrise and sunset.

Years are their own special problem, since the time to complete a single orbit, relative to the stars, isn’t divisible into an integer number of days, neither solar nor sidereal. Our calendar year is 365 24-hour days except in leap years when we give it 366. And it’s not exactly a four-minute difference between solar and sidereal time so we have to skip the leap years in century ending dates which are not divisible by 400. This is the Gregorian calendar reform. The addition of leap years was the Julian Calendar reform.

Next month we’ll divide hours into minutes and seconds, and we’ll talk about precision timekeeping.

Lunar phases for May: First Quarter on the May 8 at 8:21 p.m.; Full Moon on May 16 at 12:14 a.m. For time zones west of here, the moon is full on May 15, and this Full Moon will be accompanied by a total lunar eclipse; Last Quarter on May 22 at 2:43 p.m.; and New

Moon on May 30 at 7 a.m. All times are Eastern Daylight. The Eta Aquarid meteor shower peaks on May 5 through 7, so the moon will be providing some interference. To get a better view, find a spot away from city or town lights and look after midnight.

Mercury returns to evening twilight. Look about 30 degrees north of due west, low at about 10 degrees above the horizon. It sets by 9:30. Predawn is the time for planet watching, though these targets are also low in the sky. Venus, Mars, Saturn and Jupiter are all there to the east southeast in twilight. An uncluttered horizon will make it possible to see them.

Our midmonth view at about three hours after sunset finds the faint and likely unfamiliar constellation known as Canes Venatici, or “Hunting Dogs,” at zenith. This constellation was invented and named in 1687 by Hevelius, who mapped out what are now the official 88 constellations recognized by astronomers. In Ptolemy’s day, these stars were considered part of Ursa Major as noted in his great work the Almagest. Because the boundaries of the constellations were not official, some placed these stars in Bootes, the Herdsman, and they were known as the herdsman’s club. That name was mistranslated into Arabic where they became known as his “spear shaft with a hook.” Ironically, when the Arabic texts were translated into Latin, they were again mistranslated and became the “spear shaft with dogs.”

The association with Bootes should take your eye to Arcturus to the southeast, about 65 degrees above the horizon. To the north you’ll see Ursa Major with the bowl of the “Big Dipper” inverted. The end of the Dipper’s handle is the star Alkaid, and it lies only 15 degrees from zenith. Mizar, the middle star in the handle is a binary with Alcor. If your eyes are sharp enough you should be able to resolve these without a telescope. A modest telescope will reveal both Mizar and Alcor as close binaries. This is actually a fourstar system. The co-called “pointer stars” at the end of the bowl, Merak and Dubhe, can be used to find Polaris. Just extend the imaginary line between them toward the horizon and it will pass very close to Polaris.

If readers have questions about astronomy in particular or science in general that you would like covered in one of these columns, contact me at gspagna@rmc.edu. I have retired from Randolph-Macon College, but will continue these columns and use of this email address for the foreseeable future.

For information about viewing opportunities at the Keeble Observatory, visit the website: https://www.rmc.edu/ keeble for updates or call the Keeble Information line at 804-752-3210. If you need to speak with someone immediately, please call Dr. Michael Rodruck at 804-752-7344.

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