The Night Sky of December

Dr. Wayne Wooten
Professor of Astronomy

The last quarter moon is on December 6th. The waning crescent moon passes just south of Venus (watch both in broad daylight after sunrise!) on December 9th. The new moon is on December 12th; four months until totality on April 8th! The waxing crescent will set well before midnight on December 13th, the peak for perhaps the best annual meteor shower, the Geminids. The meteors come out of Gemini in the NE, and while there may be more after midnight, there tend be to quite a few in the evening as well, and the faint crescent will not interfere even then.

The crescent passes below Saturn on December 17th, and the first quarter moon is December 19th. The winter solstice occurs on December 21st at 10:27 to begin winter, and this is our shortest day. The waxing gibbous moon passes just left of Jupiter on December 22nd; with clear afternoon skies, can you spot Jupiter naked eye before sunset, using the moon as a guide? The Full, or Yule Moon, is the day after Christmas this year.

Mercury and Venus both are morning stars, with Mercury climbing higher in the SE dawn as the year ends, to reach greatest western elongation in January. The pair will be closest on the morning of January 12th, when they make a striking alignment with Mars below them, now emerging from behind the Sun.

Jupiter is well placed for evening viewing in Aries, dominating the eastern sky at dusk, and Saturn is getting lower in the SW in Aquarius. Observe all four giants now, as Uranus (Aries) and Neptune (Aquarius) are also visible in binoculars. Nor seen is Comet Halley. This month it is at aphelion, outside the orbit of Neptune; by January, it heads sunward again, to arrive with the naked eyes in the dawn skies of June 2061. At its best that August, it will be ten times brighter than it appeared back in 1986. Something to live for! Here is the striking conjunction of it, Venus, and the crescent moon on the evening of August 18, 2061.

The square of Pegasus dominates the western sky. South of it are the watery constellations of Pisces (the fish), Capricorn (Sea Goat), Aquarius (the Water Bearer) with Saturn now, and Cetus (the Whale). Below Aquarius is Fomalhaut, the only first magnitude star of the southern fall sky. It marks the mouth of Pisces Australius, the Southern Fish. If you want an ideal ap for learning the constellations, download “Nocturne” for Apple phones, and mount it on a tripod for 2’ exposures of the sky, which you can then annotate with star names, constellation lines, and even the mythological figures. Makes the sky come alive.

The constellation Cassiopeia makes a striking W in the NW. She contains many nice star clusters for binocular users in her outer arm of our Milky Way, extending to the NE now. Her daughter, Andromeda, starts with the NE corner star of Pegasus’’ Square, and goes NE with two more bright stars in a row. It is from the middle star, beta Andromeda, that we proceed about a quarter the way to the top star in the W of Cassiopeia, and look for a faint blur with the naked eye. M-31, the Andromeda Galaxy, is the most distant object visible with the naked eye, lying about 2.5 million light years distant.
Overhead is Andromeda’s hero, Perseus, rises. Perseus contains the famed eclipsing binary star Algol, where the Arabs imagined the eye of the gorgon Medusa would lie. It fades to a third its normal brightness for six out of every 70 hours, as a larger but cooler orange giant covers about 80% of the smaller but hotter and thus brighter companion as seen from Earth. This method of observing changes in a star’s brightness when another body (even a planet!) eclipses it is a very powerful tool in finding stars sizes, shapes (tidal distortion of Algol was confirmed by my master’s thesis at Gainesville), and exoplanets.

Look at Perseus’ feet for the famed Pleiades cluster; they lie about 400 light years distant, and over 250 stars are members of this fine group. East of the seven sisters is the V of stars marking the face of Taurus the Bull, with bright orange Aldebaran as his eye. The V of stars is the Hyades cluster, older than the blue Pleiades, but about half their distance. Their appearance in November in classical times was associated with the stormy season, when frail sailing ships stayed in port. Aldebaran is not a member of the Hyades, but about twice as close as the Hyades; distances in astronomy can be deceiving. Usually the brighter objects are closer, but exceptionally luminous objects, like Rigel in Orion, may be over 2,000 light years distant yet still first magnitude.

Yellow Capella, a giant star the same temperature and color as our much smaller Sun, dominates the overhead sky. It is part of the pentagon on stars making up Auriga, the Charioteer. Several nice binocular Messier open clusters are found in the winter milky way here. East of Auriga, the twins, Castor and Pollux highlight the Gemini. UWF alumni can associate the pair with Jason and the Golden Fleece legend, for they were the first two Argonauts to sign up on his crew of adventurers.

South of Gemini, Orion is the most familiar winter constellation, dominating the eastern sky at dusk. The reddish supergiant Betelguese marks his eastern shoulder, while blue-white supergiant Rigel stands opposite on his west knee. Just south of the belt, hanging like a sword downward, is M-42, the Great Nebula of Orion, an outstanding binocular and telescopic stellar nursery. It is part of a huge spiral arm gas cloud, with active starbirth all over the place.

Something very special will happen to Betelgeuse on the evening of December 11th. AT 8:17 p.m., observers in Key West should see this bright star fade greatly for a few seconds. The small main best asteroid Leona will cover (some!) of the huge surface of this red supergiant, one of the biggest in the galaxy. So this will, like last October 14th, be an annular eclipse, where the dark body does go through the center of the star, but is NOT big enough to cover it completely. You will probably hear a lot more about this unique event in days ahead. For us, too far north, just a close miss, alas.

Last but certainly not least, in the east rise the hunter’s two faithful companions, Canis major and minor. Procyon is the bright star in the little dog, and rises minutes before Sirius, the brightest star in the sky. Sirius dominates the SE sky by 7 p.m. and as it rises, the turbulent winter air causes it to sparkle with shafts of spectral fire. Beautiful as the twinkling appears to the naked eye, for astronomers this means the image is blurry; only in space can we truly see “clearly now”. At 8 light years distance, Sirius is the closest star we can easily see with the naked eye.

Read past issues of the Sky at Night