The waxing gibbous moon passes just below Jupiter as December begins. Then a week later, it rises as sunset as the Long Night Full Moon. Parallax will shift the moon far enough south to make it occult the Red Planet. You will see an occultation, with the Moon covering Mars for up to an hour in some places.
The last quarter moon is on December 16th. The northern hemisphere of the Earth is tilted most away from the Sun at Winter Solstice on December 21st at 3:48 p.m., the shortest day for us. The new moon is December 23rd, and the next evening, we get the thin waxing crescent passing just south of brilliant Venus in SW twilight; if it is clear enough, you also see fainter Mercury to the upper left of much brighter Venus. The first quarter moon is overhead on December 29th, to bring the year to an end.
As noted above, Mercury and Venus both are returning to the evening sky by mid December. On the solstice on December 21st, Mercury lies six degrees above far brighter Venus, but then retrogrades back toward the Sun. It passes 1.7 degrees to the right of Venus in the dusk about 40m minutes after sunset on December 29th. By the 31st, Venus climbs higher and brighter in the SW, but Mercury is disappearing in the Sun’s glare as the year begins. Venus will dominate the western sky for most of 2023.
Mars reaches opposition, passing closest to us on December 7th, the same night the full moon just misses it! It is in Taurus, and passes almost overhead around midnight, the best time to see it at high power. It lies .55 AU from us then, and shines bright orange at magnitude -1.9, brighter than Sirius, but not quite as bright as Jupiter. It spans 17" of arc then, or appears about 100X smaller than the full moon, which is .5 degree, or 1800" of arc across.
Jupiter is still well placed for evening viewing in Aquarius, due south at dusk, and Saturn is getting lower in the SW in Capricornus. So all three of the best planets for amateur viewing from Earth are well placed now; get out and observe!
The best meteor shower of the year, the Geminid meteor shower, peaks on the morning of December 14, with best observing after moonset about 3 AM. Look for a meteor a minute coming out of the NE.
The square of Pegasus dominates the western sky. South of it are the watery constellations of Pisces (the fish), Capricorn (Sea Goat) with Saturn in its tail, Aquarius (the Water Bearer) with Jupiter 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.
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.
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. Aldeberan is not a member of the Hyades, but about twice as close as the Hyades; distances in astronomy can be deceiving. Mars sits between the horns of the bull as December begins. Compare its color with that of Aldeberan.
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. You 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.
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. Below Sirius in binoculars is another fine open cluster, M-41, a fitting dessert for New Year’s sky feast.
