For amateur stargazers, December skies
All month: Mars and Jupiter still hold out in the evening, nicely placed in the southwest. As late as last October, all the planets were evening stars. Now only these two and Neptune remain. Jupiter's brightness makes it an easy target from late twilight until it sets before midnight. Mars is fading rapidly, but it is still in the range of first magnitude and the only bright object near Jupiter.
Venus is at maximum morning brilliancy in early December and nearly at greatest separation from the sun at the end of the month. Both factors favor viewing, but the southerly location of the planet doesn't. It should be visible in the southeast about dawn beginning at mid-month.
The moon occults Antares twice this month (the 1st and the 29th), about 27.5 days apart. This is the sidereal month, the interval during which the moon moves completely around the Earth. From new moon on the 1st to the next new moon on Dec. 31, nearly 29.4 days elapse, the synodic month, during which the moon returns to the same position relative to the sun.
Dec. 1: New moon is at 11:43 a.m. Eastern standard time. It occults Antares at about the same time, above the horizon over North America, but in daylight.
Dec. 2: Perigee moon (nearest Earth) is 18 hours after new moon, and it will strengthen the new moon's spring tides.
Dec. 4: Saturn is in line with the sun (conjunction) and enters the morning sky.
Dec. 5-7: The crescent moon stays up later each night and brightens, but not enough to bother the view of Jupiter and Mars, above and to its left after darkness begins.
Dec. 8: The moon is past Jupiter and Mars after sundown. First-quarter moon was at 3:01 a.m. EST.
Dec. 13-15: The stars of the Pleiades cluster are close above the moon on the 13th, and Aldebaran is easily visible in the glow of its brightness.
Dec. 16: Full moon is at 2:04 a.m. EST. It is in Gemini tonight, about midway between Aldebaran (the Bull's brightest star) to its right and Pollux and Castor (in Gemini) to its left.
Dec. 18: The moon moves into Cancer before it comes in line with Pollux and Castor.
Dec. 19: Four planets are in conjuncton today, but two-by-two, not all together. Mars catches Jupiter at about 2 a.m. EST, and it will be seen close to Jupiter's left (east) tonight. Mercury and Saturn are in line at 10 a.m. EST but too close to the sun to be seen.
Dec. 21: The sun arrives at the winter solstice at 11:02 p.m. EST, and winter begins in the Northern Hemisphere. This is the shortest day of the year, though the earliest sunset takes place about Dec. 8 and the latest sunrise about Jan. 6. The moon is directly above Leo's bright star, Regulus.
Dec. 24: Last-quarter moon in Virgo, is at 4:17 a.m. EST.
Dec. 16-27: Very bright Venus is below the rising crescent moon about dawn on both mornings.
Dec. 27-29: The moon is in conjunction with Venus late on the 27th, with Antares and Saturn on the 29th. Venus should be visible, but the others are lower and rise in a brighter sky.
Dec. 30: Another perigee moon occurs at 6 p.m. EST, another new moon at 10:10 p.m. EST. The effect of perigee will add to the tidal range, substantially increasing the high tides that occur tomorrow.