The Summer Triangle takes a bow as it prepares to exit the evening sky for another year. It’s well up in the west as night falls, with Vega, its brightest member, forming the lower right point. It drops from view before midnight. In the meantime, Gemini climbs higher into the evening sky. Look for its “twins,” the stars Pollux and Castor, low in the east at nightfall, with the rest of the constellation spreading above and to the right. The planets Venus and Mars move toward each other this month, with orange Mars to the upper left of Venus, the Evening Star. They’ll pass each other in early 2017.
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In the Sky This Month
December 9: Vega
Vega, one of the night sky’s brightest stars, is disappearing from evening view this month. Tonight, it sets around 9:30 or 10 p.m., but by month’s end it will set by about 8:30. Look for it in early evening, low in the northwest.
December 10: Capella
Capella, one of the brightest stars in the night sky, is in the northeast at nightfall and soars high overhead later on. What we see as Capella is really two stars. Each star is bigger, brighter, and heavier than the Sun.
December 11: Evening Mercury
The planet Mercury is putting in a brief evening appearance. It is quite low in the southwest 30 to 45 minutes after sunset, far to the lower right of brilliant Venus. Mercury looks like a fairly bright star, but you may need binoculars to see it.
December 12: Moon and Aldebaran
Aldebaran, the star that marks the eye of Taurus the bull, is close to the lower left of the Moon at sunset. Not long afterward, the Moon will cross in front of the star, blocking it from view for up to about an hour.
December 13: Long-Night Moon
The Moon is full tonight. December’s full Moon is known as the Moon Before Yule. It’s also known as the Long-Night Moon because it is in view longer than any other full Moon of the year. The moonlight will overpower the Geminid meteor shower.
December 14: Sun in Ophiuchus
The Sun is passing through Ophiuchus, the serpent-bearer, the thirteenth constellation of the zodiac. Ancient astrologers did not include Ophiuchus on their early charts of the zodiac. The Sun is within its boundaries for more than two weeks.
December 15: Lunar Impacts
The Moon is scarred by billions of years of impacts. They blasted out big craters and punched holes that filled with molten rock that bubbled up from below. Many of those scars are visible to the eye as the dark features on the lunar disk.