This is the Dudley Observatory Skywatch Line for Wednesday, December 11th, and Thursday, December 12th, written by Louis Suarato.
The nearly full Moon rises at 4:05 p.m., Wednesday. The Moon won’t reach its Full phase and be 100% illuminated until 12 minutes past midnight. If you look low over the southwestern horizon about an hour after sunset, you’ll see Venus and Saturn separated by 2 degrees. Venus will be the much brighter of the two planets, shining at magnitude -3.90 to Saturn’s left. Saturn’s magnitude is 0.58. Saturn sets at 6:30 p.m., followed by Venus a minute later. Venus is currently 86.5% illuminated. Saturn’s rings are currently tilted at 24 degrees. The two planets move apart after Wednesday, with Venus rising higher in the sky, while Saturn approaches the Sun. Mars rises at 4:21 a.m. in Libra. The star less than a degree to the upper right of Mars is Alpha Librae, also known as Zubenelgenubi, meaning “the southern claw”, from a time when Libra was part of Scorpius. Alpha Librae is only .33 degrees north of the ecliptic, so it is occasionally occulted by the Moon and planets. Located about 77 light-years away, Alpha Librae is a double star whose components may be gravitationally bound, but are far apart, and may have an orbital period greater than 200,000 years. Mercury rises at 5:58 a.m., but may be too close to the glow of sunrise to see.
While Venus and Saturn are setting the Moon will be rising above the east-northeastern horizon. The Moon will be at the left foot of the Gemini twin, Castor, and above Orion’s shield. As the sky darkens, you will see that the Moon is near the center of the asterism known as the Winter Hexagon, or Winter Circle. The stars comprising this asterism are Capella, in the constellation Auriga, to the Moon’s upper left; Aldebaran, in Taurus to the Moon’s upper right; Rigel, in Orion to the Moon’s lower right; Procyon, in Canis Minor, almost directly below the Moon; and Pollux, in Gemini to the Moon’s lower left.