In early 2021, Mars continues receding from Earth, drops below 10" apparent diameter, and becomes a long-standing, fading jewel in the evening sky through September next year. Its next opposition is not until December 8, 2022.
Uranus lies in Aries the Ram and is an easy binocular object at magnitude 5.7. Yet,
it’s deceptively hard to spot because it lies in a sparse region of the sky. The planet stands high in the east after sunset and sets during the early morning hours.
Use Hamal, the brightest star in Aries, and Menkar, southeast of it in Cetus the Whale, to create an imaginary triangle with Mars, which lies west of the pair. A line from Mars through the gap between Aldebaran and the Pleiades in Taurus helps you define the plane of the ecliptic. Uranus lies along this line, just west of a line between Hamal and Menkar. The Moon also helps the evening of December 24, when Uranus stands 3.6° northwest of our satellite. Don’t confuse it with a star of similar brightness, 19 Arietis, which lies a few degrees northwest of the planet. A telescope easily reveals Uranus’ planetary nature via its light bluish-green 4"-wide disk.
Venus rises soon after 5 A.M. local time in early December and stands about 10° high in the southeastern sky an hour later. It comes within 1.3° of Libra’s brightest star, Zubenelgenubi, and continues to reduce its elongation from the Sun all month. By December 31, the planet rises about 6 A.M. local time, just 90 minutes before the Sun.
A fine crescent Moon lies about 4° northwest of Venus on December 12 (closer for those in the western U.S.), providing a beautiful predawn view. The Moon occults Venus during daylight hours from locations along the western U.S. coastal states, Alaska, and Hawaii.
By late December, Venus crosses into Scorpius and is less than 12' from Beta (β) Scorpii in the predawn sky December 18. Venus reaches Ophiuchus December 22 and stands 5.8° north of Antares. Throughout the month, the planet’s gibbous phase will grow from 89 percent to 94 percent lit.
Mercury reaches aphelion and superior conjunction in the same week this month. It has the most eccentric orbit of all the planets, and aphelion occurs December 16, when it lies nearly 43.4 million miles from the Sun. It reaches superior conjunction December 20 and will reappear in the evening skies next year.
A total eclipse of the Sun occurs December 14 for locations in Chile and Argentina. Compared to 2019, the track is farther south and totality occurs near midday, with the Sun high in the sky. The longest duration of the eclipse occurs over land and lasts 2 minutes 9 seconds.