Mercury will spend most of December in the eastern pre-dawn sky descending sunward after greatest western elongation in late November. Due to Mercury's position north of the morning ecliptic, this has been an excellent appearance for Northern Hemisphere observers, and a poor one for those in the Southern Hemisphere. Early in the month, the best viewing times for mid-northern latitude observers will occur between 6 and 7 a.m. local time. By mid-December, the optimal viewing time will decrease to about 30 minutes centered on 7 a.m. local time. For the final week of the month, Mercury will be too close to the sun for observation. Viewed in a telescope during December, the planet will wax from a 71% illuminated disk on December 1 to nearly full. Meanwhile, Mercury's apparent disk diameter will shrink by 25%. On the morning of December 25, the old crescent moon will land 1.5 degrees to the left (celestial northeast) of Mercury, but the duo will be engulfed in morning twilight.
During December, Venus will continue to climb away from the sun in the southwestern evening sky. The magnitude -3.9 planet will pass from Sagittarius into Capricornus on December 19. The shallow evening ecliptic has kept Venus from appearing very high in the sky, but the planet will be carried much higher by month's end – setting up a spectacular display during early 2020. Viewed in a telescope, Venus will show a waning gibbous phase all month long. Meanwhile, its apparent disk diameter will increase slightly as its orbit carries it closer to Earth. On December 2, Venus will pass 45 arc-minutes (or 1.5 full moon diameters) to the south of the bright globular star cluster Messier 22, and on December 19, it will land only 20 arc-minutes south of another, dimmer globular cluster Messier 75. On December 10-11, Venus will pass less than two finger widths to the lower left (or 2 degrees to the celestial south) of Saturn, making a nice binocular sight and widefield photograph. On December 28, the young crescent moon will be positioned 2.5 degrees below Venus; another pretty sight and photo opportunity.
Visible in the eastern pre-dawn sky after it rises at about 4:45 a.m. local time, Mars will spend December traversing the stars of Libra while climbing away from the sun. The Earth-Mars separation will continue to decrease during December, but Mars will shine with a modest average magnitude of 1.65 and exhibit a small apparent disk size in telescopes. On December 12 Mars will pass a mere 11 arc-minutes to the upper left (or celestial north) of the pretty double star Zubenelgenubi, making a terrific sight in backyard telescopes. The old, waning crescent moon will be positioned a palm's width to the lower left (or 5 degrees to the celestial northeast) of Mars on December 23.
During early December, magnitude -1.84 Jupiter will be low in the sky and sinking into the southwestern evening twilight at dusk. But the king of planets will become lost to view altogether after about mid-month, and solar conjunction will occur on December 27, close to the December Solstice point on the ecliptic.
During December, Saturn will be visible in the southwestern evening sky — slowly moving eastward through northern Sagittarius, to the upper right of the stars that form Sagittarius' teapot-shaped asterism. During the beginning of December, Saturn will be setting in a dark sky shortly before 7:30 p.m. local time. But after mid-month, the modestly-bright magnitude 0.59 planet will become difficult to see within the evening twilight. To aid your search, on December 27, the young crescent moon will be positioned less than a palm's width to the upper left (or 5 degrees to the celestial southeast) of Saturn. But before then, on December 10-11, Venus will pass less than two finger widths to the lower left (or 2 degrees to the celestial south) of Saturn, making a nice binocular sight and widefield photograph.
Blue-green Uranus will spend December moving slowly retrograde westward among the stars of southwestern Aries. Uranus will be visible from dusk until well after midnight local time during December, shine at magnitude 5.7, and will exhibit an apparent disk diameter of about 3.6 arc-seconds. To assist you in locating the planet, which can be readily seen in binoculars and backyard telescopes on dark nights, Uranus will be sitting less than 4 degrees above (or to the celestial north) of the naked-eye star Xi (ξ) Ceti. The bright waxing gibbous moon will land 7 degrees to the lower left of Uranus on December 8 — but wait until the bright moon has moved away to observe the planet.
During December, blue Neptune will be positioned in the south and western evening sky moving prograde eastward through the stars of central Aquarius. At magnitude 7.9, Neptune will be visible in good-quality backyard telescopes. Its orbital motion will be slowly carrying it away from that constellation's naked-eye star Phi (φ) Aquarii and toward another naked-eye star named Hydor, also known as Lambda (λ) Aquarii. Neptune will be observable once full darkness arrives — but it will only be high enough for clear views until late evening in early December and until mid-evening by month's-end.
Editor's note: If you have an amazing skywatching photo you'd like to share for a possible story or image gallery, please contact managing editor Tariq Malik at email@example.com.
Monthly skywatching information is provided to Space.com by Chris Vaughan of Starry Night Education, the leader in space science curriculum solutions. Follow Starry Night on Twitter @StarryNightEdu and Chris at @Astrogeoguy.