Skylights: December 2021

The year’s final month marks the transition into winter, with the season’s most notable constellations rising into prominence during early evening hours, and, following solstice at 10:59am on the 21st, hours of darkness will begin to decrease.

If you recall using the position of the Big Dipper as a clock, note the position of its pointer stars at the 6 o’clock position relative to Polaris at 6 o’clock evening time at the beginning of the month. Twelve hours later, at 6am, they are directly above Polaris.

Early in the morning on the 2nd, step out to see the slender waning crescent Moon just 0.5° from the wide double star Zubenelgenubi in Libra. While you’re taking in this beautiful sight, note that Mars is 7° below the pair.

New Moon occurs on the 3rd, and the waxing crescent visits our evening planets early in the month, being just 3° below Venus on the 6th, 6° below Saturn on the 7th, and passing within 8° of Jupiter on the 8th and 9th.

The Full Cold Moon occurs on the 18th. December’s full Moon is noteworthy for being the highest full Moon in the northern sky all year, as it resides in the same the area of the ecliptic that the Sun does during June solstice. This creates a surreal “nighttime daylight” effect on your surroundings, especially if you can get away from lights. This is a great time for night hikes. The Moon transits the meridian at 11:42pm on the 18th, when it will be 73.6° in elevation. It is worth stepping out to enjoy December’s Full Moon.

While you’re looking at the full Moon, note its position in the Winter Hexagon, as it lies almost directly on the line between Capella and Sirius, with Orion directly below. The full Moon making its appearance in the Winter Hexagon is another indicator for the start of winter.

On its way to last quarter on the 26th, the Moon passes about 2.5° from the Beehive Cluster, M44, on the morning of the 22nd.

December is the last month to view Venus in the evening sky. Having reached its greatest elongation late in October, Venus is now moving around the part of its orbit approaching Earth, resulting in its apparent position rapidly approaching the Sun. That makes this an interesting time to watch Venus, as its angular size and phase are changing rather quickly. Through small telescopes, or even large binoculars, it will show a beautiful crescent phase, which by the end of the month approaches one arcminute in angular size. That’s about 1/30 the size of the Moon.

You’ll also notice that Venus, which has been slowly catching up to Saturn and Jupiter over the past few months, will seem to have stopped, as it reaches its stationary point on the 18th, and begins to move retrograde as it approaches inferior conjunction early next month.

That leaves Saturn and Jupiter. The two giant planets have graced our evening skies for the past half year, but their best viewing opportunity is also nearing an end. Although they’ll still be with us for a few more weeks, December is the last, best chance to get a good view of them with a telescope before they sink too low in the western sky.

Just as Venus is leaving our evening sky, Mercury is coming back. While it is in the evening sky all month, it is only far enough from the Sun to observe late in the month, and worth looking for beginning on the 26th, when Venus is nearby. Look for Mercury 6° directly below Venus on the 26th, and on the 28th, they pass within 4° of each other. A noteworthy conjunction, although it will be next to impossible to see low in twilight, the dwarf planet Pluto is just 3/4° from Mercury on the 29th and 30th.

Neptune is still in a favorable position for viewing early in the evenings through December, but it is setting rather quickly after Jupiter. It reaches its stationary point on the 2nd, and resumes its prograde motion thereafter. Use binoculars or a small telescope to locate Neptune 6° south of Kappa Piscium, the center-bottom star of the Circlet asterism of Pisces.

Uranus is our best-placed planet this month, as it is high on the meridian around 8pm local time. Located in southern Aries, about halfway between Hamal in Aries and Menkar in Pisces, you’ll see a pair of objects of similar brightness about 0.5° apart. The top is the type F (white) main sequence star 29 Arietis, and the bottom, slightly dimmer, object is Uranus, which shines steadily with a blue-green hue. Note its position change with respect to 29 Ari on subsequent nights. 

For the first time since July, Mars is now visible in a dark sky, as it is now visible in the morning sky just before twilight. While still rather dim and distant, you can track its motion from Libra into Scorpius as the month progresses, and note that it passes its “rival” Antares on the 28th, when the red giant heart of the Scorpion is just 4.5° to its lower right.

Dwarf planet Ceres is located in Taurus, and spends the month looping from the western edge of the Hyades cluster to a few degrees south of the Pleiades. Since this area is fun to explore in binoculars, don’t forget to watch Ceres whenever you’re enjoying these two star clusters.. 

There are still two good comets visible in the December sky. Comet 67P/Churyumov-Gerasimenko, the comet which was visited by Rosetta and has the now defunct Philae lander on its surface, is looping through Cancer, just over a binocular field north and slightly east of M44 in Cancer, and is expected to be around 9th magnitude.

Comet C/2021 A1 Leonard is visible in the morning sky during the first half of the month. It begins a few degrees east of Arcturus, and moves southeast almost 4 degrees per day. By mid-month it will be located in Ophiuchus in twilight. It is predicted to brighten to as low as 6th magnitude.

The Geminids, perhaps the best meteor shower display of the year, peaks on the night of the 13th-14th. The meteors appear to originate from an area of sky near Castor, which rises around 6pm. A waxing gibbous Moon could interfere with viewing, but the Geminids are known to produce a few bright meteors, which should still be visible.