Venus is December’s headliner. The planet is bright enough to see with the naked eye during daytime—and creates a spectacular pairing with a crescent moon.
The month also offers an eye-catching lineup of three evening planets all month, with a fourth joining the party before Venus checks out in early January. A few hours of dark, moonless skies coincide with the year’s best meteor shower. An extra-bright moon, a northernmost moon and a southernmost sun all arrive in a three-day span. And we get two celestial gifts on the last day of 2021: a compact dawn gathering of an old crescent moon with a planet and its namesake star; and the Dog Star reaching its high perch in time to howl at midnight.
Our evening sky map shows the positions of the naked-eye planets and the brightest stars at mid-twilight, when the sun is 9 degrees below the west-southwest horizon, nearly 45 minutes after sunset. Throughout the month, a lineup of three planets dominates the southwest quadrant of the sky. Brilliant Venus, in the southwest to west-southwest, anchors the lower right end of the lineup. In the first two weeks, Venus gleams at magnitude -4.9, as bright as it gets, and fades to magnitude -4.3 at month’s end. Jupiter, next in brightness at magnitude -2.3 to -2.1, marks the upper left end of the lineup. Jupiter’s distance from Venus is 34.5 degrees on Dec. 1, shrinking to 31.4 degrees on Dec. 14, and expanding to 37.5 degrees on Dec. 31. Saturn, at magnitude +0.7, lies between the two brighter planets.
Follow the moon at dusk Dec. 5-18. The arrangement of the moon and three bright planets is especially attractive as the moon grows from a thin young crescent, 15 degrees to the lower right of Venus on Dec. 5, through first quarter phase, when the moon is half full and 23 degrees east of Jupiter, on Dec. 10. The very best evening is Monday, Dec. 6, when the 11 percent moon appears 2.6 degrees to the lower left of Venus at dusk. Don’t miss that!
For several hours that day, we are presented an easy opportunity to spot Venus in the daytime. Try looking when the 10 percent crescent moon passes due south, 30 degrees up, on Dec. 6 about 2:19 p.m. Venus will then appear 3.2 degrees to the moon’s upper left. A telescope magnifying only 45-power would make the 24-percent crescent Venus appear the same apparent size as the moon with the unaided eye. With Venus at its brightest, a moon-Venus pairing seldom gets better than this! The moon will close in on Venus until they set, more than 2 1/2 hours after sunset, when they’re 2.4 degrees apart.
On the next evening, Tuesday, Dec. 7, Saturn will appear 5 degrees to the upper right of the 19% crescent moon. On Wednesday, Dec. 8, find Jupiter within 7 degrees to the 29% crescent moon’s upper left at dusk. Four hours later, when they’re about to set, they’ll be just more than 5 degrees apart. On Thursday Dec. 9, the 40% fat crescent moon is 10 degrees east of Jupiter.
One week later, on Dec. 16, the 96% moon is 5 degrees from the Pleiades and within 9 degrees of Aldebaran. On the next evening, find Aldebaran within 8 degrees to the lower left of the 99% moon.
Through the telescope: Venus appears larger than Jupiter all of December, because it’s unusually close to Earth. Inferior conjunction, with Venus nearly between Earth and the sun, will occur on Jan. 8. Venus grows more than 50 percent in apparent size this month, ending up more than one arcminute (1/60 degree) across. At that size, a magnification of 30-power would make Venus appear about as large as the moon with unaided eye!
Venus is bright enough to be spotted in daylight. In December 2021, find Venus to the upper left of the setting sun, by 41 degrees on Dec. 1; 36 degrees on Dec. 10; 32 degrees on Dec. 16; 26 degrees on Dec. 21; 17 degrees on Dec. 28; and 13 degrees on Dec. 31. If using an optical aid such as a telescope or binoculars, be sure to block the sun from view or wait until it sets in late December, when Venus appears close to the sun. Even binoculars easily resolve Venus’ crescent phases: 28 percent illuminated on Dec. 1; 20% on Dec. 10; 15% on Dec. 16; 10% on Dec. 21; 5% on Dec. 27; 3% on Dec. 30; and 2% on Jan. 1.
Saturn, with its ring system (still 18 degrees from edgewise), and Jupiter, with its dark cloud belts and four large satellites discovered by Galileo in 1610, are always impressive for telescopic viewing.
On Dec. 1 at dusk, look for the Summer Triangle of Vega, Altair, and Deneb, in order of brightness, well up in the western sky. A line from Vega to Altair, 34 degrees long and extended 31 degrees to the south, ends at Saturn, which is then 18.1 degrees to the upper left of Venus and 16.5 degrees to the lower right of Jupiter.
Other bright stars: Look in the south to south-southwest at dusk for Fomalhaut, mouth of the Southern Fish, 22-20 degrees to the lower left of Jupiter. Watch for yellowish Capella, the Mother Goat star, ascending in the northeast, and red-orange Aldebaran, ascending in the east-northeast to east, 31 degrees to the lower right of Capella. On Dec. 1, Aldebaran is at opposition as Earth passes between that star and the sun. Look for Aldebaran low in the east-northeast at dusk and low in the west-northwest at dawn, as shown on the evening and morning twilight maps. Although Aldebaran marks the eye of Taurus the Bull, its name translated from Arabic means “the follower,” of the beautiful Pleiades or Seven Sisters star cluster, not plotted on the maps. Aim your binoculars 14 degrees above Aldebaran as darkness falls, and enjoy!
As December progresses (or later in the evening), watch 21 degrees below and 26 degrees to the lower right of Aldebaran for Orion’s two brightest stars, 19 degrees apart: Reddish Betelgeuse, rising north of east, and blue-white Rigel, rising south of east. Within 45 minutes after you first spot them, look for the “Twin” stars of Gemini, Castor and Pollux, 4.5 degrees apart, rising farther north, 30-34 degrees below Capella.
The Geminid meteor shower reaches its peak in the predawn hours of Tuesday, Dec. 14. The bright waxing gibbous moon sets before 2:45 a.m., leaving 2 1/2 hours of dark skies for excellent viewing of “shooting stars” until dawn brightens. Meteors can light up anywhere in the sky, but the tracks of Geminids, extended backward, will all radiate from a common point not far from Castor. Morning twilight gets under way as the pointer stars of the Big Dipper stand directly above the North Star, around 5:15 a.m.
Later that same Tuesday, in the evening, we witness the least span of the three bright planets, Venus-Saturn-Jupiter—31.4 degrees. Two days later, on Dec. 16, Venus approaches to within 14 degrees west of Saturn, but no closer! Venus commences retrograde, or westward motion against the stars, on the night of Dec. 18-19.
Opposition surge: The full moon on Saturday evening, Dec. 18 will shine with extra brilliance, because it will lie just outside the Earth’s shadow and reflect much of its light back toward Earth. On the next evening, Dec. 19, witness this month’s northernmost moonrise, 32 degrees north of east, at 5:08 p.m. Northernmost moonset will take place the next morning, 32 degrees north of west, at 8:12 a.m. (Note: Our surrounding mountains will delay moonrise and hasten moonset.) Midway between those times, at 12:40 a.m. on Monday morning, the moon passes within 8 degrees south of overhead.
Winter begins on Tuesday, Dec. 21 at 7:59 a.m., when the sun stands directly over the Tropic of Capricorn. Palm Springs has its lowest midday sun of the year that day, when the sun passes directly south, 33 degrees above the horizon, at 11:44 a.m.
A fourth planet, Mercury, adds its bright presence (magnitude -0.7) the last 10 days of month. On Dec. 22, find it 12 degrees to the lower right of Venus. On Dec. 25, Mercury is 7 degrees below Venus. On Dec. 28, Mercury passes 4.2 degrees south (to the lower left) of Venus. The planets seem to be moving in opposite directions, as Mercury is emerging from the far side of the sun, and Venus is on the near side, plunging toward its inferior conjunction of Jan. 8. On Dec. 30, Mercury appears 5.5 degrees to the left of sinking Venus, and on Dec. 31, 7 degrees to Venus’ upper left.
New Year’s Eve: On Dec. 31, Venus sets in the west-southwest 65 minutes after sunset. Getting noticeably lower with each passing day, Venus now sets 7 minutes closer to the time of sunset daily. Jupiter is in the southwest at dusk, 37.5 degrees to the upper left of Venus. Saturn is 19 degrees to the upper left of Venus and nearly 19 degrees to the lower right of Jupiter. Mercury is within 7 degrees to the upper left of Venus and 14 degrees to the lower right of Saturn.
On the last day of the year, the sun reaches its highest point in the south at midday, which occurs at 11:49 a.m. as seen from Palm Springs, and about two minutes earlier as seen from the eastern Coachella Valley. Almost exactly 12 hours later, and therefore in the middle of the night, Sirius, the Dog Star, reaches its high point on New Year’s Eve, at 11:48 p.m. for Palm Springs. In the waning minutes of 2021, step outside and look toward the south. Since the moon and naked-eye planets will all then be below the horizon, Sirius will stand out as the brightest object in the sky, a fitting occasion to close the year by reading Robert Frost’s poem, Canis Major. Notice that the three-star belt of Orion, if extended, points directly to Sirius.
Earlier that evening, in bright twilight just after sunset, binoculars show Venus as a thin crescent, 2.4 percent illuminated, and just more than one arcminute across, 13 degrees to the upper left of the setting sun. (For eye safety, when Venus draws within several degrees of the sun, look just after sunset, or just after the sun’s disk has disappeared behind our local mountains.)
The four brightest stars in morning twilight in December, in order of brightness, are Sirius, visible early in the month, until it sinks below the horizon in the west-southwest; Arcturus, very high in the east to east-southeast; Vega, climbing in the northeast; and Capella, dropping lower in northwest. The only morning planet is Mars, which is faint—magnitude +1.6 to +1.5—low in east-southeast to southeast. On Dec. 18, Mars passes 1 degree south of 2.6-magnitude Beta Scorpii, northernmost of three bright stars in the head of the Scorpion. Watch for the emergence of brighter, 1.1-magnitude Antares, heart of the Scorpion, within 8 degrees below Mars that morning.
By Dec. 24, Antares is 5 degrees to the lower right of Mars. On Dec. 27 and 28, Mars passes 4.5 degrees north (to the upper left) of the star. Note their similar reddish colors, which influenced the naming of the star Antares. In a beautiful compact gathering on the morning of Dec. 31, Mars and Antares are 5.2 degrees apart, with a 7 percent waning crescent moon between and slightly above them, slightly more than 3 degrees from each. The best time to look might be one hour before sunrise, giving a viewing time for Palm Springs of 5:51 a.m. Look about 9-11 degrees above the southeastern horizon; binoculars are recommended.
Follow the moon in the morning sky Dec. 18-Jan. 1. On Dec. 20, the 98% moon is 10-11 degrees below Pollux and Castor. On Dec. 21, the 95% moon is 3.5 degrees from Pollux and 8 degrees from Castor. On Dec. 24, the 75% moon is 5 degrees from Regulus. On Dec. 28, the 34% crescent moon is 5 degrees from Spica. On Dec. 31, the 7% crescent forms a striking gathering with Antares and Mars, as described above.
Illustrations of many of these events appear on the Abrams Planetarium monthly Sky Calendar. To subscribe for $12 per year or to view a sample copy, visit www.abramsplanetarium.org/skycalendar.
Robert Victor originated the Abrams Planetarium’s monthly Sky Calendar in October 1968, and still produces issues occasionally. He enjoys being outdoors sharing the wonders of the night sky, and is hoping for the pandemic to end! Robert D. Miller, who provided the twilight charts, did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.