Bright stars and a lone bright planet are visible at dusk as December begins. The Summer Triangle of Vega, Altair and Deneb is well up in the west and getting lower as the month progresses; Saturn is halfway up in the southeast to south-southeast, with Fomalhaut, Mouth of the Southern Fish, to its lower right—and be sure to arrange for telescopic views of Saturn’s rings, tipped only 0.4° to 1° from edge-on this month
Capella, the “Mother Goat” star, is in the northeast; and reddish-orange Aldebaran is low in the east-northeast, to the lower right of Capella. Aldebaran, the eye of Taurus, the Bull, is at oppositionto the sun on Dec. 1. That night, you can also spot the star highest in south in the middle of the night, and low in the west-northwest as dawn brightens. The Arabic name Aldebaran means “the Follower,” and you can find the pursued, beautiful Pleiades, or Seven Sisters star cluster, 14° above Aldebaran in deepening evening twilight, and 31° to the lower left of a bright, 88 percent waxing gibbous moon.
Two nights later, on Dec. 3, the moon, moving through perigee, closes the distance and occults, or covers, some of the cluster’s stars. Since the moon is very bright, the best events will happen along the moon’s narrow, dark edge, and a telescope will be required to observe them: From Palm Springs, a star will be snuffed out just a few seconds before 5:25 p.m., and another will disappear just before 6:01 p.m. and reappear at 6:25 p.m.
On Dec. 4 at dusk, the full moon will appear a wide 11° north (to the upper left) of Aldebaran. On Dec. 5, this month’s northernmost moon will rise in twilight, and thereafter rise later each night, shifting farther south each time. Wait a few more evenings until the moon rises well after nightfall and examine the Pleiades through a pair of binoculars. Look for a pair of stars 21’ (0.35°) apart, within 5° south of the Pleiades. They are 13 and 14 Tauri, magnitudes 5.7 and 6.1. From Dec. 13-21, 5.6-magnitude Uranus passes closely south of the two stars. Refer to the Uranus and Neptune finder charts (Neptune is currently near Saturn) at www.abramsplanetarium.org/msta.
The Geminid meteor shower reaches its peak on the night of Dec. 13-14, with best viewing from 10 p.m. until 5 a.m. Meteors can flare up anywhere in the sky, but their tracks, extended backward, will seem to radiate from a point near Castor, one of the Twins. A waning crescent moon, present from about 2 a.m. onward, will interfere very little.
Winter begins on Dec. 21 at 7:03 a.m., as the sun reaches the southernmost point of its annual journey, directly above the Tropic of Capricorn. The moon will return to the early evening sky later that day, as a thin 4 percent crescent low in the southwest at dusk. By Dec. 26, the moon is a gravid 42 percent crescent, 3° to the upper right of Saturn, well up in the south.
Watch the waxing gibbous moon hopscotch over the Pleiades from Dec. 30-31, while its phase increases from 84 to 92 percent. At dusk on Dec. 31, the moon appears 10° to the north (upper left) of Aldebaran, while Jupiter is just rising 43° to the moon’s lower left. Majestic Orion now rises in twilight at year’s end, to the moon’s lower right. Two or so hours later, watch for the rising of Sirius in the east-southeast, in line with Orion’s belt, extended downward. If you’re in a place with unobstructed views toward the west and east-southeast, both the Winter Triangle, Betelgeuse–Procyon-Sirius, and the Summer Triangle, Deneb-Vega-Altair, can be seen simultaneously. On subsequent nights, find the stars in the same positions 4 minutes earlier each evening.
At this time of year, the entire Winter Hexagon is visible for 8 1/2 consecutive hours, taking up most of the night. Its stars of first magnitude (or close) or brighter, in clockwise order, are Sirius; Procyon; Jupiter (a temporary visitor, a planet or “wandering star”); Pollux; Castor; Capella; Aldebaran; Rigel; and back to Sirius. Betelgeuse, another first-magnitude star, lies inside. From the Coachella Valley on Dec. 1, the hexagon is in good view from 9 p.m. until 5:30 a.m., and on Dec. 31, it’s two hours earlier, or 7 p.m. until 3:30 a.m.
In morning twilight, within the hour before sunrise, Jupiter is in the west, with the “Twin” stars Pollux and Castor to its upper right. Capella is in the northwest, to the lower right of the Twins. The “Dog Stars,” Procyon and Sirius—the latter, the brightest of all nighttime stars, but not as bright as Jupiter—appear to the lower left of Jupiter and the Twins.
Below Jupiter, find red Betelgeuse, with Orion’s three-star belt farther down. Setting in the west-northwest, far to the lower right of Jupiter and Betelgeuse, is another reddish star, Aldebaran, eye of Taurus. Rigel, Orion’s foot, is already gone from the morning mid-twilight sky at the start of December, but you can catch it earlier in the morning. After Rigel, the stars Aldebaran, Sirius and Betelgeuse will all disappear below the western horizon. Remaining in the western sky at dawn through month’s end will be the “Spring Arch” of Procyon, Jupiter (a temporary visitor), Pollux, Castor and Capella.
Regulus, highest of the first-magnitude stars in the southwest quadrant of the sky on December mornings, marks the heart of Leo, the Lion. Look for Regulus 36° to 39° to the upper left of Jupiter.
In eastern half of the sky on December mornings, golden Arcturus climbs high in the east to southeast, with Spica of Virgo 33° to its lower right, in the southeast to south. Blue-white Vega is in the northeast to east-northeast, nearly 60° to the lower left of Arcturus. Denebrises in the far northeast, to the lower left of Vega.
Mercury puts on its year’s best morning showing low in the southeastern sky in first three weeks of December, and sinks to the horizon at mid-twilight by month’s end. Watch for fainter, first-magnitude Antares emerging during the last two weeks of the month. It’s 6° to the lower right of Mercury Dec. 17-21, moving to the upper right of Mercury thereafter, to 10° on Christmas morning, and to 18° on Dec. 31.
Follow the moon at dawn: On the morning of Dec. 4, the full moon appears low in the west-northwest, 6° to the upper left of the Pleiades and 11° right of Aldebaran. On Dec. 7, a 90 percent waning gibbous moon appears near Jupiter, Pollux and Castor, and on Dec. 10 at 63 percent, near Regulus. On Dec. 14, a 25 percent crescent moon appears very near Spica. On Dec. 17, find an easy 6 percent crescent moon low in the southeast, with Mercury 10° to its lower left. Finally, on Dec. 18, use binoculars to find a slender 2 percent crescent moon 7° to the lower right of Mercury and 2° below Antares.
Bring in the new year in a Sirius way. In the middle of the night of Dec. 31 to Jan. 1, the brightest star, Sirius, passes directly south almost exactly 12 hours after the sun’s midday passage through its highest point, solar midday, on Dec. 31, at 11:49 a.m. in Palm Springs. On the night of Dec. 31, find the Dog Star well up in the southern sky as the New Year begins. Observers in Southern California have an added bonus: The second-brightest star, Canopus, reaches its high point in the south just 22 minutes before Sirius does. Observers in the Coachella Valley will find it 3° to 4° up due south when it’s highest, provided mountains don’t block the view.
The Astronomical Society of the Desert will host a star party on Saturday, Dec. 13, at Sawmill Trailhead, a site in the Santa Rosa Mountains at elevation 4,000 feet; and on Saturday, Dec. 20, at the Santa Rosa and San Jacinto Mountains National Monument Visitor Center. For more information, including maps and directions to the two observing sites, visit astrorx.org.
Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968 and still helps produce an occasional issue. He enjoys being outdoors sharing the beauty of the night sky and other wonders of nature. Robert Miller, who provided the evening and morning 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.
