December’s evening views begin with Venus near its brightest and highest in the southwest; the Summer Triangle of Vega-Altair-Deneb high in the west; and Fomalhaut, mouth of the Southern Fish, just east of due south.
At the start of December, the only bright objects in the eastern sky at mid-twilight are Capella, the mother goat star, in the northeast, and Aldebaran, eye of Taurus, very low in the east-northeast. Around Dec. 1 each year, that star is up all night from dusk to dawn. Wait a few minutes to allow the evening sky to darken a bit, and you’ll notice the compact Pleiades, or Seven Sisters cluster, 14 degrees above Aldebaran. The scene is beautifully described in Alfred Lord Tennyson’s “Locksley Hall”: “Many a night I saw the Pleiades / rising thro’ the mellow shade / glittering like a swarm of fireflies / tangled in a silver braid.” Don’t miss the view of the Pleiades through binoculars!
By month’s end, Venus is much lower in the west-southwest, because it’s heading toward inferior conjunction, nearly between Earth and the sun, on Jan. 11. The Summer Triangle is lower, too, and Fomalhaut crosses west of south, both owing to Earth’s revolution around the sun. Capella and Aldebaran rise higher in December for the same reason. Betelgeuse and Rigel, Orion’s brighter shoulder and foot, appear above the eastern horizon four minutes earlier each evening during twilight as the year’s end draws near. Between them, note the vertical line of three stars—Orion’s belt! Poet Robert Frost in “The Star-Splitter” (about a farmer who set fire to his house to collect insurance money to buy a telescope) describes what you can observe here in the Coachella Valley at this time of year: “You know Orion always comes up sideways / throwing a leg up over our fence of mountains …”
In the final days of December, bright Jupiter appears above our evening twilight horizon. From a good vantage point, you can spot Venus and Jupiter simultaneously. As Venus approaches the horizon, watch for Jupiter to rise in the opposite direction. Of course, you can spot Jupiter on any date this month just by looking late enough in the evening.
Don’t miss Venus near the crescent moon early on Thursday evening, Dec. 5. It’s also a great chance to spot Venus in the daytime, all afternoon! Here are times for each Sunday when Venus is directly south, with its height above the horizon in degrees, as seen from the Coachella Valley: Dec. 1, 2:40 p.m., 32 degrees up; Dec. 8, 2:28 p.m., 33 degrees up; Dec. 15, 2:10 p.m., 34 degrees up; Dec. 22, 1:44 p.m., 36 degrees up; and Dec. 29, 1:09 p.m., 38 degrees up. Many roads in the valley run north-south, which will help you face the right direction. Aim your binoculars at a distant mountain, and focus. Then point toward Venus, and observe its current crescent phase, getting thinner and larger in size as this month progresses! (Take care that you do not aim binoculars at the sun.)
Our morning twilight map (below) shows the sky nearly 45 minutes before sunup. On Sunday, Dec. 1, five solar-system bodies are easily visible. From west to east, they are Jupiter, about halfway from horizon to overhead in west; Mars, even higher in the south-southeast; and a clustering of Saturn, the old crescent moon, and Mercury low in the east-southeast. From the Coachella Valley, Saturn is 4 degrees above the thin moon, and Mercury appears within 4 degrees to the moon’s lower left. There are less than 35 hours until the invisible new moon (on Dec. 2 at 4:22 p.m.), so the moon will be gone by Monday morning. Mercury drops out of sight in December’s second week, on its way to the far side of the sun on Dec. 28.
The two brightest objects in our morning sky are steady Jupiter, sinking west toward west-northwest, and twinkling Sirius, until it sets in the west-southwest.
On Dec. 1, Earth is heading in the direction of 10 degrees east (left) of Regulus. As the Earth curves around the sun in the next five months, it will overtake the slower-moving outer planets: Jupiter in early January, Mars in early April, and Saturn before the middle of May. As a result, these planets, along with their background stars, will progress toward the western horizon in our morning sky, and will appear above our eastern horizon in the early evening sky as Earth passes them in turn.
On Dec. 2, Mars appears midway between Regulus and Spica, 27 degrees from each.
If you enjoy chasing very thin crescent moons in twilight, there’ll be three chances in coming weeks: (1) A young crescent on the evening of Tuesday, Dec. 3, age 25 hours, very low in west-southwest 28 degrees to the lower right of Venus; (2) a very old crescent on the morning of Dec. 31, 21 hours before new, very low in the east-southwest, 20 degrees to the lower left of Antares; and (3) an extremely young crescent in the very early evening on Jan. 1, age 14 hours, within 8 degrees to the lower right of Venus. (My personal record for a youngest moon sighting is 13.5 hours past new.)
The Geminid meteor shower is largely spoiled by bright moonlight this year. The best viewing with the least moonlight will occur on Friday, Dec. 13 from 3:30 to 5:30 a.m., and on Saturday, Dec. 14, from 4:30 to 5:30 a.m.
Comet ISON will pass within 725,000 miles of the sun’s surface on Thanksgiving Day and make a sharp turn to the north, or upper left, of the predawn sun. By Dec. 6, it will rise as morning twilight begins. Visit www.pa.msu.edu/abrams/ison/ for updates on the comet.
The Astronomical Society of the Desert will host a skywatching session on Saturday, Dec. 7, from 5 to 8 p.m. at the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument on Highway 74, about four miles south of Highway 111 in Palm Desert. For more on the club’s activities, visit astrorx.org. The Whitewater Preserve is hosting a skywatch on Friday, Dec. 20, from 5:30 to 9 p.m., preceded by afternoon viewing of the crescent Venus. I hope to see you there, and at some of the WildLights evenings at the Living Desert, where we’ll show the crescent Venus and other sky phenomena.
Robert C. Victor was a staff astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. 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.