In evening twilight in December, the Summer Triangle is well up in the west, getting lower as the month progresses. Its brightest member is blue-white Vega, at its northwest (lower right) corner. Altair marks the southern point of the Triangle, and Deneb the northeast corner, above Vega. Follow the Summer Triangle within the first hour after sunset until mid-January, when Altair sinks into the twilight glow.
Solitary Fomalhaut, marking the mouth of the Southern Fish, drifts low across the southern sky in December’s evening twilight. From late in December’s second week into early January, try to find Mercury very low in the southwestern twilight glow; binoculars make the search easier.
Yellowish Capella climbs in the northeast, while to its lower right, ascending in the east-northeast to east, we find red-orange Aldebaran, eye of Taurus the Bull. This star is at opposition to the sun each year around the start of December, so as we gaze at that star, we face almost directly away from the sun. Low in the east below Taurus, rising into view during twilight in late December, we find Orion’s two brightest stars: reddish Betelgeuse marking one shoulder, and blue-white Rigel marking his upraised foot. Robert Frost, in the opening lines of his poem “The Star Splitter,” described the scene: “You know Orion always comes up sideways. Throwing a leg up over our fence of mountains … .” Rising just a bit later from Southern California are Pollux and Castor above it, the bright stars of Gemini, the Twins.
In December’s morning twilight, Venus, in the southeast, ranks first in brilliance. Next is Jupiter, high in the southern sky. Third is twinkling Sirius before it sets in the west-southwest, and next is a nearly three-way tie between Arcturus very high in the east to southeast, Vega ascending in the northeast, and Capella sinking in the northwest.
Before Rigel sets south of west, look for the Winter Hexagon. In clockwise order beginning at Sirius, its other members are Procyon, Pollux (with Castor), Capella, Aldebaran, Rigel and back to Sirius, with Betelgeuse inside. At month’s end, all that remains of the Hexagon in morning twilight is an arch, in order from west to northwest: Procyon, Pollux (with Castor) and Capella.
Regulus marks the heart of Leo the Lion, chasing the Hexagon across the sky. Regulus is within 0.5 degrees north of the ecliptic (plane of Earth’s orbit). Following Regulus and in line with it is an almost straight lineup of the planets Jupiter, Mars and Venus, and finally Saturn, emerging in the southeast by the middle of December. Blue-white Spica, only 2 degrees south of the ecliptic, appears not far off the lineup of planets: Venus passed 4.2 degrees north of Spica on Nov. 29; contrastingly colored, dim, red Mars will pass 3.6 degrees north of that star on Dec. 23. By the latter date, Antares will have just emerged, some six degrees to the south (lower right) of Saturn.
One additional star appears on our December morning twilight chart: Deneb, rising in the far northeast late in the month, to the lower left of Vega.
Watch for these events:
Friday, Dec. 4, morning: Jupiter is 5 degrees to the upper right of the moon.
Saturday, Dec. 5, morning: Mars is 5-6 degrees to the lower left of the moon.
Sunday, Dec. 6, morning: Spica is five degrees to the lower right of the moon.
Monday, Dec. 7, morning: Spica is midway between Venus and Mars, 10 degrees from each. A spectacular close conjunction of the crescent moon and Venus occurs in morning twilight. Continue observing after sunrise and witness a daytime occultation of Venus by the moon. From Palm Springs, binoculars and telescopes show the leading sunlit edge of the moon covering Venus well up in the south-southeast to south at 8:09 a.m., with the trailing dark edge of moon (invisible in daylight) uncovering Venus well up in south-southwest at 9:59 a.m. Outside that interval, when Venus isn’t covered, this is a great chance to use the moon to help locate Venus in the daytime!Telescopes show Venus in gibbous phase.
I will hold three different sky watches, if the sky is clear, on the morning of Monday, Dec. 7: The first will be held from 5:15 to 5:45 a.m. to view the pairing in a dark sky before sunrise, as well to observe Jupiter, Mars and the bright stars; the next is from 8 until 8:10 a.m., to watch the bright edge of the moon cover Venus; and the last is from 9:55 to 10:05 a.m., to watch the invisible dark edge of the moon uncover Venus. All three watches will be held in Palm Springs, on the pedestrian bridge crossing over Tahquitz Creek, at Camino Real between North and South Riverside drives, three blocks north of Cahuilla Elementary School.
After Dec. 7, the waning moon can be followed for two or three additional mornings. On Thursday, Dec. 10, 40 minutes before sunup, try for the very thin old crescent, only 20-21 hours before new, very low in the east-southeast. Binoculars will be helpful for spotting it, and possibly emerging Saturn, rising within 3 degrees to the moon’s lower right.
Also watch for Saturn 6.2 degrees north of Antares (the minimum distance) on Monday, Dec. 21; Mars 3.6 degrees north of Spica (the minimum distance) on Wednesday, Dec. 23; and the moon 3 degrees to the lower right of Jupiter on the year’s final day.
Illustrations of these events appear in the Sky Calendar. For more information, visit www.abramsplanetarium.org/skycalendar.
The Astronomical Society of the Desert will host a star party on Saturday, Dec. 5, at dusk at Saw Mill Trailhead, at elevation 4,000 feet in the Santa Rosa Mountains. The society also hosts monthly star parties at the more conveniently located Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument; the next is on Saturday, Dec. 19, from 5 to 8 p.m. Get more information at www.astrorx.org.
Robert C. Victor was a staff astronomer at Abrams Planetarium at 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 and the planet orbit 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.