Only 37.6 years to go! Halley’s Comet, according to calculations by NASA/JPL, is expected to reach its aphelion, or greatest distance from the sun, at a location farther than the most distant planet Neptune, on the evening of Dec. 8, 2023. The comet then begins its journey back toward the inner solar system.
On July 28, 2061, it will reach its perihelion, or closest approach to the sun, inside the orbit of Venus, and will reach its closest approach to Earth on the very next day. The comet will be visible to the unaided eye in July-August 2061. Urge your young friends and family to plan to see it!
The annual Geminid meteor shower, usually the strongest of the year, is timed ideally in 2023, with the moon just two days past new and absent during the shower’s peak nights. Expect the greatest numbers of meteors from about 10 p.m. local time on Dec. 13, until the first light of dawn on Dec. 14, and again on the following night. Meteors can occur anywhere in the sky, but the trails of Geminids from asteroid Phaethon, traced backward beyond where they became visible, should radiate from a common point in the starry background, near Castor in Gemini.
Winter begins on Dec. 21 at 7:27 p.m., as the sun stands directly over the Tropic of Capricorn, giving Northern Hemisphere residents their lowest midday sun and shortest day.
The four brightest “stars” at dusk in December are brilliant Jupiter, in the east-southeast to southeast; Mercury, very low in the west-southwest in month; and two stars of zero magnitude: Vega, in the west-northwest to northwest, and Capella, ascending in the northeast.
Objects that are first magnitude or brighter include Saturn, in the south-southwest to southwest, with Fomalhaut, mouth of the Southern Fish, to its lower left; Altair and Deneb, completing the summer triangle with Vega; and Aldebaran, eye of Taurus, rising north of east. On Dec. 2, Aldebaran is at opposition as Earth passes between that star and the sun, so the star is up all that night.
Late in the month, the stars Betelgeuse and Rigel of Orion (identified with the vertical line of three second-magnitude stars marking his belt in between) appear low in the eastern sky at dusk, with Castor and Pollux rising farther left, in the northeast.
Jupiter gleams at magnitude -2.8 to -2.6 at dusk. In Aries, Jupiter shifts 1.4° west-southwest until Dec. 30, when it ends retrograde. A telescope reveals its disk 48” to 44” (arcseconds) across, and four bright satellites discovered by Galileo in 1610. Saturn glows near magnitude +0.9 at dusk in December. Against stars of Aquarius, Saturn progresses 2.1° east-northeast, away from 4.3-magnitude Iota in Aquarius. Telescopically, Saturn’s rings appear 38” to 36” in east-west extent, while 10.2° to 9.2° from edge-on. Mercury begins December at magnitude -0.5 very low in the west-southwest at dusk; binoculars help find it in twilight. Mercury reaches greatest elongation, 21.3° east of the sun on Dec. 4, and for a few days around Dec. 7 hovers 4° up in mid-twilight. Mercury fades to magnitude 0.0 by Dec. 11, and to magnitude +0.8 by Dec. 14. Mercury retrogrades Dec. 12-Jan. 2, shifting as much as 1.4° westward daily. This fast relative motion enables sky watchers to spot Mercury in the morning sky by the end of this month!
Faint solar system bodies: Uranus, of magnitude 5.6 to 5.7 and retrograding nearly 0.9° in December, is 14° to 13° east-northeast of Jupiter, 11° to 12° west-southwest of the Pleiades, and 2.3° to 2.8° southwest of the 4.3-magnitude star Delta in Aries. Neptune, of magnitude 7.9 and ending retrograde in Pisces on Dec. 6, is 24° to 22° east-northeast of Saturn, and 1.6° to 1.4° west-southwest of the 5.5-magnitude star 20 in Pisces. Don’t confuse Neptune with the 7.3-magnitude star HIP 117112, located 57’ (0.95°) west-southwest of 20 Psc. Fainter Neptune will pass 9’ (arcminutes) north-northwest of the 7.3-magnitude star on Jan. 22. Finder charts for Uranus and Neptune for binocular users appear at www.abramsplanetarium.org/msta.
Minor planet No. 4 Vesta reaches opposition at magnitude 6.4 on Dec. 22. During the night of Dec. 14-15, find Vesta 12’ (arcminutes) northeast to 8’ north-northeast of the 4.6-magnitude star Chi-2 in Orion’s club. Vesta is then moving west by 0.26° per day. The 4.4 magnitude star Chi-1 in Orion’s club is 2 1/4° west of Chi-2 and 13° north of Betelgeuse. On the evening of Dec. 23, Vesta passes 22’ north of 4.4-magnitude Chi-1 Ori, but in a bright moonlit sky.
The brightest “stars” at dawn in early December are Venus in the southeast; Sirius, the Dog Star, in the southwest to west-southwest; Arcturus, in the east; Vega, rising in the northeast; and Capella, in the northwest. Find Spica within 5° of Venus on Dec. 1 and 2; otherwise use the Big Dipper’s handle to “follow the arc to Arcturus and drive a spike to Spica.” Regulus, heart of Leo, is high in the south-southwest at dawn as December begins. Early in the month, look for the spring arch topped by two stars 4.5° apart, Pollux and Castor of Gemini. To the Twins’ lower right is Capella. To the lower left of the Twins are Procyon and Sirius. Below the Arch are Aldebaran, eye of Taurus, and Orion’s Betelgeuse and Rigel. Rigel is the first of all these bright stars to set. Look after Vega rises and before Rigel sets, and you can see 11 stars of first magnitude or brighter, not counting Castor of magnitude +1.6, and the planet Venus.
Venus, of magnitude -4.2 to -4.0, rules the morning! Don’t miss the conjunction of the moon and Venus on Dec. 9. Venus progresses east by 1.2° per day, while its gibbous phase fills out from 68% to 78% in December, and its disk shrinks from 17” to 14”. Mercury, brightening to magnitude +1.0 by Dec. 30, appears to the lower left of Venus, by 22° on Dec. 30, and by 20° on Dec. 31. Mars, low in bright twilight and fainter at magnitude +1.4, is 5° to 6° below Mercury those two mornings, requiring binoculars. Both improve visibility in January.
If you have a vantage point with unobstructed views, you can catch Venus and Jupiter 5° above opposite horizons, about three hours before sunrise, on Dec. 2. Look after Venus rises and before Jupiter sets. The chance to see both at once is slipping away; by Dec. 11, Jupiter will set before Venus rises, and your next chance to view these two brightest planets simultaneously will occur in evenings for 4 1/2 months beginning on Nov. 4, 2024.
The waning gibbous moon, in the morning sky, appears 5° east of Pollux in Gemini on Dec. 1 and within 6° east of Regulus in Leo on Dec. 4. The waning crescent moon appears within 2° of Spica in Virgo on Dec. 8, and within 4° of Venus on Dec. 9. The last chance to view the old moon will be in morning twilight on Dec. 11, as a 2 percent crescent low in the east-southeast to southeast, 25° lower left of Venus.
On Dec. 19, the moon is just past first quarter phase, when it is 90° east of the sun and appears half-full. The moon’s position against the stars of Pisces tonight is close to where the sun will appear three months from now, near the beginning of spring. On Dec. 21, the 75 percent waxing gibbous moon appears 6° to the upper right of bright Jupiter. On the following night, the 83-percent moon will appear 8° to Jupiter’s lower left. On Dec. 23, binoculars will help spot the Pleiades in the glare of the 91 percent moon. Look in the same field, 4° to the moon’s lower left. On Dec. 24, note Aldebaran, eye of Taurus and “follower” of the Pleiades, within 9° to the lower right of the 96 percent moon. On Dec. 25, use binoculars to spot 1.7-magnitude Elnath, or Beta in Taurus, 1-2° to the upper left of the 99-percent moon. The moon is full on Dec. 26 at 4:33 p.m. From a location with unobstructed views, try to see the sun and moon simultaneously shortly before sunset that evening, and shortly after sunrise on Dec. 27.
Of the five first-magnitude stars within the zodiac, only Pollux can’t be hidden by the moon, since the star is 6.7° north of the plane of Earth’s orbit. The moon’s orbit is inclined about 5° to Earth’s, not enough to cover the star from any place on our planet. In 2023-24, the moon comes close to Pollux at each pass, at intervals of about 27.3 days. (For example, on the morning of Dec. 28, the 97 percent moon passes only 2° south of Pollux.) The moon’s orbit slowly precesses, or wobbles, changing its orientation in a cycle of 18.6 years, so in 8 to 10 years, the moon each month will pass some 12° south of the star. Enjoy the close passes of the moon to Pollux while you can! The four brightest stars the moon can occult are Aldebaran, Regulus, Spica and Antares.
The Abrams Planetarium Sky Calendar is available by subscription from www.abramsplanetarium.org/skycalendar. Each monthly issue consists of a calendar page illustrating events, and an evening sky map. For $12 per year, subscribers receive quarterly mailings, each containing three monthly issues.
Robert Victor originated the Abrams Planetarium monthly Sky Calendar in October 1968 and still helps to 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.
