In December, Jupiter and Saturn appear strikingly close to each other, low in the southwestern sky at dusk. Jupiter’s crawl past Saturn, occurring at intervals of about 20 years, is the least-frequent pairing of naked-eye planets. After their very close pairing—just 0.1 degree apart on Dec. 21—their next conjunction will occur in 2040, with the next after that in 2060. However, on both of those occasions, the planets will get no closer than 1.1 degree apart.
In other words: We are about to witness their closest pairing since 1623! The youngsters among us can wait until 2080 to see the two giant planets as closely paired again.
The sky frequently offers up pairings of the moon with the five planets whose brightness and motions have attracted attention since ancient times; pairings and groupings of planets; and pairings of the moon and planets with the five stars of first magnitude within the well-known belt of zodiac constellations.
Evening planets: As mentioned above, Jupiter and Saturn form a striking, close pair in the southwestern sky at dusk, within 2.2 degrees on Dec. 1, closing to just more than 6 arcminutes (0.1 degrees) apart on Monday, Dec. 21, and reopening to nearly 1.2 degrees apart on Dec. 31. From Dec. 12-29, the two giant planets appear within a degree, easily fitting together within a low-power telescope field. Higher magnifications can show closer views of Jupiter’s cloud belts, the system of four Galilean satellites, and Saturn’s rings, 21 degrees from edge-on—all within one field! The pairing of Jupiter and Saturn in Capricornus on Dec. 21 is their closest since their conjunction in Cancer in 1623. Jupiter-Saturn pairings occur at intervals of about 20 years, moving more than two-thirds of the way eastward (or nearly one-third of the way westward) around the zodiac on each successive occasion. At their next two pairings—in Virgo, on the morning of Oct. 31, 2040, and in Taurus, within 5 degrees south of the Pleiades on the evening of April 7, 2060—they’ll appear 1.1 degrees apart. This month, Jupiter, at magnitude -2.0, is the brightest evening “star,” following the sun over the west-southwestern horizon by 3.3 hours on Dec. 1; by 2.3 hours on Dec. 21; and 1.7 hours on Dec. 31. Saturn, at magnitude +0.6, is about one-tenth as bright.
Mars is high in the southeast sky at dusk. Fading from magnitude -1.1 to -0.2 this month as the Earth pulls away, the red planet ranks next in brightness after Jupiter among early evening’s “stars.” It claims first place when Jupiter sets, but then drops back to second when Sirius appears in the east-southeast. All month, Mars is going 10 degrees east against the faint constellation Pisces; binoculars will help you enjoy these passages: Mars is 1 degree south of 4.3-magnitude Epsilon Piscium on Dec. 4; within 6 degrees north of the gibbous moon on Dec. 23; 4.5 degrees south of 3.6-magnitude Eta Piscium on Dec. 31; 2.5 degrees north of 4.3-magnitude Omicron Piscium on Jan. 1; and 1.6 degrees north of 5.7-magnitude Uranus on Jan. 20.
Meteor shower: The peak of the Geminid meteor shower is Sunday night, Dec. 13-14. Saturday night’s count will be lower, with fainter meteors. Monday night’s count would be even lower, but possibly including some brighter meteors. Each night, the meteor count should increase until the 1 a.m. hour, when the shower radiant, near the star Castor in Gemini, passes just south of overhead. It might be fun to be in place in the early evening to watch for “Earth-grazers,” coming into the atmosphere nearly parallel to the earth’s surface. On Sunday, the radiant rises in northeast about one and a quarter hours after sunset.
Stars: The Summer Triangle of Vega, Altair and Deneb is well up in the western sky at dusk, getting lower as the month progresses. Capella is ascending in the northeast, while Fomalhaut goes from south to south-southwest. Aldebaran, at opposition on Dec. 1, begins the month low in the east-northeast, ascending into the east, making way for the rising of Orion’s Betelgeuse and Rigel (with his three-star belt between them) in the east. About two hours after Orion’s rising, follow his belt down to the east-southeastern horizon, and watch for the rising of Sirius, the Dog Star.
Morning planets: Venus, rising 2.2 to 1.5 hours before sunrise, shines at magnitude -4 in the east-southeast to southeast in twilight. Going east 1 1/4 degrees daily, Venus passes 1.4 degrees north of 2.8-magnitude Alpha Librae on Dec. 3; within 0.2 degrees north of 2.6-magnitude Beta Scorpii on Dec. 18; and 5.6 degrees north of first-magnitude Antares on Dec. 23. Mercury, 17 degrees to the lower left of Venus on Dec. 1, quickly drops into brighter twilight. Superior conjunction—when Mercury and the Earth are on opposite sides of the sun—occurs Dec. 19.
Stars: Of winter’s bright stars in the western morning sky, only the arch of Procyon, Pollux, Castor and Capella remains visible by month’s end. Regulus is high in the southwest, to the upper left of the arch. Arcturus is high in the east, with Spica in the southeast to south-southeast, and Vega in the northeast. Deneb appears to the lower left of Vega. Antares emerges in the southeast late in month.
Planets near the moon: On Saturday, Dec. 12, at dawn, don’t miss the 6 percent old, waning crescent moon, 3 degrees to the upper right of Venus. On Dec. 16 at dusk, catch a 2.4-day-old, 7 percent young waxing crescent moon within 5 degrees of and below Jupiter and Saturn. In this event, just five days before the planets’ Dec. 21 closest pairing, Jupiter and Saturn will appear one moon diameter—or just over one-half degree—apart! On Dec. 23 at dusk, catch the nine-day-old, 69 percent waxing gibbous moon within six degrees south of Mars.
To check for the post-pandemic resumption of star parties hosted by the Astronomical Society of the Desert, visit the club’s website at www.astrorx.org. Rancho Mirage Library’s Observatory is currently closed as well, although the library is open for checking out books during limited hours; visit www.ranchomiragelibrary.org/observatory.html for updates.
I originated the Abrams Planetarium’s monthly Sky Calendar in October 1968, and still produce issues occasionally. For subscription information and a sample, visit www.abramsplanetarium.org/skycalendar.
Wishing you clear skies!
Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing informal sky watching opportunities for folks in and around Palm Springs. Robert D. Miller 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.