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For those of you who like to listen to your stories, click below to hear Susan Trishel Monsøn reading Chris Vaughan’s summary of the planets movements this month.
Following its superior conjunction with the Sun at the end of November, Mercury will enter the western post-sunset sky in December. But for mid-northern latitude observers, its position several degrees south of the plane of the Solar System, plus the severely tilted evening ecliptic, will prevent the swift planet from climbing out of the bright twilight until the second half of December. Observers in the tropics and in the Southern Hemisphere will see Mercury somewhat earlier in the month. On December 29, Mercury will pass four degrees to the south of much brighter Venus, and they’ll be binoculars-close from December 27 to 30. The best viewing period for those inner planets will start at around 5 p.m. local time, but you’ll need a cloud-free, unobstructed southwestern horizon. Viewed in a telescope during the latter part of December, Mercury will show a waning gibbous phase and a disk diameter that swells to 5.9 arcseconds at month’s end. Ensure that the Sun has fully set before turning optics toward Mercury.
After a lengthy stay in the western sky during early evening, Venus will spend December swinging sunward. The increasingly upright ecliptic will finally let the planet shine in a darkened sky for mid-northern latitude viewers. On December 3, Venus will reach its maximum brightness of magnitude -4.66 for the current apparition. Viewed through a telescope during December, our sister planet will show a waning crescent phase that will end December as a mere 2.5-per-cent-illuminated sliver. Meanwhile, Venus’ apparent disk diameter will swell dramatically from 38.7 arcseconds to one arcminute. Venus will move prograde eastward through the stars of eastern Sagittarius until December 18, and then it will enter a retrograde motion period. On December 29, Mercury will pass four degrees to the south of much brighter Venus, and they’ll be binoculars-close from December 27 to 30. (Be sure to wait until the Sun has fully set before turning optics towards either planet.) On December 6, the slim crescent of the young Moon will shine three degrees to the south of Venus, close enough for them to share the view in binoculars. On the evenings around December 17, Venus will reach its minimum angular separation from Jupiter of 31 degrees, with Saturn positioned between them.
During December, magnitude 1.62 Mars will become observable in the southeastern pre-dawn sky as it slowly climbs away from the Sun. Viewed in a telescope in December, the red planet will display a modest four-arcsecond-wide disk. In December Mars will be beginning its year-long trek to a bright opposition in December, 2022. In the interim, it will pass very close to many planets and deep sky targets. On December 1, the planet will rise in central Libra at about 5:30 a.m. local time. The following morning, the slim crescent of the old Moon will shine seven degrees to the northwest of it. Observers in most of Mongolia, northeastern China, parts of eastern Russia, Japan, most of Micronesia, northern Polynesia and Hawaii can see the Moon occult Mars. The planet’s eastward prograde motion will carry it into Scorpius on December 16. After passing between the scorpion’s claw stars Acrab and Dschubba — and very close to the wide double star Omega1,2 Scorpii on December 18-19 — Mars will shine only 1.6 degrees north of the globular cluster Messier 80 on December 23. It will pass less than five degrees north of its rival, the reddish star Antares on December 27-28, and then end the month in eastern Ophiuchus. The old crescent Moon will shine less than five degrees to the west of Mars on December 31.
The earlier sunsets of December will extend evening Jupiter-viewing time. Look for the planet shining as a very bright, white dot in the lower third of the southern sky after dusk. It will sink below the west-southwestern horizon at around 10 p.m. local time on December 1 and then before 9 p.m. by month’s end. All month long, Jupiter will travel prograde eastward, starting December above the Sea-Goat’s tail stars in eastern Capricornus and then entering next-door Aquarius on December 14. Fainter, yellow-hued Saturn, shining to Jupiter’s west, will increase its separation from Jupiter by two degrees, to 18.5 degrees on December 31. Views of Jupiter in amateur telescopes will show dark equatorial bands across its disk, which will shrink in size from 38.2 to 35.3 arcseconds during the month. The Great Red Spot will appear for a few hours every second or third night. Two of the round, black shadows of Jupiter’s Galilean satellites will cross Jupiter’s disk, one of them with the Great Red Spot, on December 10. From December 8 to 9, the crescent Moon will move past Jupiter. On the evenings around December 17, Venus will close to within 31 degrees of Jupiter, with Saturn positioned between them.
During December, Saturn will become visible after dusk as a creamy-coloured magnitude 0.7 object shining to Jupiter’s lower right in the lower part of the southwestern sky. But the ringed planet will become less satisfying as a telescope target when its elongation from the Sun decreases toward year end. Compared to the stars of western Capricornus that surround it, Saturn will be treading slowly eastward all month. Viewed in a telescope, Saturn will display a mean apparent disk diameter of 15.7 arcseconds, and its rings will subtend 36.6 arcseconds. Several moons can be readily seen arrayed around the planet, especially the brightest one, Titan, which takes 16 days to complete an orbit. Saturn’s rings will be tilting more edge-on to Earth every year until the spring of 2025. This year they are already closed enough for Saturn’s southern polar region to extend beyond them. On December 7-8, the crescent Moon will pass five degrees to the celestial south of Saturn.
Magnitude 5.7 Uranus will be observable in binoculars and telescopes from after dusk until the wee hours during December. Its small, blue-green dot will be moving slowly retrograde westwards in southern Aries, 11.5 degrees southeast of that constellation’s brightest stars, Hamal and Sheratan, and only five degrees from the star Mu Ceti to its south. On December 14 and 15, the very bright and nearly-full Moon will shine to the southwest and southeast of Uranus, respectively.
The distant and blue-colored planet Neptune will be observable in the southwestern sky in evening during December while the magnitude 7.9 planet travels very slowly eastward among the stars of northeastern Aquarius. If the sky is very dark, Neptune can be seen in binoculars. To locate it, find the north-south grouping of five medium-bright stars Psi, Chi, and Phi Aquarii (or ψ, X, and φ Aqr). Neptune’s non-twinkling speck will sit three degrees to the north-northeast of the top star, Phi. Viewed in a telescope, Neptune’s apparent disk size will be 2.3 arcseconds. Your best views will come immediately after dusk, when the blue planet will be highest in the south.
Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the Universe together.