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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 for December.
Except for a few minutes before sunrise on the first two or three days of the month, Mercury will not be observable during December. After it passes superior conjunction with the Sun on December 20, Mercury will return to the southwestern evening sky, but its orbital position south of the shallow evening ecliptic will keep the planet too close to the horizon for observing until it swings farther from the Sun in January.
During December, Venus will continue to shine at a very bright magnitude -3.9 in the southeastern sky before sunrise — but it will be in the final stages of a lengthy pre-dawn apparition. Because the planet will be shifting sunward, it will rise 2.5 hours before the Sun on December 1, but only 90 minutes before sunrise at month-end. Meanwhile, the planet will traverse the stars of Libra until December 17, dash through the northern section of Scorpius from December 18 to 21, and then finish the month in southern Ophiuchus. Viewed in a telescope during December, Venus will exhibit a waxing, nearly fully-illuminated phase and an average apparent disk diameter of 11 arc-seconds. On December 11-12, the delicate sliver of the old crescent Moon will pass Venus, occulting the bright planet at approximately 22:00 GMT for observers in easternmost Russia, Hawaii and western North America.
During December, Mars will continue to be conveniently positioned for observing from dusk until several hours after midnight. It will reach its maximum elevation, more than halfway up the southern sky, in early evening, and then it will descend as it’s carried west by Earth’s rotation. Because Earth will be increasing its distance from Mars, the Red Planet will diminish in brilliance by half, fading from magnitude –1.1 on December 1 to magnitude –0.24 on December 31. Telescope views of the planet will show Mars’ apparent disk diameter shrinking from 15 to 11 arc-seconds. At the same time, Mars will be traveling eastward across the V-shaped constellation of Pisces until early January. On December 23, the waxing gibbous Moon will pass five degrees to the south of Mars.
Jupiter & Saturn
After a lengthy chase, Jupiter will finally catch and overtake more distant Saturn during December in a “Great Conjunction.” As the month opens, the two planets will appear in the lower part of the southwestern sky for about two hours following sunset. Magnitude -2 Jupiter, which will shine 10 times brighter than magnitude 0.64 Saturn, will pop into view first as the sky darkens. Between December 12 and December 29, the two planets will be less than one degree apart, allowing them to be viewed together in the eyepiece of backyard telescopes. At their minimum separation on December 21 — their closest since 1623 — the planets will sit only 0.1 degrees apart, making it a terrific opportunity to capture their markings, moons and Saturn’s rings in a single photograph taken through a telescope. Jupiter will span a 33 arc-second disk diameter, comparable to the apparent width of Saturn’s rings. On that evening, they will sit quite low in the southwestern sky, and appear as a single point of light to your unaided eyes. At mid-month, Saturn will lead the pair out of the stars of Sagittarius and into Capricornus. On the nights before and after December 13, the two planets will pass only a degree north of the magnitude 9.9 globular star cluster Messier 75. By the end of December, the viewing window for the two planets will become quite short as they slide sunward into the western twilight. A pretty crescent Moon will hop past them on December 16-17.
During December, blue-green Uranus will be visible all night long while it travels slowly westward in southwestern Aries, about 10 degrees south of Aries’ brightest star Hamal and 5 degrees north of the stars that form the top of Cetus’ head. Try to view Uranus in mid-to-late evening, when it’s more than halfway up the southern sky. The bright Moon will pass a few degrees south of Uranus on December 24, showing you where the planet is, but take advantage of the moonless first half of the month to try seeing the magnitude 5.7 planet with unaided eyes or binoculars.
Neptune will be available for observing in the evening sky during December. The best time to view the distant planet will be as soon as the sky darkens fully, when the planet will be about halfway up the southern sky. From dark sky locations the magnitude 7.9 planet can be observed in good binoculars and backyard telescopes. Look in eastern Aquarius, about 0.8 degrees to the east of the medium-bright star Phi Aquarii, or φ Aqr. Both the planet and that star will appear together in the field of view of a telescope at low magnification (red circle).
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.