In December's night sky we bid a fond adieu to Jupiter, which dominated our evening sky since late spring. We still can get a glimpse of it, very low in the southwest right after sunset during the first week of this month, but during the second week it disappears into the evening twilight sky.
Recall that Jupiter had a close encounter with Venus just before Thanksgiving. Now it's Saturn's turn. Saturn approaches the dazzling planet during the first 10 days of December with conjunction on Dec. 11. But during the second half of the month, Saturn gradually descends into the sun’s afterglow; you might still catch a brief peek of it on Dec. 27, when the planet will appear close to an exceedingly thin crescent moon.
In the predawn morning sky, Mars still appears disappointingly dim, ranking only with stars of the second magnitude. Mercury, also a morning object, appears much brighter, but closer to the sun and is visible during the first half of the month, before it descends into the dawn twilight and disappears.
In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm's length measures roughly 10 degrees. Here, we present a schedule below which provides some of the best planet viewing times as well directing you as to where to look to see them.
Planet Viewing Guide
Mercury – will be visible during the first two weeks of December, shining as a bright yellow-orange star low in the east-southeast sky about 45 minutes before sunrise. After mid-month, this speedy little world's altitude dramatically lowers and it becomes too deeply immersed in the bright morning twilight to be seen. It will be in superior conjunction with the sun on Jan. 10.
Venus – becomes dramatically more visible for viewers at mid-northern latitudes in December, shining brightly low in the southwest at dusk. The interval between sunset and Venus-set increases very respectably from about 1¾ hours to 2½ hours during the month. Telescopes show its relatively small disk as only slightly out of round – decreasing from 89% to 82% lit over the course of the month. On Dec. 11, Venus will be in conjunction with Saturn (see Saturn for more details). Then, on Dec. 28th, an eye-catching celestial tableau awaits skywatchers as darkness falls this evening. Look toward the southwestern sky you’ll easily see a lovely crescent moon and shining about 3° above it, will be brilliant Venus . . . a sort of post-Christmas celestial ornament of this holiday season.
Mars – can be readily identified on Dec. 23, a couple of hours before sunrise. Just look toward the east-southeast to see a skinny waning crescent moon. That orange-yellow 2nd-magnitude star shining about 5 degrees to the moon's upper right is Mars, now in the zodiacal constellation of Libra, the Scales.
Jupiter – is visible with difficulty low in the southwest early in the month, then fades into invisibility against the bright twilight sky by midmonth. On Dec. 27th, Jupiter is in conjunction with the sun and transitions into the morning sky.
Saturn – has been a planet of summer, since it currently resides in Sagittarius. Now it is departing into the sunset as if fleeing the impending cold of a new winter season. At the beginning of December it is still high enough up to see fairly well when twilight fades, but by Christmas it sets before the end of twilight.
On Dec. 11, shortly after sunset we'll have a view of two planets, low in the west-southwest sky passing each other in the twilight; one heading up and the other going down. The planet Venus, blazing at magnitude -3.9, will be passing 1.8 degrees to the lower left of a much dimmer (magnitude +0.6) Saturn.
Finally, on Dec. 27 is a challenge: About a half hour after sunset, look toward the southwest horizon for a hairline-thin crescent moon, 3% illuminated and 1½ days past new. Scanning with binoculars will certainly help. If you find it, search about 5° to the moon’s lower right for what likely will be your last view of Saturn before it disappears from the evening sky.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications in New York's lower Hudson Valley. Follow us on Twitter @Spacedotcom and on Facebook.