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This December, the dance of planets continues in the predawn sky, while the fall 2015 evening “planet drought” ends with the return of Mercury to the mid-December evening sky.
Also this December, the reliable Geminid meteor shower and the Southern Cross and the Big Dipper return to our skies. And we will have the first full moon on Christmas since 1977. The moon will be full at 1:12 a.m. in Hawaii on Christmas this year.
Throughout the month, the planets Venus and Jupiter remain striking in the hours before dawn. Venus shines at minus-4.4 magnitude, much brighter than any star. Jupiter shines at minus-2 magnitude; this is 30 times dimmer than Venus but still the sky’s second-brightest dot. Saturn joins the predawn gathering at the end of the month.
Jupiter rises in the east at 1:15 a.m. in early December and is about two-thirds of the way up the eastern sky by daybreak; Venus follows its rising by 2-1/2 hours in early December.
Venus comes up in early December at 3:45 a.m. and is about a third of the way up in the eastern sky by dawn. Venus is about 35 degrees below Jupiter, or more than the width of three fists held at arm’s length.
In between the two blazing dots of Venus (the lower one) and Jupiter (the upper one), look for the pale faint dot of Mars. At the start of the month, Mars appears closer to Venus than to Jupiter. At second magnitude, Mars is hundreds of times fainter than Venus.
On the morning of Dec. 6, look for the waning crescent moon between Mars and Venus.
In the morning sky, Venus and Jupiter appear to pull farther apart as the month goes on; by midmonth Venus will be 50 degrees below Jupiter in the morning sky, the width of five palms.
On Dec. 15, Jupiter will rise at 12:30 a.m. and will be high in the south at daybreak. Venus does not rise until 4 a.m. so you will only have a couple of hours to see the two planets together before day breaks at 6:15 a.m. Mars remains in between the two dots of light, still faint.
For the first half of December, there are no planets visible to the naked eye in the evening sky.
Mercury returns to the evening sky in mid-December. Look for Mercury low in the west at dusk, around 6:20 p.m. on Dec. 15; it sets quickly, by 6:45 p.m., but shines brightly at minus-0.6 so you might be able to catch it. The planet sets a little later each night and goes down at 7 p.m. by Christmas.
Around New Year’s Eve, look for a string of four planets in the sky just before dawn. Around 6 a.m., find the two bright planets first. Look for Jupiter now high in the south; blazing Venus is about 20 degrees above the eastern horizon. Mars is now almost exactly halfway between the two brighter planets Venus and Jupiter, and improving in brightness; on Dec. 31, the Red Planet shines at 1.27 magnitude.
Then, look down below Venus in the east; the planet Saturn has re-emerged and is now about 10 degrees to the lower left of Venus and shines at 0.5 magnitude. Saturn does not rise until 5 a.m., so you have a little over an hour from then until dawn to catch all four planets. Make sure you have a flat eastern horizon to see Saturn shimmering just above the eastern horizon.
Mercury remains steady for the reminder of the month — emerging low in the west at dusk, shining at minus-0.6 and not setting until 7:15 p.m. at month’s end.
Geminid shower
The Geminids are one of the most reliable shooting-star showers, with as many as 120 shooting stars an hour. The peak occurs on the nights of Dec. 12-13 and 13-14, from 12:01 a.m. to dawn each night. (Stay up late on the 12th, and the peak begins right after midnight on the 13th; or stay up late on the 13th, and the peak begins right after it passes midnight.) Conditions are promising this year since there will be no moon to interfere.
The constellation of Gemini is about halfway up in the eastern sky by midnight in mid-December; it’s to the left of the famous Orion constellation.
Cross, Dipper return
The Southern Cross comes back every year to Hawaii’s December skies. For nearly half of every year, July through November, the sun and the Cross are in the sky at the same time, and we can’t see this famous constellation from the islands then.
In early December, look to the south-southeast around 6 a.m. The Southern Cross will clear the horizon by 5:45 a.m., giving you 15 minutes to catch it before day breaks. By the end of December, it rises at 3:45 a.m. and is due south at 6 a.m. as day breaks.
The Southern Cross is at its highest when it is due south, but it is still low. The bottom, brightest star, Acrux, is only 6 degrees above the horizon when the Cross is due south, no farther than the distance between Acrux and the top star of cross, Gacrux. (Six degrees is the width of three fingers held at arm’s length.) The four points of the cross are marked by three fairly bright stars and one, much fainter, that marks the right side of the crossbeam.
The Big Dipper, missing in early fall, also shines in the December morning sky. The Dipper is up by 2 a.m. in early December and by midnight at the end of the month.
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