The Full Cold Moon arriving Dec. 29 will be the last full moon of the year, ushering in the winter constellations and notably, three naked-eye planets in the evening sky.
The moon becomes officially full at 10:28 p.m. EST (0328 Dec. 30 GMT), according to NASA’s SkyCal site. For New York City observers, the moon will rise Dec. 29 at about 4:12 p.m. and set at 7:52 a.m. local time on the morning of Dec. 30.
According to the Old Farmer’s Almanac, the full moon in December is called the Full Cold Moon, and it’s pretty clear why, at least if you live in the Northern Hemisphere’s mid-latitudes. This particular Cold Moon will rise accompanied by Mars, which will be high in the southern skies just after sunset, as well as Jupiter and Saturn, which will be low in the west coming out of the “great conjunction,” which occurred Dec. 21.
Related: Best night sky events of December 2020 (stargazing maps)
Planets to see
The great conjunction was when Jupiter and Saturn approached within one-tenth of a degree of each other on Dec. 21, appearing as a “double star” in the evening sky. The two planets move against the background stars slowly, with Jupiter completing a circuit every 12 years while Saturn takes 29 years to do so. During a conjunction they pass each other in the sky and share the same celestial longitude, but Jupiter takes long enough that this only happens approximately every 20 years in any case. This particular conjunction will be the closest since Galileo’s time, the 16th century.
By Dec. 29 the two planets will still be close — about a degree apart, appearing to trail the sun in the southwest. Jupiter will set over New York City at 6:24 p.m. local time, about four minutes after Saturn.
Mars, meanwhile, will be in the constellation Pisces and making its way in to Aries. Mars takes only 687 days to circuit the sun, so it spends approximately two months in each of the constellations of the zodiac — though this can vary a lot because the borders of said constellations do not divide the zodiac into 12 perfectly even pieces. Mars is easily spotted because of its reddish color, apparent even when it has to compete with bright city lights.
Venus, meanwhile, will still be a “morning star” in the constellation Ophiuchus, rising at 5:50 a.m. local time in New York on Dec. 30. Sunrise is not until 7:19 a.m. local time, and the planet will be at 13 degrees altitude by then. Venus is bright enough that it remains visible even as the sky becomes lighter, and a fun challenge is to see how long you can still spot it as sunrise approaches. Mercury, meanwhile, is simply too close to the sun to see — at sunset on Dec. 30 it is only 2 degrees above the horizon and won’t be a visible evening star for a few days.
Full moon names
Full moon names are a product of where the people that name them live and the lives they lead. This lunation will be the thirteenth of the year, and one of the names given by the Ojibwe people is Mnidoons Giizis, or “Big Spirit Moon,” associated with rites of purification.
In China, the winter lunation that corresponds with the solstice is called Dōngyuè, “Winter Month”, while in the Southern Hemisphere the Maori called the December-January lunation Kohi-tātea, ” Fruits are now ripe, and man eats of the new food of the season,” according to the Encyclopedia of New Zealand — because in the Southern Hemisphere, December is in midsummer.
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