The moon becomes officially full on Dec. 22 at 12:49 p.m. EST (1749 GMT), according to NASA. The moon will be in the constellation Taurus and will rise about 15 minutes after sunset. The full moon happens to occur a day after the winter solstice. The last time a full moon coincided with the solstice was in 2010, and the next will be in 2094.
One night before the full moon becomes full, skywatchers can see the nearly full moon snuggle up to the bright star Aldebaran. According to NASA, at 2:31 a.m. EST (0731 GMT) on Dec. 21, the moon will share the same celestial longitude as Aldebaran, a situation called a conjunction. Given the late winter sunrise and early sunsets in the Northern Hemisphere, the moment of conjunction will be visible to observers across the United States, and as far west as Hawaii, where the moon will rise at 4:29 pm. on Dec. 20. [The Brightest Planets in December's Night Sky: How to See them (and When)]
Those located in most of Europe won't catch the conjunction itself, as it will happen after the moon sets the morning of Dec. 21. For example, the moon sets in London at 6:09 a.m. on Dec. 21, or 22 minutes before the conjunction occurs. Icelanders will see the moon set at 8:58 a.m. local time, about 1.5 hours after the conjunction. (The sun doesn't rise there on that day until 11:22 a.m.)
Full moons are so bright that they tend to overwhelm fainter objects in the night sky, even from dark-sky locations. In fact, the full moon casts distinct shadows and it is almost bright enough to read by. That said, joining the moon in the December sky will be a few planets, and some will pass close to each other, making for striking observational pairs.
One pair will be Jupiter and Mercury, which are in conjunction the day before the full moon on Dec. 21 at 9:41 a.m. EST (144 GMT), according to In-the-Sky.org. The two will pass within less than 1 degree of each other, or a bit less than two lunar diameters. From New York City, the two planets will be difficult to see, as they will be close to the horizon, about 10 degrees up (approximately the width of a closed fist). The pair will rise before the sun, at about 5:40 a.m., but the sun rises only 1.5 hours later at about 6:55 a.m.
Seeing the conjunction of Jupiter and Mercury will be easier closer to the equator. In Puerto Rico, for example, the two planets will get to 14 degrees in altitude when the sun is about 6 degrees below the horizon, otherwise known as civil dawn, which occurs at about 6:35 a.m. At that point the sky is so light that even the brightest planets are difficult to spot.
On the night of the full moon, Mars will be visible after sunset in the western sky. For observers in New York, Mars will rise at 11:39 a.m. and set at 11:17 p.m., and will be in the constellation Pisces, according to heavens-above.com calculations. At about 6 p.m., well after sunset for the eastern U.S., the planet will be about 45 degrees above the horizon and distinct because of its red hue and location in a relatively faint constellation.
Venus, meanwhile, will grace the predawn skies, rising at 3:29 a.m. and reaching an altitude of about 26 degrees an hour before sunrise (which will be at 7:17 a.m. that day). The planet is bright enough that it will be distinct even in light-polluted locations.
On Dec. 22, Saturn will rise at 7:59 a.m. and set at 5:10 p.m. That means at moonrise, Saturn will be low in the sky, only about 3 degrees above the western horizon. For a skywatcher to catch it at all will require a flat horizon and clear weather.
Other planets won't be visible on the days around the full moon, as they will be below the horizon. One exception is Uranus, which will be in the eastern half of the sky about 28 degrees above the horizon in the constellation Aries. But Uranus can be hard to catch without binoculars or a small telescope – it never gets brighter than about magnitude 5.5, which for most people is near the limit of what they can see without optical aids.
The full moon will also share the sky with the annual Ursid meteor shower. The Ursids are named after the constellation Ursa Major, where they appear to originate. The point of origin, or radiant, of the shower will be about 30 degrees above the northern horizon at local midnight, but the full moon will make it difficult to spot many meteors. (The radiant happens to be in a spot that for mid-northern latitudes never drops below the horizon).
The December full moon is often called the Full Cold Moon, according to the Old Farmer's Almanac, and it's not hard to see why if you live in North America or Europe. While the names for the full moon in the United States and Canada are adapted from Native American terms, nations in the Americas had diverse names and traditions.
According to the Ontario Native Literacy Project, the Ojibwe (or Anishnabeg) peoples called it Mnidoons Giizis, the Big Spirit Moon or Blue Moon. (This is not the same as the “blue moon” that is a second full moon in a single calendar month). The Cree called it the Thithikopiwipisim, or Hoar Frost Moon.
The Haida of the Pacific Northwest called December's full moon the Snow Moon, or Ta'aaw Kungaay, while the Tlingit called it Shanáx Dís, meaning "unborn seals are getting hair," according to the Tlingit Moon and Tide Teaching Resource published by the University of Alaska at Fairbanks. (The name Snow Moon is also often associated with February's full moon.)
Among the Hopi, whose ceremonial life revolved around the lunar and solar cycles, the lunation just before the winter solstice was the Sparrow-Hawk moon, as noted by Janet Sharp of Washburn University in her study of Hopi mathematical concepts and teaching that appeared in the February 2015 edition of the Journal of Mathematical Culture.
In the Southern Hemisphere, December is during the summer, and the Māori of New Zealand described the lunar months in November to December as Hakihea, meaning "birds are now sitting in their nests," according to the Encyclopedia of New Zealand.
In China’s traditional lunar calendar, the December lunation is the 11th month of the year. Called Dōngyuè, meaning "winter month," it marks the winter solstice. (The month actually begins on Dec. 6, because the lunar and solar calendars fall out of sync).