AMoon Cross

AMoon Cross

Moon Cross

Coming up this weekend, it’s the December full moon! In North America, we call this full moon by one of several nicknames: the Full Cold Moon, Moon Before Yule, or the Long Night Moon. Whichever name you use, the moon will cross high overhead in the Northern Hemisphere’s sky (while making a low arc from the Southern Hemisphere). For all of us, it’ll light up these chilly nights. Our nearest neighbor will appear to be full throughout this weekend. But the exact moment it’s full is 4:35 UTC on December 19 (10:35 p.m. CST on December 18). That’s the moment when the moon is directly opposite the sun as seen from Earth. It’s when we see the entire, fully illuminated day side of the moon.


This full moon happens one day after it reaches the farthest point in its orbit around Earth, called apogee. At apogee in December, the moon is 252,000 miles (400,000 km) away. That’s about 12,000 miles (19,000 km) more than the average moon-distance from Earth. As a result, on the night of full moon – December 18-19, 2021 – the moon is measurably smaller in our sky than usual. Your eye probably won’t notice any difference, although the moon may appear less bright than you remember it at other months this past year. Many skywatchers are familiar with the term supermoon. That’s a particularly large and bright full moon, near perigee, or its closest point to Earth. December’s full moon is the opposite of a supermoon. We might call it a mini-moon or micromoon.Every full moon stays more or less opposite the sun. And the full moon’s path through the night is also opposite the sun’s path through the day. In fact, the moon’s path roughly follows the sun’s daytime path from six months ago, or six months from now. You can see this happening as you watch the December full moon rise to nearly the top of the sky, just as the sun does near the June solstice.

Here’s another way to look at it. In the Northern Hemisphere, the December solstice has the least amount of daylight of the year. Since there’s still about 24 hours in a day no matter how much daylight there is, the shortest day means it must also be the longest night. In order for the moon to stay up all night and remain roughly opposite the sun, it needs to take a longer path across the sky. The higher an object crosses the sky, the longer its path and the longer it stays above the horizon. Since it’s opposite the sun, every full moon rises and sets opposite the sun. And so every full moon rises in the east around sunset. From there, it climbs to its highest and southernmost point for the night around local midnight. This is halfway between your local sunset and sunrise. Then, it continues downward toward the horizon until it sets in the west around sunrise. So a moon that’s in the sky all night long is a full moon, or close to it. (Source: earthsky.org)


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