Over the weekend, night sky gawkers were treated to a supermoon, in December known as the “Full Cold Moon.”

Over the weekend, night sky gawkers were treated to a supermoon, in December known as the “Full Cold Moon.”

While the moon's average distance is 238,000 miles from Earth, its orbit isn't perfectly circular, so that distance varies a small amount. When it reaches apogee, or its farthest distance from Earth, on Dec. 19, it will be 252,651 miles away. That's a difference of 30,516 miles — but the moon's distance from Earth can vary more than that, according to Space.com.

Supermoons don't happen every month because the moon's orbit changes orientation as the Earth goes around the sun. So, the long axis of the moon's elliptical path around the Earth points in different directions, meaning that a full (or new) moon won't always happen at apogee or perigee, according to Space.com.

The perigee for December's supermoon, for example, wasn't the closest this year; that happened May 25, when the new moon was 221,958 miles away from Earth. However, that was not a full moon so it did not qualify as a supermoon.

December's supermoon was the first of three in the next two months. On the first and last day of January the full moon will also occur near the moon's arrival at perigee, according to NASA, which billed the line up as a supermoon trilogy. The Jan. 31 supermoon is also the second full moon of January, making it a Blue Moon, and also occurs during a total lunar eclipse.

Supermoons don't happen every month because the moon's orbit changes orientation as the Earth goes around the sun. So, the long axis of the moon's elliptical path around the Earth points in different directions, meaning that a full (or new) moon won't always happen at apogee or perigee, according to Space.com.

Thanks to Ron Rudolph for the amazing photos.