More a popular term than a scientific one, a supermoon occurs when a full lunar phase syncs up with an especially close swing around Earth. This usually happens only three or four times a year and consecutively, given the moon’s constantly shifting, oval-shaped orbit.

The lunar phase calendar year is wrapping up with November’s sky closing out the year’s last supermoon, part of a series that began in August. Supermoons in the lunar calendar are slightly closer to Earth than the average full moon, which makes them ...