A supermoon occurs when the full moon is at its perigee, the point in its orbit where it is closest to Earth. This makes the moon appear up to 14% larger and 30% brighter than when it is at its farthest point, or apogee. While visually impressive, it is a normal astronomical event.
How often do supermoons occur?
The Moon's orbit is elliptical, so its distance from Earth varies between about 356,500 km (perigee) and 406,700 km (apogee) — a difference of about 50,200 km. A supermoon occurs when a full moon coincides with perigee (or very close to it). This happens roughly 3-4 times per year, though the exact definition varies. Some astronomers require the moon to be within 90% of its closest approach to count as a supermoon.
Supermoon vs. regular full moon
The size difference between a supermoon and a regular full moon is about 14% in diameter and 30% in brightness. This is noticeable to careful observers — the moon does look larger and brighter near the horizon during a supermoon. However, the difference is subtle enough that many people can't tell without a side-by-side comparison. The "supermoon" term was coined by astrologer Richard Nolle in 1979, not by astronomers.
Check our Moon Phase Calendar to see when the next supermoon occurs and what phase the moon is in tonight.