What Is A Supermoon?

What makes a moon super?

Timing with the Moon's orbit. According to NASA, the Moon's orbit isn't a perfect circle but an ellipse. During every 27-day orbit, the Moon reaches both its perigee (closest point, about 226,000 miles from Earth) and apogee (farthest point, about 251,000 miles from Earth). A supermoon occurs when a full moon coincides with perigee. Although supermoon isn't an official astronomical term, it's typically used for any full moon within 90% of perigee. Throughout most of Earth's year, perigee and full moon don't overlap.

How much bigger does it look?

Up to 14% larger and 30% brighter. At perigee, the full moon can appear up to 14 percent larger in diameter and 30 percent brighter than the faintest full moon of the year (a micromoon, occurring at apogee). However, a 14% size increase is difficult to notice without comparing two moons side by side, which is impossible in practice since 30+ days pass between full moons. The brightness difference is more noticeable. Many supermoon photographs that show dramatically large moons use telephoto lenses with distant foreground objects, not the supermoon effect itself.

How often do they happen?

Three to four times per year, always consecutively. The Moon's orbit wobbles slightly because of gravitational influences from the Sun and Earth, so supermoons tend to come in groups of three or four consecutive full moons each year. NASA notes that supermoons only happen three to four times a year, and always appear consecutively. Months with supermoons vary year to year, with seasonal preferences shifting over time. The exact distances of each event differ slightly because perigee distance varies.

What about the moon illusion?

It's a separate phenomenon. The famous effect where the moon looks much bigger near the horizon than overhead is the moon illusion, an optical and perceptual effect happening in our brains. It has nothing to do with whether a moon is a supermoon. Any full moon can trigger the moon illusion when low on the horizon. Photographs of full moons rising over landscapes often combine both effects, using telephoto lenses to create dramatic compositions. Scientists still don't fully understand why the moon illusion happens.