Supermoon: the next supermoons and how much bigger they look

What a supermoon is

The Moon’s orbit around Earth is an ellipse, not a circle. NASA gives the closest point, perigee, as about 226,000 miles (363,300 km) away and the farthest point, apogee, as about 251,000 miles (405,500 km). The exact distances change a little from orbit to orbit. A full moon can happen anywhere along that orbit. When it falls near perigee, it is called a supermoon.

“Supermoon” is not an official astronomical term, as NASA points out, and EarthSky notes that the International Astronomical Union has never defined it. Where the line falls depends on whom you ask.

Who defines it, and how

Richard Nolle (1979). Smithsonian Magazine credits Richard Nolle, an American astrologer, with coining the word in 1979. On his own website, Nolle defines a supermoon as a new or full moon that happens when the Moon is at 90% or more of its mean closest approach to Earth (perigee). EarthSky explains that he measures that 90% against the closest and farthest distances of the whole year. That gives a fixed threshold for the year. Nolle’s definition includes new moons, but a new moon can’t be seen. Today the word is used almost only for full moons.

Fred Espenak. According to EarthSky, the eclipse expert and 30-year NASA veteran applies the same 90% idea to each individual orbit. That counts more full moons as supermoons. In EarthSky’s 2018 example, Espenak’s method gave three supermoons and Nolle’s gave two.

NASA. NASA’s supermoon page says the term usually means a full moon within about 90% of perigee. It notes that supermoons come three or four times a year, always in a row.

Because of these differences, one publisher may call a full moon a supermoon while another does not. This site uses a single definition, stated with the table above, and computes every full moon against it. A supermoon on our pages is therefore a supermoon by that rule, which may differ from the label in a news story.

How much bigger and brighter does it look?

NASA’s figures are the ones most often quoted. A full moon at its closest can appear up to 14% bigger and 30% brighter than the faintest full moon of the year, the one near apogee, sometimes called a micromoon. Those are the extremes. A typical supermoon compared with an average full moon shows a smaller difference.

In practice, NASA says, a supermoon can be hard to tell apart by eye. There is no ordinary full moon beside it to compare with. The extra brightness is easier to notice than the extra size, as a slightly brighter moonlit landscape. Side-by-side photos taken with the same lens at perigee and apogee show the difference clearly.

Supermoons also come with slightly higher tides. NOAA explains that when a new or full moon coincides with perigee, which it says happens about six to eight times a year, the result is a “perigean spring tide.” How much higher depends on the location. NOAA says it is not uncommon for high tide during a perigean spring tide to run more than a foot above the high tides of spring tides near apogee. With storms or seasonal high water, that can contribute to minor flooding in low-lying areas.

How to watch a supermoon

Watch at moonrise. The table above gives the time for major cities on the best evening. Rising near sunset and seen against the horizon, any full moon looks huge. That is the Moon illusion, which NASA explains as a perception effect: a photo shows the Moon the same width low and high. It is also the main reason supermoon photos look so dramatic. Photographers shoot the rising Moon with a long telephoto lens from far away, so that a distant building or hill looks small beside it.

For your own comparison, photograph a supermoon and an ordinary full moon a few months later. Use the same camera, zoom setting and height above the horizon, and the size difference will show up. Our moon calendar shows where each full moon falls through the year.