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What is a supermoon?

You may have seen headlines about an upcoming supermoon, but what exactly does that mean? 

A supermoon happens when the full Moon appears larger than usual. This takes place roughly once a year when the full Moon falls near the point where the Moon is closest in its orbit to Earth. 

The Moon’s 27-day orbit around the Earth is not perfectly circular. Its distance to Earth varies between 363,300 and 405,500 kilometers (226,000 and 253,000 miles). When the Moon is at its closest point to Earth, it appears up to 14% bigger and 30% brighter to observers on Earth.

The Moon's Changing Size
The Moon's Changing Size The Moon's 27-day orbit around the Earth is not perfectly circular, so its apparent size in the night sky varies. NASA's LandSat 8 spacecraft captured the left image in December 2013 when the Moon was near its closest point to Earth. LandSat 8 captured the right image in July 2014 when the Moon was near its farthest point from Earth.Image: NASA / USGS

What is a sturgeon supermoon?

The full Moon in August is known as the sturgeon Moon because it happens in the northern hemisphere when sturgeon fish were supposedly abundant in the United States' Great Lakes and Lake Champlain. Different cultures have different traditional names for full Moons throughout the year. When a sturgeon Moon coincides with the Moon's closest point to the Earth, it's called a sturgeon supermoon.

The Moon at Arches National Park
The Moon at Arches National Park Image: Navid Baraty

Does the Moon sometimes look bigger for other reasons?

A Moon that looks especially big isn't always a supermoon. Whether or not the Moon is near its closest point to Earth, a full or nearly full Moon looks extra large when you see it near the horizon. 

This is a trick of the brain called the Moon illusion, and scientists aren’t sure exactly what causes it. One common explanation is that because you’re used to seeing the Moon high in the sky all by itself, when you see it low on the horizon next to familiar foreground shapes like trees and buildings, your brain perceives it as being larger than normal.