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Supermoon 2017: how to watch (and why)

A slightly larger, brighter full moon will appear in the sky Sunday night.

The Cold Moon Rises Over Cornwall Matt Cardy/Getty Images
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

On Sunday, December 3, the full moon will appear slightly larger and brighter than normal in the sky when it rises in the early evening. It will be the last (and only) “supermoon” of the year. And while it’s not as spectacular of an event as a solar or lunar eclipse, it’s as good of a reason as any to step outside Sunday night and get a close look at Earth’s natural satellite.

What is a supermoon? Why does it happen? And is it all that special?

NASA/JPL-Caltech

The moon’s orbit around Earth is not a perfect circle. It’s an ellipse, a saucer shape that’s longer than it is wide. That means as the moon follows this orbit, it’s sometimes closer to the Earth and sometimes farther away. At perigee, the closest spot in its orbit to the Earth, it’s around 31,068 miles closer to Earth than at apogee, when it’s farthest away.

NASA/JPL-Caltech

Meanwhile, we see different phases of the moon — full, crescent, waxing, and waning gibbous — depending on if the sun-facing side of the moon is facing the Earth.

Ernie Wright/NASA

A supermoon is when these two cycles match up, and we have a full moon that’s near its perigee. The result is that the full “super” moon appears slightly larger and slightly brighter to us in the sky. This occurs about one in every 14 full moons, Jim Lattis, an astronomer at the University of Wisconsin Madison, notes.

The supermoon doesn’t have any astronomical significance other than it makes for a slightly larger target for backyard astronomers to look at. In fact, the term originates from astrology, a pseudo-science that tends to make big deals out of insignificant coincidences.

The difference between a normal full moon and a supermoon isn’t all that much. Neil deGrasse Tyson has called the frenzy around supermoons overblown. “If you have a 16-inch pizza, would you call that a super pizza compared with a 15-inch pizza?" he said on the StarTalk radio show. For the most part, NASA explains, the differences between a normal full moon and a super moon “are indistinguishable” to the human eye.

You may want to check out the supermoon around the time it rises. (It’s at 5:15 pm on the East Coast of the United States. Check out your moonrise time here.) When the moon is near the horizon, an optical illusion makes it appear absolutely huge. Like so:

A beautiful, golden harvest moon rises over Falmouth Harbor, Maine Moment RF

And if you miss this weekend, know there will be supermoons in 2018. The first two full moons of 2018 will occur near the perigee, making them super as well.

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