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Wednesday’s “supermoon,” explained in one chart

The full moon will be a bit bigger and brighter than usual tonight. Go outside!

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 Wednesday night, there will be a full moon in the sky. This isn’t that special: We see one full moon every month (and sometimes twice in one month!).

But tonight might be a better night than most to gaze upon our “natural satellite.” The moon will be near its closest approach to Earth, making it a “supermoon”; that means it will appear very slightly larger in the sky. And it will be the last supermoon we can see until 2020. (Coincidentally, Wednesday is also the vernal equinox, or the first day of spring, another astronomical milestone worth celebrating.)

How much more “super” will this moon be?

Well, not that much. The size difference between supermoons and “micromoons” (when a full moon is farther from the Earth) would probably be imperceptible to the human eye if they were side by side. The supermoon appears just 14 percent bigger. What you might notice tonight is that the moon will be brighter — about 30 percent brighter than a micromoon.

Here, we try to visualize the difference.

Javier Zarracina/Vox

Want to see it for yourself? Head over to TimeAndDate.com/Moon/ to find out when the moon rises and sets in your area.

Why does the moon’s apparent size change?

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. As the moon follows this orbit, it’s sometimes closer to Earth and sometimes farther away. At perigee, the closest spot in its orbit to 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 Earth.

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 in the sky. This occurs about one in every 14 full moons, Jim Lattis, an astronomer at the University of Wisconsin Madison, notes.

The difference between a normal full moon and a supermoon isn’t all that significant. 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 supermoon “are indistinguishable” to the human eye.

The supermoon doesn’t have any astronomical significance other than making for a slightly larger object for backyard astronomers to look at. That’s enough for us.

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