Supermoon This Sunday: Look Out the Window to See Brightened Sky

June 21, 2013 — -- Full moons come and go each month, but supermoons come only once a year. And this Sunday is the supermoon's time to shine.

At 7 a.m. ET, the moon will not only be bright and full, but it will also be at its closest distance to Earth. If it's a clear night (or day), take a step outside for the largest full moon that you'll see until August 2014.

But wait, why isn't there a supermoon more often? After all, the moon orbits Earth every month. If the moon is at its closest point now, it should be at the closest point next month too. Bruce McClure and Deborah Byrd at earthsky.org write that the supermoon's coming only once a year depends on how "month" is defined.

If you're watching the moon change phases from full to new and back to full again, you're dealing with the synodic month. The phase of the moon is determined by its position relative to Earth and the sun. If Earth is sandwiched directly between the moon and the sun, you get a full moon. The synodic month is a little more than 29 days long.

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If you're on the moon and are recording how long it takes to make one revolution around Earth, you're dealing with the anomalistic month. Since Earth isn't in the exact center of the moon's elliptical orbit, there is a point at which the moon is at its farthest point from Earth (the apogee) and a point at which it's at its closest point (the perigee). The anomalistic month is a little more than 27 days along.

That two-day difference causes the two different types of months to be out of sync. For this weekend's moon, the full moon happens to coincide with the moon's perigee, resulting in the supermoon. Similarly, there will be times when the full moon occurs at the moon's apogee, where it will look smaller than usual.

"Smaller than usual" isn't actually that different, though. According to space.com, there's only a 12 percent difference in distance between the moon's perigee and apogee.

But If you want to snap your own photo of a gigantic moon, try doing so close to sunrise. The moon hasn't magically grown in size, but it will be near the horizon. You'll see it looming over trees and buildings, which will make it appear bigger to our own eyes.

Editor's Note: A previous version of his story misstated the positioning of the moon in a full moon. The story has been corrected to read if the Earth is sandwiched directly between the moon and the sun, you get a full moon.