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The only supermoon of 2017 is rising on December 3. But what is a supermoon? And why do we only see one or two a year?

Supermoons are a rare type of full moon. They appear up to 14% bigger and 30% brighter than normal. While the full moon rises about once per month, supermoons can only occur a few times per year — max.

That's because the timing has to be just right. The Moon takes about 27.32 days to orbit Earth. During its orbit, the moon passes through two points: Perigee and apogee. Perigee is about 30,000 miles closer to Earth than apogee.

A supermoon can only occur at perigee-syzygy, when the moon is full and at perigee simultaneously. This is rare because as the Earth revolves around the sun. the moon's orientation to Earth stays mostly the same. This changes where the Moon is in orbit during each full moon. But supermoons would even more rare if the moon didn't precess. Over many years, the moon's precession changes its orientation slightly.

It's often hard to differentiate a supermoon and a regular full moon. The best time to watch a supermoon is when it's low, near the horizon. So you can fully appreciate this rare moment in space and time.