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. The Moon takes about 27.32 days to orbit Earth. 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.