Mark your calendars: 2017's first and only visible supermoon is taking place on Sunday, December 3.A supermoon is when a full moon appears bigger than it typically does because it's closer to Earth. In fact, the moon must be at least 222,443 miles away from Earth to be considered a supermoon.Technically, there were three other supermoons this year, per National Geographic, but because each occurred during the "new moon" phase when the moon is covered by shadows, this will be the only one visible in 2017.University of Arizona professor Gurtina Besla told NPR that the best time to see this month's supermoon is at 3:45 a.m. ET on Monday, December 4, when it'll appear to be seven percent larger and 16 percent brighter than usual.Don't feel like setting an alarm for the middle of the night? The moon will actually appear to be bigger just after sunset on December 3 than when it's higher in the sky, thanks to an optical effect called "moon illusion."But if you miss this one entirely, you won't have to wait long until the next supermoon. Both full moons next month—one on January 2 and another on January 31—will also be close enough to enjoy supermoon status, as well.