When the “Full Cold Moon” rises on Sunday night (Dec. 3). It will also be the first (and last) “supermoon” of 2017.
Supermoons happen when a full moon approximately coincides with the moon’s perigee, or a point in its orbit at which it is closest to Earth. This makes the moon appear up to 14 percent larger and 30 percent brighter than usual.
The moon becomes totally full at 10:47 a.m. EST (1547 GMT) on Sunday (Dec. 3). It will officially reach perigee the next day (Dec. 4) at 3:45 a.m. EST (0845 GMT), when it is 222,135 miles (357,492 kilometers) away from Earth. [Supermoon Secrets: 7 Surprising Big Moon Facts]
What is a Super Moon?
While the moon’s average distance is 238,000 miles (382,900 km) from Earth, its orbit isn’t perfectly circular, so that distance varies a small amount. When it reaches apogee, or its farthest distance from Earth, on Dec. 19, it will be 252,651 miles (406,603 km) away. That’s a difference of 30,516 miles (48,110 km) — but the moon’s distance from Earth can vary more than that.
The perigee for December’s supermoon won’t even be the closest this year; that happened May 25, when the not-so-super new moon was 221,958 miles (357,208 km) away from Earth. That date didn’t coincide with a full moon, though, so it didn’t qualify as a supermoon.