Due to the moon's egg-shaped orbit, our natural satellite is at perigee—its closest approach to Earth—about once a month. The term "supermoon" was coined in 1979 to describe a full moon that coincides with perigee. (Apogee, when the moon is farthest from us, also occurs on a roughly monthly schedule.)
At its closest on Saturday, the moon was 221,801 miles (356,955 kilometers) from our planet—and just minutes away from of the official full moon phase, at 11:35 p.m. ET.
"As a consequence, this translates into it appearing as much as 16 percent bigger and 30 percent brighter than other full moons of 2012—not a huge amount, but definitely noticeable," said Geza Gyuk, an astronomer at the Adler Planetarium in Chicago.
—With reporting by Andrew Fazekas