By Ed Payne, CNN
March 18, 2011 2:24 p.m. EDT
(CNN) — If the moon looks a little bit bigger and brighter this weekend, there’s a reason for that. It is. Saturday’s full moon will be a super “perigee moon” — the biggest in almost 20 years. This celestial event is far rarer than the famed blue moon, which happens once about every two-and-a-half years.
“The last full moon so big and close to Earth occurred in March of 1993,” said Geoff Chester with the U.S. Naval Observatory in Washington. “I’d say it’s worth a look.”
Full moons look different because of the elliptical shape of the moon’s orbit. When it’s at perigee, the moon is about 31,000 miles (50,000 km) closer to Earth than when it is at the farthest point of its orbit, also known as apogee.
“Nearby perigee moons are about 14% bigger and 30% brighter than lesser moons that occur on the apogee side of the moon’s orbit,” the NASA website says. This full moon will rise in the east at sunset and should look especially big at that time because of what’s known as the “moon illusion.”
“For reasons not fully understood by astronomers or psychologists, low-hanging moons look unnaturally large when they beam through trees, buildings and other foreground objects,” according to NASA.