March 20, 2011

Saturday's full moon was 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. 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's 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. This full moon rose in the east at sunset and looked 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. Even so, Saturday's full moon was still 211,600 miles (356,577 km) away. The next one occurs in 2029.