If a moonbeam shines in your eyes as you lie in bed on Saturday night, you might want to step outside and take a look. This will be the only "super moon" of 2012--as much as 30 percent brighter and 14 percent wider than other, ordinary full moons this year, according to NASA.
That's because Saturday will be the only night this year that the moon is both full and at perigee. Perigee is the point during its orbit when the moon is closest to earth.
At perigee, the moon is about 31,000 miles (50,000 kilometers) closer to earth than when it's at the most distant point in its orbit (apogee). So there's a big difference in brightness between the biggest and smallest moons. The average distance between the earth and moon is 238,000 miles (383,000 kilometers).
Of course, the best time to see this giant moon will be early evening, just after moonrise, not late at night. For some reason, which scientists studying perception do not fully understand, the moon looks biggest when it is low on the horizon.
This fact has been recognized since ancient times, but has not been satisfactorily explained, even today. By actual measurement, as opposed to the eye's perception, the moon is really 1.5% smaller when on the horizon than at its zenith, because it's one radius of the earth, about 4,000 miles, further away.
Last year's super moon, which occurred on March 19, 2011, was just a bit closer to earth (221,567 miles), than this year's (221,802 miles). But the small resulting difference in brightness would not be detectable with the naked eye.
Moonrise on Saturday evening here in Athens will be at 8:11 PM. It's the perfect night and time for a romantic hilltop picnic with the one you love. Start packing your basket now!