January 9, 2009

According to NASA, the biggest full moon of 2009 is coming this weekend, January 10 and 11. It's called a "Perigree Moon,' and it's as much as 30% brighter than lesser moons.

Johannes Kepler explained the phenomenon 400 years ago. The Moon's orbit around Earth is not a circle; it is an ellipse, with one side 50,000 km closer to Earth than the other. Astronomers call the point of closest approach "perigee," and that is where the Moon will be this weekend.

Perigee full Moons come along once or twice a year. 2008 ended with one and now 2009 is beginning with another. NASA reports that "It's the best kind of déjà vu for people who love the magic of a moonlit landscape."

January is a snowy month in the northern hemisphere, and the combination of snow + perigee moonlight leads to a brighter moonlight. NASA says you can read a newspaper, ride a bike, write a letter, and at the same time count the stars overhead.

For a report on the pedigree moon from NASA Science news click here.