This week's full Moon is the biggest full Moon of 2007. It's no illusion. Some full Moons are genuinely larger than others and Thursday night's will be a whopper. Why? Read the answer below.

©Unknown
Left: A big, bright perigee Moon. RIght: A lesser apogee Moon.

The Moon's orbit is an ellipse with one side 30,000 miles closer to Earth than the other. The full Moon of Oct. 25-26 is located on the near side, making it appear as much as 14% bigger and 30% brighter than lesser full Moons we've seen earlier in 2007.

In the language of astronomy, the two ends of the Moon's orbit are called "apogee" and "perigee." Apogee is the farthest point, perigee the nearest: diagram. This week's full Moon is a "perigee Moon" with extra-high "perigean tides."

©SpaceWeather.com

The Moon is 14% bigger, but can you actually tell the difference? It's not so easy. There are no rulers floating in the sky to measure lunar diameters. A fun experiment: Take a friend outside Thursday night and ask if they notice anything unusual about the Moon. Explain perigee after they answer.