Rare 'supermoon' shines brighter than usual tonight

Saturday night's moon will appear larger and brighter than any in 18 years. Here's why.

AP/Dmitry Lovetsky
The city landmark, a weathercock in the form of a ship fixed atop a spire of the Admiralty building, is silhouetted on the rising full moon in St.Petersburg, Russia, Friday, Feb. 18, 2011.

Wolves may have a bit more incentive to howl Saturday, when the moon will appear bigger and brighter than it has in 18 years. Despite one astrologer’s fears that the “supermoon” will cause a natural disaster, NASA insists there is nothing pernicious or supernatural about it.

It all comes down to the moon's oval orbit. At one end of the orbit – the perigee – the moon is about 31,000 miles closer to Earth than at its farthest point, the apogee.

It’s quite rare for a full moon to coincide with a perigee. The most recent example was the near supermoon in 2008, when the full moon occurred four hours before perigee. Saturday’s occurrence is even closer to the full moon and perigee lining up.

IN PICTURES: The full moon

"The full Moon of March 19th occurs less than one hour away from perigee – a near-perfect coincidence that happens only 18 years or so," Geoff Chester of the US Naval Observatory said in a NASA report.

POST YOUR MOON PHOTO: Take a picture of tonight's 'supermoon' and tag it to our fan page.

But that’s not all. From one orbit to the next, perigees and apogees vary. Saturday’s perigee will bring the moon 221,566 miles away, about as close as it ever gets.

According to NASA, perigee moons are about 14 percent larger, and 30 percent brighter than apogee moons, though it’s hard to tell the difference with the naked eye. For the best view of the supermoon, NASA has suggested viewing it when it’s lower in the sky.

IN PICTURES: The full moon

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