"Supermoon": Biggest Full Moon in 18 Years Saturday

It may not be faster than a speeding bullet, but tomorrow the moon will make its closest approach to Earth in 18 years—making the so-called supermoon the biggest full moon in years.

And despite Internet rumors, the impending phenomenon had no influence on the March 11 Japan earthquake and tsunami (see pictures).

The monthly full moon always looks like a big disk, but because its orbit is egg-shaped, there are times when the moon is at perigee—its shortest distance from Earth in the roughly monthlong lunar cycle—or at apogee, its farthest distance from Earth.

Likewise, because the size of the moon's orbit varies slightly, each perigee is not always the same distance away from Earth. Friday's supermoon will be just 221,566 miles (356,577 kilometers) away from Earth. The last time the full moon approached so close to Earth was in 1993, according to NASA.

The March 19 supermoon, as it's called, will be visible "pretty much any time during the night," said Geza Gyuk, astronomer at the Adler Planetarium in Chicago.

"Look for the full moon as it rises above the eastern horizon as the sun sets below the western horizon—it will be a beautiful and inspiring sight," he said via email.

(See "Year's Biggest Full Moon, Mars Create Sky Show [2010].")

Though the supermoon will be about 20 percent brighter and 15 percent bigger than a regular full moon, the visual effect may be subtle, added Anthony Cook, astronomical observer for the Griffith Observatory in Los Angeles.

"I doubt that most people will notice anything unusual about this full moon," Cook said.

"Because the total amount of light is a little greater, the biggest effect will be on the illumination of the ground—but not enough to be very noticeable to the casual observer."

Japan Earthquake Not Linked to Supermoon

Such a lunar close encounter can cause slightly higher than normal ocean tides and localized flooding—especially if there is already a storm surge, astronomers say.

A supermoon may even have some impact on seismic activity because of the stronger gravitational interaction between the moon, the sun, and Earth.

Even so, there is no clear evidence that any of these phenomena influenced the Japan earthquake and tsunami.

(Read more: "Can the Moon Cause Earthquakes?")

"The earthquake in Japan happened when the moon was close to its average distance to Earth—there was nothing extreme about its position or phase," Cook said.

"While some earthquakes seem to have tidal connections, this isn't one of them."

(Take a moon myths and mysteries quiz.)

There's no need to get worked up over a supermoon, Adler Planetarium's Gyuk added.

"We survived 2008 [an almost supermoon year] and 1993 just fine," he said by email.

"Just keep in mind even this 'extreme' supermoon is not really that extreme!"

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