Super Full Moon to Grace the Skies Tonight

Full moon on March 19 will be biggest in nearly 20 years.

March 19, 2011— -- When you step outside tonight, don't forget to take a good look up at the sky.

Assuming clouds don't get in the way, you'll get to gaze at the biggest full moon in nearly two decades.

During what some skywatchers are calling the "supermoon," the moon won't just be at its closest approach to Earth in its elliptical orbit, it will be closer than it has been in 18 years.

"It's going to be big and really bright," said NASA astronomer Dave Williams. "It should be noticeably brighter than a normal full moon."

Full moons come in different sizes because of the elliptical shape of the moon's orbit -- one side of the ellipse is about 31,000 miles closer to Earth than the other. When the moon is closest to Earth (at its perigee), it is 14 percent larger and 30 percent brighter than when it's farthest from the planet (at its apogee).

For weeks, the rare full moon has sparked interest online, with astrologers and amateur astronomers speculating that the "supermoon" could lead to unusual weather. After Japan's earthquake, some even wondered if the supermoon contributed to the event.

'Supermoon' Not Connected to Earthquakes, Natural Disasters, Scientists Say

In a post earlier this month, Accuweather blogger MarkPaquette said the phrase "supermoon" originated on the website of astrologer Richard Nolle and spread to astronomers online.

According to Nolle's definition, a new or full moon at 90 percent or more of its perigee (or closest approach to Earth) qualifies as a "supermoon." Tonight's full moon won't just be a supermoon but an extreme supermoon, he said, because the moon will be almost precisely at its closest distance to Earth.

According to "new age" forecasts, he said, the supermoon brings strong earthquakes, storms or unusual climate patterns.

"There were supermoons in 1955, 1974, 1992 and 2005," Paquette wrote. "These years had their share of extreme weather and other natural events. Is the Super Moon and these natural occurrences a coincidence?

But scientists emphasize that there is no connection between the moon's position and extreme weather or natural disasters (like Japan's earthquake) here on Earth.

"There's nothing really special about this," NASA's Williams told

Scientist: 'Supermoon' Will Be Biggest Moon You'll Ever See

The moon orbits the Earth every 29-1/2 days, so it reaches perigee more than once a month. The orbit of the moon changes slightly over time, so the distance between Earth and the moon also changes -- but only slightly, Williams said.

Tonight, it will probably be only about half a percent closer than it ever is every 18 years, he said, which is a "very, very small amount."

Even this super-close full moon will still be 221,567 miles away, according to NASA.

And though the gravitational effect of the moon causes tides (when the moon is closer, the tides are slightly larger), he said there's "no scientific reason whatsoever" to expect that this supermoon will result in floods or other extreme conditions.

It's likely that the moon won't even look that different to the human eye (although scientists say the moon looks larger when it's closer to the horizon). Still, Williams recommended that you make the extra effort to take a look.

"This is the biggest full moon that you will ever see," he said. "You will see this moon again, but this is as big as it gets."