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On March 19th, a full Moon of rare size and beauty will rise in the east at sunset. It's a super "perigee moon"--the biggest in almost 20 years.

"The last full Moon so big and close to Earth occurred in March of 1993," says Geoff Chester of the US Naval Observatory in Washington DC. "I'd say it's worth a look."

Full Moons vary in size because of the oval shape of the Moon's orbit. It is an ellipse with one side (perigee) about 50,000 km closer to Earth than the other (apogee): diagram. Nearby perigee moons are about 14% bigger and 30% brighter than lesser moons that occur on the apogee side of the Moon's orbit.

"The full Moon of March 19th occurs less than one hour away from perigee--a near-perfect coincidence1 that happens only 18 years or so," adds Chester.

A perigee full Moon brings with it extra-high "perigean tides," but this is nothing to worry about, according to NOAA. In most places, lunar gravity at perigee pulls tide waters only a few centimeters (an inch or so) higher than usual. Local geography can amplify the effect to about 15 centimeters (six inches)--not exactly a great flood.

Indeed, contrary to some reports circulating the Internet, perigee Moons do not trigger natural disasters. The "super moon" of March 1983, for instance, passed without incident. And an almost-super Moon in Dec. 2008 also proved harmless.

Okay, the Moon is 14% bigger than usual, but can you really tell the difference? It's tricky. There are no rulers floating in the sky to measure lunar diameters. Hanging high overhead with no reference points to provide a sense of scale, one full Moon can seem much like any other.

The best time to look is when the Moon is near the horizon. That is when illusion mixes with reality to produce a truly stunning view. For reasons not fully understood by astronomers or psychologists, low-hanging Moons look unnaturally large when they beam through trees, buildings and other foreground objects. On March 19th, why not let the "Moon illusion" amplify a full Moon that's extra-big to begin with? The swollen orb rising in the east at sunset may seem so nearby, you can almost reach out and touch it.

Don't bother. Even a super perigee Moon is still 356,577 km away. That is, it turns out, a distance of rare beauty.

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The shimmering muse of poets and artists will reach full brilliance at perigee -- or the point in the moon's orbit when it's nearest Earth -- for the first time in 18 years.

If it's not cloudy, Saturday skygazers will be able to glimpse the "supermoon." And while it might yank on some tides, the supermoon won't trigger natural disasters or werewolf uprisings.

As it cozies in to a mere 220,000 miles away, the moon will appear 14 percent larger and 30 percent brighter than a most distant full moon.

"It's not something you would notice unless you were really looking for it," said Ben

Burress, an astronomer at the Chabot Space & Science Center in Oakland. "Looking at the moon is a very subjective thing sometimes. It looks huge when it's near the horizon, but that's just a funny trick the brain plays on you."

Because the moon's orbit is elliptical, with one end of the ellipse closer to Earth than the other, it reaches perigee once each month. "It just doesn't happen to be full every time," Burress said.

And while the moon does affect the tides, differences are marginal along the California coast, said Steve Anderson, meteorologist with the National Weather Service. "There's no need to worry about the tide, but people need to be more worried about the high surf," Anderson said.

producing superswells? Nope. "They're from a storm in the gulf of Alaska," Anderson said. "It's been spinning around up there for the past week."

So stay dry and look east just after sunset on Saturday to see the supermoon rise -- if it's not cloudy.

Supermoon expected Saturday, but will we see it? - San Jose Mercury News

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