Super Full Moon - Presented by Science@NASA.

Mark your calendar.

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 1983," says Geoff Chester of the US Naval

Observatory in Washington DC.

"I'd say this is 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

kilometers closer to Earth than the other (apogee).

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 coincidence that happens only

once every 18 years or so," says Chester.

A perigee full Moon brings with it extra-high "perigean

tides," but this is nothing to worry about.

In most places, lunar gravity at perigee pulls tide waters

only a few centimeters (that is an inch or so) higher than usual.

Local geography can amplify the effect to about 15

centimeters (or 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 December 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 - even a super 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

super 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 Moon is still 356,000 kilometers away.

That is, it turns out, a distance of rare beauty.

For more information about moons - super and otherwise -

visit science.nasa.gov.