A "Super Moon" will rise in the east at sunset tonight. This unusually large full moon -- known as a super "perigee moon" -- will be the biggest in almost 20 years, according to NASA.
"The last full moon so big and close to Earth occurred in March of 1993," says Geoff Chester of the U.S. Naval Observatory in Washington D.C. "I'd say it's worth a look."
Why the "Super Moon" label? "A 'Super Moon' is a situation when the moon is slightly closer to Earth in its orbit than on average, and this effect is most noticeable when it occurs at the same time as a full moon," says James Garvin, chief scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
It's so close because the moon's orbit around the Earth is not a perfect circle, but an ellipse, according to Weather Channel meteorologist Tim Ballisty.
When the Moon's orbit takes it the farthest away from Earth, the moon is at its "apogee," Ballisty says. When it makes its closest approach, the moon is at its "perigee."
So not only will it be a full moon tonight, but it will also be at its perigee. This month's perigee will put the moon about 8% closer to Earth than usual, and about 2% closer to Earth than the average lunar perigee.
The best time to look at the full moon is when it's near the horizon. That is when illusion mixes with reality to produce a truly stunning view, NASA reports. Low-hanging moons look unnaturally large when they beam through trees, buildings and other foreground objects.
According to NASA and NOAA, although a perigee moon brings with it extra-high "perigean tides," it's nothing to worry about. In most places, lunar gravity at perigee pulls tide waters only about an inch or so higher than usual. Local geography can amplify the effect to about six inches.
Don't be concerned about other natural disasters, either: The Super Moon of March 1993, for instance, passed without incident. And an almost-Super Moon in December 2008 also proved harmless.
"The effects on Earth from a supermoon are minor," says NASA's Garvin, "and according to the most detailed studies by terrestrial seismologists and volcanologists, the combination of the moon being at its closest to Earth in its orbit, and being in its 'full moon' configuration (relative to the Earth and sun), should not affect the internal energy balance of the Earth, since there are lunar tides every day."
At its closest on tonight, the moon will pass by Earth at a distance of about 221,567 miles.