Tonight, if the full moon, rising in the East, strikes you as unusually large, you'll be right. It will loom larger than usual. Though it's hardly a scientific term, it will be what's known as a "supermoon."
If the weather is clear where you are, it should be a sight to see. It happens because -- despite what our senses tell us -- the moon does not orbit us in a perfect circle. It follows a slightly elliptical path every month. At 11:35 p.m. EDT, say astronomers, it will come within 221,802 miles of us -- coincidentally about one minute before it's at its fullest.
The "supermoon" of March 19, 2011, seen from England. Ben Birchall/PA/AP Photo.
The result: When the moon is closest to Earth, it appears 14 percent larger and 30 percent brighter than when it's farthest from us. Two weeks from now, on the opposite side of its orbit, it will be about 252,000 miles away.
Remember the numbers. They'll come in handy at the end of this story.
"The full Moon has a reputation for trouble," wrote Tony Phillips, an astronomer who maintains NASA's Science News site. "It raises high tides, it makes dogs howl, it wakes you up in the middle of the night with beams of moonlight stealing through drapes.
"If a moonbeam wakes you up on the night of May 5th, you might want to get out of bed and take a look."
But is there really anything "super" about it, beyond the spectacle? Astronomers say emphatically not. It may increase high tides in places by a few inches, but that's about it.
For all the folklore about full moons and madness ("lunacy"), social scientists have searched in vain for correlations between the moon and crime rates or admissions to psychiatric institutions.
So if the weather cooperates (check our weather page for a forecast), look outside -- fearlessly -- and drink in the moonlight. Many people say the best time to look is early evening, just as the moon is coming up over the horizon. You'll see the moon, pumpkin-colored, slowly rising in the East -- and, boy, will it look large.
It is, in fact, no larger in the sky than when it's overhead, but our minds fool us, perhaps because we have a reference point -- something on the horizon -- that we lack when it is high among the stars.
"For instance," said Phillips in an email, "when you see the moon in close proximity to a tree, your brain will miscalculate the distance to the moon, mentally bringing it closer (like the tree) and thus making it bigger. It seems so real, but this beautiful illusion is all in our minds."
We said it would be useful to remember the moon's changing distance from Earth, and here's why:
Two weeks from now, on the afternoon of May 20, the moon will be in its new phase, passing between Earth and the sun.
The result will be a solar eclipse -- one that will be visible in a strip of the Western United States, stretching southeastward from the Oregon-California border to Lubbock, Texas.
Annular eclipse of Jan. 15, 2010.
It should be a spectacular sight, visible from Reno, Albuquerque, Zion National Park and parts of the Grand Canyon -- but it won't be the kind of eclipse you most often see pictured.
Recall that on Saturday the moon, at perigee, will appear 14 percent larger? On May 20, it will seem 14 percent smaller -- in other words, not quite enough to block the sun.
The result will be what's known as an annular eclipse. A blindingly-bright ring of sun ("ring" in Latin is "annulus") will surround the black disc of the moon.
Still worth a trip if you're inclined, but a bit less "super" than other such sights. It's the price we'll pay for a big, bright "supermoon" this weekend.