How big is that supermoon anyway?

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Robert Michael / AFP - Getty Images

A nearly full moon rises behind the cross of the Frauenkirche in the German city of Dresden in May 4.

By Alan Boyle

Tonight's "supermoon" is the biggest and brightest full moon of the year, due to the fact that the moon is near the closest point in its orbital path around Earth. But just how much bigger and brighter does it look? That's a tricky question.

Most reports say the moon looks 14 percent bigger than usual, which is close to the truth but isn't quite right. They also say it's 30 percent brighter than usual, which isn't right, either. James Garvin, chief scientist at NASA's Goddard Space Flight Center, ran the numbers to come up with an explanation that seems to make the most sense.

First of all, it's important to note that the moon itself is not getting significantly bigger or smaller. There's a scientific debate over whether the moon is slowly shrinking or spreading out. But in either case, the change isn't noticeable on human time scales.

The difference in the moon's apparent size is basically a function of how close it is to Earth in its elliptical orbit. That orbit isn't changing on human time scales, either. It just so happens that tonight, the moon is coming closest to Earth at the same time that it's going full. Because the moon and the sun are precisely opposite each other, relative to Earth, tonight's ocean tides may be a bit higher than typical — but again, the effect is nowhere near big enough to worry about.

So how noticeable is the visual effect? Here what Garvin told me in an email today:

• "The biggest predictable effect on the brightness of the full moon is how close the moon is to Earth.  With everything else the same, a full moon is about 30 percent brighter when the moon is closest to Earth in its orbit (called perigee) compared to a full moon when the moon is farthest from Earth in its orbit (called apogee).  Today’s full moon is at perigee."
• "Also, when the moon is high in the sky (as it is now), we are closer to the moon by approximately the radius of Earth compared to when the moon is on the horizon. (Note: Earth’s radius is about 6,371 kilometers)."
• "Since the distance from the center of Earth to the center of the moon is on average about 384,403 kilometers, the radius of the earth is about 6,371 kilometers, and brightness changes as the square of the distance, being closer to the moon by about the radius of the earth increases the brightness of the full moon by about 3 percent."  
• "Thus the present supermoon is, at maximum, only about 9 to 10 percent larger in an angular (appearance) sense than a typical full moon and is also brighter (by a few percent), making it appear 'super.'"

"Meanwhile, our intrepid Lunar Reconnaissance Orbiter continues its remarkable mapping of our nearest celestial neighbor, coming up (in June) on its three-year anniversary of being in lunar orbit with its amazing array of 7 instruments," Garvin added.  "As of now, the data returned from LRO (over 300 trillion bytes) is larger than all of the rest of the data acquired for planets in the solar system combined (except for Earth, of course)."

Which just goes to show that every day is a "super moon" day for the Lunar Reconnaissance Orbiter and its science team. Check out NASA's Web site for more wisdom from James Garvin.

A NASA video explains the science behind the "supermoon."

Geoff Chester, an astronomer at the U.S. Naval Observatory, says the moon appears 14 percent larger in angular size when it's at the closest point in its orbit, compared with its appearance when it's farthest away from Earth. That's not 14 percent larger than average. That's 14 percent larger than the minimum apparent size.

"You'd be very hard-pressed to detect that with the unaided eye," Chester told The Associated Press. Seasoned skywatchers, however, say they can definitely tell the difference. Can you? Take a look at the moon tonight — before, during or after the moment of maximum fullness at 11:35 p.m. ET — and tell us what you see.

Update for 6:45 p.m. ET: Bad Astronomy's Phil Plait observes that the moon's angular size is roughly equivalent to that of a dime as seen from 6 feet away. You can bet I'll have a dime taped onto a south-facing window tonight to make the observations. Also, tonight's supermoon will be a little less super than last year's supermoon, because the moon is about 240 miles farther away at peak fullness than it was in March 2011. For what it's worth, next year's supermoon will be imperceptibly smaller than this year's. I wonder if there'll be perceptibly less hype.
More about the supermoon:

• How to see the supermoon — and meteors, too
• How to plan your supermoon snapshot
• Supermoon rises over Greek temple
• Five moon mysteries
• Five moon myths