Full moon may look bigger over weekend

Moon at 'perigee'

March 18, 2011|By Frank D. Roylance, The Baltimore Sun

The buzz on the Internet has been all about a scary "super moon" this weekend.

Seer Kit Karson, at PsychicCosmos.com writes, "Get ready for what could be moderate to severe weather patterns, increased seismic activity, tsunamis and more volcanic eruptions than normal."

But cooler scientific types say there's no evidence the unusually close full moon that rises Saturday evening will do anything of the sort. It may appear a bit bigger and brighter, they say, but not by much.

"There's no reason to expect any calamities or quakes or anything really bad," said Jim O'Leary, director of the Maryland Science Center's Davis Planetarium. "It's happened many times before … and hasn't caused anything bad in the past."

The event is what astronomers call a "perigean full moon." The moon will be precisely full (but out of view from Baltimore) at 2:10 p.m. By coincidence, that's only 50 minutes before it reaches "perigee" — its closest approach to the Earth for the month, and the year, at 221,566 miles.

It's the shortest time between the two events since March 1993. The moon will finally be visible from Baltimore at moonrise, at 7:37 p.m. Saturday.

The combination of a full moon, perigee and an optical illusion that makes all full moons look large on the horizon may make this moonrise all the more startling.

"I think the moon probably will look bigger to folks that haven't looked in a long time," O'Leary said.

Astronomers say the difference between the moon's distance at perigee, and at "apogee" — the farthest point in its monthly orbit around the Earth — is about 31,000 miles. That means the moon's diameter will appear about 14 percent wider than at apogee. And if skies are clear, it may appear up to 30 percent brighter than the usual full moon.

The differences are caused by the lopsided elliptical shape of the moon's orbit, passing a little nearer to Earth on one side and farther on the other.

And because it's the moon's gravity that causes the ocean tides, this closer approach adds a few inches to the water level at high tide. That comes on top of a high tide that is normally higher anyway when the moon is full, and the sun, Earth and moon are lined up in a row.

But "unless you have a storm, and strong onshore winds, you may not even notice it," O'Leary said. The forecast calls for clear skies and calm winds.

frank.roylance@baltsun.com

Maryland weather blog: Frank Roylance on meteorology

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