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Transit and Eclipse

June 1, 2012

If you’re the kind of person who likes to have your ducks in a row, your posture arrow straight, and your marching bands military precise, then you’ll love an astronomical doubleheader that’s coming up early next week — a partial lunar eclipse on Monday, and a transit of Venus across the face of the Sun on Tuesday. Both events are the result of precise astronomical alignments.

A partial lunar eclipse occurs when the full Moon dips into Earth’s dark inner shadow. The shadow darkens part of the lunar disk, so it looks like some cosmic force has taken a “bite” out of the Moon.

Lunar eclipses happen an average of two or three times a year. But there are 12 or 13 full Moons in a year, so most months there’s no eclipse at all. That’s because the Moon’s orbit is tilted with respect to Earth’s orbit around the Sun. Most months, the Moon passes a little above or below the shadow. Eclipses occur when the full Moon is crossing the plane of Earth’s orbit, producing a precise alignment.

Transits of Venus are even less common. The last one took place eight years ago, but the next one won’t happen until 2117.

Venus crosses between Earth and the Sun once every 19 months or so. But like the Moon, Venus’s orbit is tilted a bit compared to Earth’s orbit. So during most of these crossings, Venus misses the Sun. Only when the geometry is just right does the little planet pass across the face of the Sun — a rare alignment that’s coming up next week.


Script by Damond Benningfield, Copyright 2012


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