Thursday, July 9, 2009

Lunar and Solar Eclipses

We had a lunar eclipse on July 7 and there's a solar eclipse on July 21. The lunar eclipse was only visible from the western U.S. and points further west into the Pacific, but the next one on August 5-6 will be visible on the east coast, as well (in the evening just after sunset). The solar eclipse will be visible in China but not in the western hemisphere.

A lunar eclipse occurs when the earth is directly between the sun and the moon, thus blocking any direct solar radiation. This leaves the moon illuminated only by "earthlight", which makes it glow an eery red. This is because the bouncing back and forth through atmosphere causes the shorter, bluer wavelengths to be absorbed, leaving the longer red wavelengths.

A solar eclipse occurs when the moon is directly between the sun and the earth. Since the moon's orbit is not right on the ecliptic plane (the plane on which the earth revolves around the sun), this doesn't happen every month. You can guess the time of an eclipse (lunar or solar), however, by looking at a calendar that has both the date and the time of the full and new moons. A lunar eclipse only occurs during a full moon and a solar eclipse only occurs during a new moon because those are the only times when the moon crossing the ecliptic plane coincides with the earth or moon blocking the sun.

There are many solar and lunar eclipse calculators online. NASA has a good one:

Solar Eclipse:

And this one comes from the Naval Oceanography Portal:

Lunar Eclipse:

As a rule, partial eclipses are more common than total eclipses and lunar eclipses are more common than solar eclipses. I've seen several total lunar eclipses, but only two partial solar eclipses (I would have seen a third one when I was a kid, but heavy cloud cover hid it).

In addition to total and partial lunar eclipses, there are also "penumbral" eclipses, in which the moon darkens a little from the earth's shadow but not much as in a partial/total eclipse. Both this month and next month's eclipses are penumbral.

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