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Penumbral lunar eclipse Friday: So what's a penumbral eclipse, anyway?

Penumbral lunar eclipse: On Friday, October 18, the moon will pass through the edge of Earth's shadow, resulting in a 'penumbral eclipse.' It's a subtle effect, and won't be visible in much of North America.

This series of photographs shows the Dec. 20, 2010 total lunar eclipse as seen from the Pine Barrens of New Jersey and won a NASA/JPL photo contest. Friday night's penumbral eclipse will look like the first photo in the series (top left) for a few minutes around the maximum, 7:51 p.m. EDT.

Keith Burns/NASA/JPL

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Friday's lunar eclipse will be the least dramatic of the three kinds of lunar eclipse: total eclipse, partial eclipse, and penumbral eclipse.

What's the difference? Location, location, location.

Total lunar eclipse

In a total lunar eclipse, like the one seen in the picture above (see a bigger version here), the sun, moon, and Earth are perfectly aligned, and the Earth's shadow completely covers the moon. As the eclipse starts, and the three bodies are moving into perfect alignment, the Earth's shadow sweeps across the moon's face until it is totally darkened, at which point the moon appears to have an unearthly reddish color, due to the sunlight filtering through Earth's atmosphere and painting the moon with a sunset-red glow. That lasts a few minutes or hours, depending on the Earth's and moon's orbits, and then the Earth's shadow sweeps off the moon.

Partial eclipse

During a partial eclipse, the Earth is just a smidge out of alignment with the moon and sun. This results in only a piece of the moon getting hidden by Earth's shadow, so observers from Earth see a round notch "missing" from the full moon, leaving a shape like a cookie that's been bitten.

Penumbral eclipse

Penumbra comes from the Latin for next-to-the-shadow, because the moon will be only slightly shadowed. During a penumbral eclipse, the sun, moon, and Earth are a bit further out of perfect alignment than in a partial eclipse, and suddenly it matters that the sun doesn't produce a pinpoint of light, like a laser, but a huge, broad beam, like a floodlight a million miles across. During a penumbral eclipse, the sunlight is partially blocked, but not enough to cast more than a smudge of a shadow. Imagine someone holds up a basketball in front of a floodlight. Sure, you'll block some light, but not enough to make a lot of difference.


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