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Dennis Mammana: Catch a Glimpse of Friday’s Penumbral Lunar Eclipse

See a penumbral lunar eclipse after dark this week. Click to view larger
See a penumbral lunar eclipse after dark this week. ( illustration)

If you’re looking for a celestial challenge this week, you’re in luck.

On the night of Friday, Feb. 10, you might want to pay close attention to the full moon as it rises in the east shortly after sunset. If you do, you’ll be treated to a celestial phenomenon that few stargazers will even notice: a penumbral eclipse of the moon.

Depending on where you live, you may see the northern side of the moon appear the slightest bit darker shortly after it rises. For sky watchers on the West Coast, the deepest part of the eclipse will occur just as the moon is rising around 5:44 p.m. Pacific Standard Time.

This will not be what one might think of as a typical lunar eclipse, so regardless of where you live, only those who know what is happening will have much chance of noticing anything at all.

The reason is that the Earth, as well as every other solid object in the universe, casts two shadows into space: a dark inner shadow (the umbra) and a light outer shadow (the penumbra).

You can demonstrate this for yourself by finding a bright light bulb and casting the shadow of your hand onto a wall. Look carefully and you’ll see that your hand actually casts two shadows. The darkest part of the shadow is what we call the umbra, and since the light source is not a point, the edges of the umbra tend to blur a little, producing a faint outer shadow that we know as a penumbra.

The sun, like the light bulb, is also an extended light source, subtending about one-half of a degree in the sky. So when the Earth’s shadow is cast into space, it, too, appears lighter and fuzzier along the edges. And it’s this penumbral shadow that the moon will drift through on Friday night.

The maximum eclipse — our best chance of seeing anything at all — will come shortly after moonrise when the moon enters deepest into the penumbral shadow and just skirts the outer edge of the Earth’s dark inner (umbral) shadow.

If you miss this one — and many of us may because it will be quite subtle — our next chance to see a deeper lunar eclipse will come in August, when the moon will pass directly into the Earth’s umbral shadow, creating a partial eclipse of the moon. This, however, will be visible only to sky watchers in the Eastern Hemisphere.

The next lunar eclipse visible to part of North America will occur in January. While this will be a total lunar eclipse, only those in the western half of the continent will have a good view of it. After that, the next total lunar eclipse visible to the United States won’t occur until January 2019.

In the meantime, don’t miss the penumbral show on Friday night!

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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