Keen sky-watchers may notice that October's full moon, known traditionally as the Hunter's Moon, will lose just a bit of silvery luster tonight as part of its southern limb gradually dims. What is happening is that Earth's outer, pale shadow is falling on the outer edge of the lunar disk.
The celestial phenomenon, known officially as a penumbral lunar eclipse, may be a bit tricky to catch in the sky with the eye, however, since the shading effect will be quite subtle.
Armchair astronomers can watch a live feed of the eclipse, thanks to SLOOH. The Internet-based space-tracking service is broadcasting the eclipse with its robotic telescopes on the Canary Islands (map) starting at 4:30 p.m. EDT (23:30 UT).
"Penumbral lunar eclipses are the most subtle of all eclipses, but SLOOH can use techniques to bring out the shadow and reveal that something is indeed happening," said Bob Berman, an astronomer and columnist with Astronomy magazine. "Eclipses have always caught the fancy of the public ... They inspire fear, awe, superstition, you name it."
Here's what else you need to know about this lunar cover-up event:
What is a lunar eclipse?
A lunar eclipse occurs when the sun, Earth, and moon align. During total lunar eclipses, the entire moon is engulfed in Earth's darkest shadow. But during partial eclipses, the moon never completely goes dark or turns red—only a portion of its disk appears to darken slightly. (Read about a total lunar eclipse in 2011.)
"This is an eclipse where the circumstances place the moon only inside of the very light outer shadow of the Earth called the penumbra, rather than the darker inner shadow known as the umbra," said Larry Ciupik, an astronomer at Adler Planetarium in Chicago, Illinois.
"That outer shadow cone is so light that one normally barely notices the darkening moon as the eclipse progresses."
Who will see it?
In eastern North America, the eclipse will already be underway when the moon rises, with the deepest part of the event occurring at 7:50 p.m. EDT (23:50 UT). At that time, a slight shading will appear along the southeastern edge of the moon. Folks in the Mountain and Pacific time zones will probably miss out, however, because the moon will rise locally well after maximum eclipse is reached.
The eclipse will also be visible throughout South America, across Europe, and in Africa on Friday night. Meanwhile sky-watchers in Asia will get to witness the partial lunar eclipse at dawn on October 19. (Take a moon myths and mysteries quiz.)
All the stages of the eclipse will be visible across the Indian Ocean, central Asia, western Australia, Africa, and Europe.
When is the best time to watch?
The entire duration of the eclipse will be 4 hours and 10 minutes, commencing at 5:45 p.m. EDT (21:45 UT) tonight.
The most readily visible part of the eclipse, however, will be at 7:50 p.m. EDT (23:50 UT), when Earth's shadow is at its deepest/maximum coverage—a bit more than 75 percent of the lunar disk.
Why do lunar eclipses, even if they are partial, have such a hold on sky-watchers?
Although we understand the mechanics of lunar eclipses, they still have the power to amaze, "perhaps because eclipses are a direct illustration of those celestial mechanics," said Paul Cox, SLOOH host and outreach coordinator.
"There is something deep within us that is struck with awe when we see nature's 'big events'—and eclipses of any variety certainly fall into that category."
What if I miss this one?
Generally, between two and four lunar eclipses occur each year, so no need to fret. Next year, two much more dramatic total lunar eclipses will occur, one on April 15 and one on October 18. (Read about a 2012 lunar eclipse that occurred during a supermoon.)
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