That’s why the upper part of the full moon will appear to shine normally; no part of the Earth’s silhouette will appear to block any part of the sun from this portion of the moon.
By contrast, near the moon’s lower limb in the rugged southern highlands is the crater Clavius, the third-largest crater on the moon’s visible near side. From here, Earth will appear to cover about 70 percent of the sun’s diameter; consequently, the brilliant illumination of the surrounding lunar landscape will turn somewhat more somber.
This diminished effect of the glare of sunlight on the moon’s surface is precisely what those living in the eastern part of North America will be trying to detect during the deepest phase of the eclipse on Friday evening.
Can others see the shadow?
Here’s an experiment: At the peak of the eclipse (7:50 p.m. EDT; 6:50 p.m. CDT), bring a friend or family member outside with you who knows nothing about the eclipse and ask him or her to look at the moon and see if anything looks unusual.
About that time, even a casual observer — if he or she is looking hard enough — should be able to note a slight diminution of light corresponding to a "smudged" or "soiled" appearance of the moon’s lower limb.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Originally published on SPACE.com.
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