HomeA Penumbral Lunar Eclipse
By Richard Callwood III

 

On Friday, February 10, 2017, the full Moon will be slightly darker than usual.
What is a penumbral lunar eclipse? It helps to explain what an eclipse is. A lunar eclipse occurs when the Sun, Earth, and Moon line up such that the Earth casts a shadow on the Moon. But because the Moon’s orbit around the Earth is not precisely in the same plane as the Earth’s orbit around the Sun, this alignment is rarely perfect.
A shadow has two layers: a central dark area, called the umbra, in which the light source is completely blocked, and a fuzzy outer area, called the penumbra, from which the light source is partly visible. The Earth’s umbra and penumbra are shown in the illustration.
A total lunar eclipse occurs when the Moon is entirely within the umbra. These are the “blood moons” we’ve seen in recent years. During a partial lunar eclipse, only part of the Moon is in the umbra. But this Friday, no part of the umbra will be cast on the Moon, making this a penumbral eclipse. This will make the eclipse a challenge to see, as the Sun is so bright that it is hard to tell the difference even when it is partially blocked.

Blood Moon, April 4, 2014, East End, St. Thomas. ©karlcallwood

Blood Moon, April 4, 2014, East End, St. Thomas. ©karlcallwood

The Moon will pass closest to the Earth’s umbra at about 8:44 p.m. Atlantic Standard Time. At this time, the north-northeast side of the Moon should appear noticeably darker than the rest of the Moon. A sharp-eyed observer may be able to notice the darkening about an hour and a half on either side of deepest eclipse.

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