On Friday evening (Oct. 18), the moon will undergo an eclipse of minor importance — a "penumbral" lunar eclipse.
For nearly four hours on Friday, at least some part of the southern portion of the moon will be within Earth's pale penumbra, our planet's faint outer shadow. Penumbral lunar eclipses are rather subtle events that are usually difficult to detect unless at least 70 percent of the moon’s diameter shaded.
So the penumbra may be marginally detectable over the moon’s southernmost limb for just a short period centered on the time of the deepest phase/greatest eclipse (7:50 p.m. EDT, or 2350 GMT, Friday). At that moment, the penumbra will cover 76.5 percent of the lunar disk. [Shadowy Moon: Penumbral Lunar Eclipse Photos]
Those living in the eastern half of North America are well-placed to see this celestial event. But farther west, in the Mountain and Pacific time zones, the deepest phase of the eclipse occurs before the moon rises and during the late afternoon hours when the sun is still above the horizon, so no evidence, however slight, of this eclipse will be visible.
Meanwhile, the Indian subcontinent and the western half of China might notice some vestige of faint shading before the moon sets early on Saturday morning (Oct. 19). It is a delicate spectacle for eastern South America, all of Europe and Africa as well as western Asia. At mid-eclipse, the moon will appear in the zenith from Ouagadougou, the capital of the African nation of Burkina Faso.
Penumbral eclipse from the moon
It might be easier to understand why the penumbral shadow of Earth is so faint by imagining being on the moon when Friday’s event takes place.
An astronaut on the moon during this time might see an eclipse of the sun, but it would all depend on our hypothetical moonwalker's lunar location. For example, no eclipse takes place from Aristarchus, a brilliant impact crater in the lunar northwest, as that part of the moon remains completely untouched by the penumbra.