Space.com - 10/18/13, Joe Rao
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.
