A Penumbral Lunar Eclipse

Everyone loves viewing a total eclipse of the Moon. But have you ever seen a “penumbral” eclipse of the Moon? If not, this month presents an ideal opportunity for witnessing one.

The eclipse of the Moon that we all are familiar with is that which happens when our lovely satellite passes into, through, and out of the Earth’s dark inner shadow—known as the “umbra.” That darkness is surrounded by a much lighter outer shadow or halo known as the “penumbra.” (This may remind those who have seen photos of sunspots, which have a dark inner core surrounded by a much larger pale halo.) On the late evening of July 4th into the early hours of July 5th, the Moon will pass through the outer penumbra of the Earth, entirely missing the umbra itself.

At 11:07 p.m. EDT on July 4th, the upper (northern) one-third of the Moon will pass through the bottom (southern) edge of the penumbra. It will stay within it for 2 hours and 45 minutes, finally exiting at 1:52 a.m. on the 5th. Maximum coverage is at 12:30 a.m. on the 5th. The Moon will be well up in the sky at that time, making it easier to glimpse the faint shadow that when it is just rising or setting when our atmosphere largely masks it presence.

This is mainly a naked-eye event—not one suitable for telescopes! These typically spread out the Moon’s image too much making it difficult to see the edge of the penumbra. (An exception here is the Scientifics’ Astroscan Millennium Telescope, which has a low enough magnification and wide enough field of view to take in more than the entire Moon. See scientificsonline.com) But your Scientifics’ binoculars will be perfect for the occasion, offering optimum contrast between the brightness of the Moon and the dimness of the shadow. Look for an eerie appearance to the top part of the Moon, especially at mid-eclipse.

The Moon will be full and opposite the Sun in the sky (that set as the Moon rose), which must always be the case during an eclipse. This is when the dark inner cone of the Earth’s huge shadow extends out into space and across the path of the Moon’s orbit. Interestingly, there are two motions of the Moon going on as you watch this event unfold—one of them real and the other only apparent. The first is the never-ending eastward orbital motion of the Moon itself (right to left in the sky as you watch). The other is the slow turning of the entire sky westward (left to right), carrying the Moon with it as the Earth itself rotates. We’re all familiar with this “diurnal” or daily motion, but most don’t realize that our satellite is moving roughly its own diameter every hour, circling the Earth once a month and resulting in the phases of the Moon.

For most of our readers who do manage to glimpse July’s penumbral eclipse, this will be a first. Congratulations! Once seen, it will prepare you for enjoying this subtle aspect of future total eclipses of the Moon—all of which have the dim outer shadow of the Earth encompassing the umbra.

— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of 10 books on
stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.