Get Set For Friday’s Penumbral Lunar Eclipse

Shady doings on the moon. Watch for the moon to be eclipsed by Earth’s outer shadow Friday evening (Feb. 10) as shown here in an earlier eclipse. Credit: Fred Espenak

Friday evening, the Full Snow Moon will look a little weird when it rises in the eastern sky at sundown. That’s because for much of the U.S., the moon will already be in eclipse. Our favorite and only natural satellite slides through Earth’s outer shadow or penumbra that evening.

Lunar eclipses — penumbral, partial and total — always occur at Full Moon, when the Moon, Earth and Sun line up in a row in that order. Only then does the Moon travel through the shadow cast by our planet. Because the moon’s orbit is tilted 5.1 degrees with respect to Earth’s orbit, it normally passes above or below the shadow, and we don’t experience an eclipse. Other times, as on Friday, it passes through just the outer part for a penumbral eclipse. Credit: Starry Night with additions by the author

During a partial or total lunar eclipse, the full moon passes first through the Earth’s penumbra, before entering the dark, interior shadow or umbra. The penumbra is nowhere near as dark as the inner shadow because varying amounts of direct sunlight filter into it, diluting its duskiness. The penumbral shadow doesn’t take a “bite” out of the moon the way the umbra does but shades it instead. To your eye, the moon appears dimmer and blunted but not blackened.

This is what Earth’s shadow looks like projected against the sky. Times are shown for key moments during the eclipse: start, greatest or maximum eclipse and eclipse end. Credit: F. Espenak, NASA’s GFSC with additions by the author

The leading edge of the moon (east-facing of left side) first enters the penumbra on Friday at 4:34 p.m. Central time, an hour before sunset for the Midwest. Most of us won’t see the eclipse until moonrise nearly an hour later. By 5:30 p.m. local time, when the moon is staring at us from low in the eastern sky, you’ll  detect the shadow as a subtle darkening of the left half of the lunar disk.

By 6:44 p.m., when the moon is deepest in the penumbra and closest to the umbra, the top half of the moon will appear noticeably darker than the bottom. As the moon rises ever higher in the sky, it moves through the outer shadow traveling at 2,290 mph, finally exiting the penumbra at 8:53 p.m.

This diagram shows an approximation of the Sun’s position and size as viewed by an observer at the center of the lunar disk during Friday’s penumbral eclipse. More sunlight shines across the Moon early in the eclipse, making the penumbral shadow very pale, but at maximum eclipse (right), half the sun is covered and the Moon appears darker and duskier as seen from Earth. During a total lunar eclipse, the sun is hidden completely. Credit: Bob King with Earth image by NASA

From start to finish, the eclipse lasts more than 4 hours, but the best part is the hour and a half centered on maximum eclipse when the penumbral shading will be most obvious. That would be from 6-7:30 p.m. Central; 7-8:30 p.m. Eastern; 5-6:30 p.m. Mountain and around moonrise Pacific Time. If you can’t devote that amount of time, at least check out the scene between 6:30-7 p.m. Here are the particulars:

Eclipse begins: 4:34 p.m. CST (22:34 p.m. UT)
Maximum eclipse (moon deepest in shadow): 6:44 p.m. (00:43 UT Feb. 11)
Eclipse ends: 8:53 p.m. (2:53 UT Feb. 11)

This map shows where you can view the penumbral eclipse. Most of the U.S. will see at least part of the event. Credit: F. Espenak, NASA’s GFSC

The southern or bottom half of the moon is home to the pasty white lunar highlands, the first solid crust to form on the cooling moon 4.5 billion years ago. The northern half of the moon has more of the darker “seas”, areas that originally were huge impact basins that later filled with dark-colored lavas. At maximum eclipse, the northern or darker half of the moon will plow deepest into the penumbra, which should make the shading more dramatic than at other penumbral eclipses.

This is a simulated view of the moon at maximum eclipse Friday evening low in the eastern sky alongside the familiar asterism, the Sickle of Leo. Created with Stellarium

Try recording the event on your phone. Your best photo opportunity will be when the moon first comes up and the light from the lunar disk balances with the ambient light of twilight. Once the sky is dark, it’s more difficult to get a good photo without the phone camera completely overexposing the bright moon.

Here on Earth, the planet’s shadow can grow after sunset to cover the entire sky, but at the moon’s average distance of 239,000 miles, the combined penumbra and umbra span just 2.5° of sky or about the width of your thumb held at arm’s length. Stick out your thumb while watching the eclipse to get a feel for it.

I’m hopeful we’ll get a break in the forecasted clouds Friday evening as I, like you, face east after sunset into the shadow. In case of bad weather, astronomer Gianluca Masi will webcast the eclipse live starting 4:15 p.m. CST (22:15 UT). Clear skies and happy skywatching!

** Want to know more about eclipses and all the cool stuff you can see on the moon with just your naked eye? Pick up a copy of my recently published book, Night Sky with the Naked Eye, at Amazon or Barnes and Noble. It covers lots of different night sights — no equipment required!