Tomorrow morning (Nov. 30), when you should really be asleep, the moon will slip into the Earth's outer shadow called the penumbra. This is a called a penumbral eclipse, and it's the second one this year. The first occurred on July 4-5 but was barely noticed by skywatchers because so little of the moon passed into shadow.
During a total lunar eclipse the moon passes through the umbra; in a penumbral eclipse it only crosses the outer or penumbral shadow. (NASA)
The Earth casts dual shadows: a dark, inner umbra where the ball of the planet completely blocks the glaring solar disk, and an outer penumbra where the globe only partially blocks the sun, creating a partial shadow. If you could stand in the umbra and look back toward the Earth, the sun would be completely blocked from view. But if you stood in the penumbra and looked back, the globe would only partially block the sun the same way the tree partially blocks the sun in the middle panel of the illustration (see below).
Because the moon's orbit is tilted with respect to the sun-Earth plane, most months it passes either above or below Earth's shadow, and we don't get an eclipse. (NASA)
Total and partial lunar eclipses occur only at full moon when the moon lines up on the opposite side of the sun from the Earth and enters the cone of shadow stretching out behind the planet. At least two and as many as five lunar eclipses can occur in a calendar year. Because the moon's orbit is tilted 5.1 degrees with respect to Earth’s, the full moon normally passes above or below Earth’s shadow, and no eclipse occurs. Only when the lineup is nearly precise does the moon enter the shadow.
You can demonstrate how eclipses work by taking a walk in the forest on a sunny day. The dark inner core of a tree's shadow mimics the Earth's umbra. Its fuzzy edges define the penumbra, where partial sunshine mixes with deep shadow. From left to right: a total lunar eclipse, where the tree trunk (a.k.a. Earth) completely blocks the sun from view; a penumbral eclipse, where the sun is partially hidden by the trunk, and no eclipse, with the sun fully exposed. (Bob King)
While the umbral shadow takes a dark bite out of the moon, the penumbral shadow, filtered by partial sunlight, looks like a dusky, gray shading over part of the moon. It's subtle. But unlike July's eclipse, during Monday's event, 83% of the moon's diameter will dip into the penumbra. Shading should be apparent over the right side (northern half) of the moon with the naked eye.
The eclipse is visible across the United States, Canada, Central and South America (occurs at dawn in the eastern half of the continent), Australia and East Asia.
The moon travels deep into the penumbra Monday morning, Nov. 30. At maximum eclipse, around 3:44 CST, shading on the upper right side of the moon should be obvious. (Fred Espenak, NASA GSFC with additions by the author)
The show begins at 1:32 a.m. CST Nov. 30 and ends at 5:53 a.m., but the penumbra will only be obvious for about an hour between 3:10-4:10 a.m. CST. The VERY BEST time to see the shadow will be around 3:44 a.m. CST (4:44 a.m. Eastern; 2:44 a.m. Mountain and 1:44 a.m. Pacific), the time of maximum eclipse.
Here's the view on Monday morning facing west during the best part of the eclipse. (Stellarium)
The hour is early and cold for many of us, so here's what I suggest. Set your alarm for the time of peak eclipse, stumble out of bed and take a look through a west-facing window. If you're feeling bolder you'll step outside for a clearer view and maybe even to take a picture. For much of the U.S. the eclipsed moon will hang in the western sky in Taurus the bull not far from its brightest star, Aldebaran.
After you've finished watching, slide back into bed and imagine drifting into your own personal eclipse, starting with a penumbral dozing-off that melts into an umbra of deep sleep. Clear skies and happy Z-z-z-z.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.