The Planetary Society Blog
By Emily Lakdawalla
Penumbral Lunar Eclipse Tonight
Jul. 6, 2009 | 17:53 PDT | Jul. 7 00:53 UTC
by Anne Verbiscer
 Anne Verbiscer is a Research Associate Professor at the University of Virginia. She studies the surfaces of icy bodies in the outer Solar System and has been involved with the Cassini mission to Saturn since 2007. Currently a visitor at Southwest Research Institute, she has been enjoying the past year living in the mountains above Boulder, Colorado.
At 8:37:51 UTC on July 7th, the Moon will be eclipsed, but just barely, by the Earth's penumbra for about two hours. This penumbral eclipse, the second of four lunar eclipses in 2009, will be so slight that the difference in brightness between the eclipsed and uneclipsed Moon will be invisible to the unaided eye.
The penumbral eclipse on July 7, 2009
Credit: timeanddate.com
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So, while Earthbound observers won't be in for much of a show tonight, it's nevertheless interesting to consider the view of the Earth from the Moon during an eclipse. In addition to an eerie reddish hue cast across the lunar surface by sunlight refracted through the Earth's atmosphere, future lunar explorers will have simultaneous views of every sunrise and sunset on Earth as the planet is encircled by a red ring. One such lunar explorer, Japan's Kaguya spacecraft, already captured such a view on February 10, 2009, in a geometry remarkably similar to that of tonight's eclipse.
Kaguya spacecraft's view of the Earth during a penumbral lunar eclipse
Credit: JAXA / NHK
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More information can be found at the JAXA web site, including a not-to-be-missed movie embedded in the page.
July and December 2009 Eclispes
Eclipses seen from the Moon on July 7 (left) and December 31 (right) 2009.
Credit: Dave Seal / NASA Solar System Simulator / Anne Verbiscer
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The Solar System Simulator view of the Earth from the Moon during tonight's penumbral eclipse (left) compared with the view during the upcoming partial eclipse in December (right) illustrates the difference between penumbral and partial eclipses. One might not think there was an eclipse tonight because the view is from the center of the Moon. Tonight's eclipse is very shallow. The Earth's penumbral shadow just brushes the Moon, so observers at the lunar equator will not see an eclipse, only those near the lunar north pole. The partial eclipse in December seen from the same spot does show the Sun hidden by the Earth's limb. Now observers on the lunar equator are in the penumbral shadow of the Earth (and are by definition seeing a partially eclipsed Sun.)
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