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Dr. Neal Sumerlin keeps us abreast of happenings in the night sky and the progress of the new Belk Astronomical Observatory.

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Lunar Eclipse Update

Posted on 02/18/2008

Absolutely true to form, the weather forecast for Wednesday night (February 20) is a bit iffy about the prospects of clear skies. Of course, that is the night when a total lunar eclipse will be visible (maybe!) from the entire contiguous United States.

Quick facts: the partial eclipse begins at 8:43 p.m. EST, the total eclipse at 10 p.m. EST. Totality ends at 10:52 p.m. EST, and the partial phase of the eclipse ends at 12:09 a.m. EST.

A bit more detail: lunar eclipses occur when the moon moves into the shadow of the Earth. This can only happen when the sun, the Earth, and the moon are in a straight line, in that order. This diagram, while definitely not to scale, will give you the idea.

diagram of lunar eclipse

If you think about this geometry, you will realize such an alignment can only occur when the moon is full, that is, fully illuminated by the sun from our perspective. So why does it happen only rarely - the next total lunar eclipse is not until December 2010 - and not every time the moon is full?

If the plane of the Earth's orbit around the sun were exactly aligned with the plane of the moon's orbit around the Earth, it would. But these are slightly offset from each other, by only 5 degrees, and this is enough to make lunar eclipses the exception rather than the rule.

The Earth is of course much larger than the moon, and so is its shadow. The moon is completely enveloped in that shadow during a total lunar eclipse, and the eclipse is visible from any spot on Earth where the moon itself is visible at that time. For a total solar eclipse, the lineup is the sun, the moon, and the Earth, in that order. Since the moon's shadow is much smaller than the Earth's, it only reaches a small portion of the Earth's surface, and a total solar eclipse is visible only in a rather small area. People travel great distances to see total solar eclipses, and most of us have never seen one. Travel isn't really necessary for lunar eclipses, and most of us have seen several.

I always like to have my students imagine what they would see if they were standing on the moon during this event. On the moon one would be observing a total solar eclipse as the Earth slid in front of our very own star. But the fact that the Earth has an atmosphere makes for what must be a truly spectacular sight. A totally eclipsed moon has a color that ranges (depending on atmospheric conditions) from a deep reddish-brown to a rather bright yellow-orange. This is because the Sun's light is refracted through our atmosphere and "bent" enough to reach the otherwise shadowed parts of the moon. If the Earth were simply a solid ball of rock with no surrounding air, the moon would be completely dark in an eclipse. But standing on the moon and looking back at Earth, one would be bathed in that reddish-orange light coming from the rim of the eclipsing Earth. Every sunset and every sunrise on Earth would be sending its light your way, courtesy of the blanket of air that sustains our biosphere.

Blog and journal content is produced by an individual. All opinions are those of the individual writer and may not reflect those of Lynchburg College.