Wednesday, December 15, 2010

Guest Post - Lunar Eclipses

Today we have a guest post by Tom Miller, a member of the Cache Stargazers, an astronomy club in Cache Valley, UT & ID. The club meets on the 2nd Friday of each month in the Physics Conference Room at Utah State University. When the weather allows, they often get to use the USU Observatory after the meeting.

And now, Tom's Post:


Why Total Lunar Eclipses look the totally awesome way they do

(Explanation of Lunar Eclipse Colors)

by, Tom Miller

To really understand and visualize the how and why of the sometimes vivid colors of the partially and especially totally eclipsed moon (which we'll hopefully see next week at December's full moon), just imagine yourself on the moon during the eclipse, watching that same eclipse from the lunar perspective, reversed from Earth. From the moon you would be watching a total solar eclipse with the sun going behind the Earth. Remember, the Earth appears 4 times larger across in the lunar sky than the moon appears in the Earth sky! But the sun still appears almost practically the same size as the sun and moon appear from Earth.

Relax and imagine what you would see, watching the sun disappear (or lets even say "set") behind the Earth in your jet-black lunar sky. Less and less of the sun lights up your lunar surroundings as the sun slides slowly behind the Earth in your alien sky. As the moment of totality approaches from where you relax on the moon, your surroundings grow darker grey with some slight yellowish and orange color now added to the moon-scape around you. If you block the glaring bit of remaining sun from your view you can see the rest of the Earth before totality, or at least a thin brightening ring of sunset defining where the dark Earth is.

As the sun finally and fully disappears (that part can take about an hour) behind the Earth in your lunar sky, you now clearly see that bright, full and very thin ring of sunset brightly surrounding whole darkened "New Earth" phase of the Earth. You would now be able to see some city lights doting the night side of the Earth which fully faces you. The sunset ring near the edge of the Earth where the sun just disappeared looks much brighter and more yellow because the sun just "set" there, while parts of the whole sunset ring around the Earth further from there look less and less bright and more orange and even reddish, much like a sunset looks on Earth, also depending on the amount of smoke, dust and other sunset reddening factors in Earth's atmosphere at the time, as you would expect to see on Earth at sunset as the sun slips a little further below the horizon of Earth.

And that's why the moon looks the very nice yellow to orange to even reddish colors it does during a total lunar eclipse, lit by a bright ring of Earth sunset as seen from the moon. From Earth you will watch this lunar eclipse cross part of the sky as the Earth turns from beginning to end of lunar eclipse. But, from the moon, the Earth would stay at a fairly stationary spot in your lunar sky while the sun appears to move behind the earth, making just over one 360 degree circle of the lunar sky per Earth month while the Earth remains fairly stationary while going through the same phases we are familiar with the moon going through as seen from Earth.


Thanks! Tom.

If you have any questions, please e-mail Tom

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