The Half-Astrophysicist Blog

My, What a Big Moon We Have

Friday the 12th not only marks the last full Moon of 2008, but also the largest and brightest. The Moon’s orbit around the Earth isn’t a perfect circle, but rather an ellipse. It’s distance from the Earth varies by about 30,000 miles from its closest approach to farthest point in its orbit. That’s about 14%, so the Moon does look a little bit larger when it is closer to the Earth. The image below from the NASA press release illustrates this nicely.
The two images were taken with the same camera and same magnification.  Therefore, the difference in sizes is due to the Moon’s distance!

In reality, it is kind of hard to see the difference without a camera.  The problem is that there just isn’t much in the sky to compare the size of the Moon to.  And even though the Moon appears larger in the sky than almost any other object, it still is only about half the size of your pinky at arms length (try it if you don’t believe me!)  Seeing a 14% change in an object that size is not easy to do with the naked eye.

However, you can do it with many off the shelf digital cameras.  Take a picture of the full Moon tomorrow night. Take another picture in a few months and compare them (be sure to use the same zoom setting!) and you should be able to see the difference.

December 11, 2008 - Posted by | Uncategorized

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