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Big Moon, Little Moon

Week of July 6, 1998

In past snacks, I have talked about how the Moon's orbit is an ellipse, and how this effects our view of the Moon. One effect is called "libration", which is an apparent rocking of the Moon a bit in its orbit. This allows us to see a bit more than the 50% of the Moon we usually see.

But a much simpler effect is that because the Moon has an elliptical orbit, sometimes the Moon is closer to the Earth than other times. This has a host of weird effects on the Earth (the most interesting are probably due to its gravity and tides), but again we may have overlooked something simple: a nearby object looks bigger than the same object when it is farther away. This is of course true for the Moon as well.

It takes the Moon about a month to orbit the Earth. During that time, it reaches perigee (closest approach to the Earth) at some instant, and apogee (farthest distance) at some other instant about half an orbit (or about two weeks) later. The Moon is roughly 400,000 kilometers away (230,000 miles), and the average change in distance between perigee and apogee is about 40,000 kilometers, or 10%! That's a lot, and it means there is a noticeable size change in the Moon as well.

image of moon and apo and perigee Antonio Cidadao, whose images have graced these Bitesized pages in the past, has once again made a stunning image using relatively simple equipment. These two images of the Moon were taken when the Full Moon was at perigee (on the left) and when Full Moon was at apogee (right). The difference is clear!

Antonio's web site has lots of images, and another I should point out: he took a series of images across an entire lunar orbit (called a "lunation"), showing the changing appearance of the Moon from new through full and back to new. Besides the obvious phases, look for the change in apparent size, as well as the libration. Amazing! He has a web page devoted to images he took using a QuickCam, and the Moon animations are at the bottom.

©2008 Phil Plait. All Rights Reserved.

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