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Name: Colleen
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A

I teach 6th grade science. What is a clear way to explain to students why the moon appears to be different sizes when viewed from Earth? Is there a fun way to model this for kids as well?

Check out the web site

for a very nice discussion of the moon size illusion.

Dave Kupperman

It is not clear whether this refers to the "phases" of the moon, or to the apparent size difference of the moon close to the horizon compared to its apparent size when overhead. There are many demonstrations illustrating the phases of the moon on various websites. Search the term: "phases of the moon demonstrations" and take your pick. The apparent size difference of the moon at various locations in the sky is trickier, but more interesting because the result is counter intuitive. First, the effect is greater when the moon is full, or almost full. Its apparently larger size when it is near the horizon is an optical illusion. This can be demonstrated by taking a photograph of the moon near the horizon using a modest telephoto lens on the camera -- a 10X zoom on a digital camera should work well. Then repeat the experiment at the same settings when the moon is high in the sky. Processing the images -- say using computer digital imaging software -- will reveal that the size of the disk is the same in both positions. This effect is discussed in detail on any number of websites on lunar effects.

Vince Calder

Dear Colleen,

Although the Moon does vary in size from perigee to apogee (closest and farthest points to Earth in the Moon;'s orbit) I am not sure of how to illustrate this to children. While it is very hard to see the change in the size of the Moon as it moves round Earth, the change is obvious during solar eclipses. In an annular eclipse the Moon, at apogee, is smaller than the apparent diameter of the Sun and we see an annular or "ring" eclipse. Near perigee, the Moon is bigger and we see a total eclipse.

I looked up "Moon sizes demonstration" on Google and got some interesting results. Best of luck!

David Levy

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