“Mad Science” Mini-Lessons

1. The Moon Is a Round Sky Rock


Astronomy; phases of the moon; shape of the moon; orbit of the moon; relationship of earth, moon, and sun; scientific inference; evidence


Each student needs a small sphere: berry or marble, or even a pea could work.  At my village I used mthula fruits.  It’s best if they can hold the sphere on a stick, so that it isn’t in the shadow of their hand holding it.


This must be done when the moon is visible in the daytime sky.  This means in the afternoon around first quarter and in the morning around third quarter.  (Around new moon, the moon is too close to the sun to be seen in the sky and around full moon, the moon is visible only at night, being above the horizon in daytime only in late evening before the full moon and in early morning after the full moon.) All students stand where the moon is visible.  Instruct students to hold the sphere in front of the moon so that the sun illuminates it.  The phase of the sphere is the same as the phase of the moon!

Why is this?

Just like the hand-held sphere, the moon is a sphere illuminated by the sun.  The sun is so far away, and the moon so close by comparison, that the angle of the sun to the moon is the same as the angle of the sun to the sphere.  (The sun is so big that its diameter is greater than the distance from earth to the moon.)

The phase is only the same when the sphere is right in front of the moon, along the line from the moon to the observer’s eye.  Whenever the sphere is in the sun, half its surface is illuminated by sunlight.  But we only see the part of the illuminated surface that is in front of our eyes.  We see the full disk of the sphere illuminated only when the sun is behind us.  We see half the disk when the sun is 90° from the line of sight to the sphere, and we see only a sliver (crescent) of the sphere when the sphere is back-lit by the sun.  It is the same with the moon.  We see the full round disk of the moon (full moon) when the moon is on the opposite side of the earth from the sun, and so on.

This would not work if the moon were not spherical.  You can test other round shapes and see that only a sphere gives the same light-and-shadow pattern as the moon.  So, you now know and have demonstrated that the moon is a round sky rock illuminated by the sun.

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Copyright © 2004, Richard Barrans
Revised: 14 December 2016; Maintained by Richard Barrans.
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