Why the Moon Changes Shape

This image illustrates why the moon changes shape.  To understand it, you must first remember three things.

1.  The moon has no light of its own; all its light is a reflection of sunlight.  In this image, the sun is shining from the right.  You can see its light on the right side of the moon shapes and on the right side of the earth shape.

2.  The moon is rotating around the earth. This complete rotation takes about a month—29.5 days to be exact.  In this image, the moon’s position on the first day of the rotation is at number one.  It moves around the earth to position number eight and ends the complete rotation back at position number one.

3.  We see the moon at night much more than in the day.  To understand this image, imagine that you are standing on the dark side of the Earth—the left side—and looking up into the night sky.


Now, imagining yourself on the dark side of the earth when the moon is at the beginning of its rotation in position number one, you can see that there is no moon in the night sky because the night side of the earth is facing away from the sun and the moon.  This is the perfect time to study the stars, and some say these are the nights when the creatures who are hunted at night can breathe a little easier.

Next, imagine that the moon’s orbit takes it to position two.  Now the bright side of the moon begins to become visible on the dark side of the earth.   At first we can see only a fingernail sliver of the moon’s bright side.  As the moon’s orbit brings it farther around to the night side of the earth, we can see more and more of its bright side.  A crescent moon appears and then a half moon.

Halfway through the moon’s orbit, the earth will be directly between the sun and the moon.  Now the lighted side of the moon is completely visible in the night sky.  We see a full moon.  Many of the stars are blotted out by the moon’s bright light and the hunted creatures of the night seek darker corners to hide.

As the moon continues through its orbit, its shape reverses from full, to half, to crescent, to fingernail as it passes through positions six through eight and returns to its starting point between the earth and the sun where once again it is invisible to us.  The orbit is complete.  The month is over. And the process begins again.