Published: 2007-06-23 11:27:17
Updated: 2007-06-23 11:27:17
Posted June 23, 2007
MIKE MOSS SAYS: Shannon, The phases of the moon are not caused by something casting a shadow on the moon, but are instead a matter of the angle between the sun, the moon, and the earth. The sun always lights up the half of the moon that faces toward it, and since the moon revolves around the earth about one every 29 and a half days, we get to see it at a little bit different angle each day. When the moon is farther away from the sun so that earth is almost lined up between the sun and the moon, the lit side of the moon will face toward us and we will see it as full or gibbous (more than half full). When the moon is lined up so that it is between us and the sun, the dark side of the moon will face toward us and we see it as new or crescent, and when it is lined up to one side or the other so that it is about the same distance from the sun as we are,we see it from the side so that it appears "half full" (these are the "first quarter" and "last quarter" moons).
There is a nice lesson online that concludes with a helpful simulation that shows how the moon, earth and sun are positioned throughout a lunar month, and how the moon appears from the earth at each of those points. The site is
You can go through the entire lesson if you like, or just scroll down and click on "Activity 3" to open a window with the full simulation where you can either click to a certain phase of the moon to see the alignment involved, or let the animation proceed on its own by clicking the "run simulation" button.