Except for the sun, the moon is the most notable object in the sky. We consider the moon to be a nighttime object because we notice it more often in the dark. However, the moon is up during the day half the time. We also notice that the moon undergoes phases. That is to say the shape of what we see changes as time passes. The phases and the day and night appearances are closely related.

The phases of the moon are determined by the alignment of the sun, earth, and moon. In the drawing below, assume the sunlight is coming from the right. The moon labeled start will be the first moon we consider. From that point on we will move in a counterclockwise manner. This view is taken from above the north pole of the earth. Notice that all the moons have the same side illuminated. But the earth does not always see the illuminated side.

The start position is called the new moon, since it begins the phases. In this phase the moon is between the sun and the earth. (Though not directly between. Keep in mind this is a two dimensional drawing in three dimensional space. The position of the moon could be toward you on the page or away from you.) The illuminated side is facing away from the earth and therefore is not visible to us. In addition, this moon is in the sky when the sun is in the sky. Even if a sliver is showing, it is not likely to be visible because sunlight drowns out the sliver of light.

The diagram below shows two people, Jay and Jana, representing this configuration. Jay is going to be the earth. His arms outstretched are representative of the horizon. He will always face the moon, Jana, so that she is overhead on the meridian (above Jay's nose). (In reality, the same side of Jay, Earth, would not always face the moon. But the same side of Jana, the moon, does always face Earth.) Consider what time it would be for people viewing the moon from Jay's point of view. You do so by thinking of where the sun is located with respect to his horizon. The sun is at the location of the camera. Next to the picture of Jay and Jana is a drawing of what the moon looks like from the earth. (Ignore the two year old with the two older children.)

In all the following pictures, the camera represents the view from the sun.

Note the back of Jana's head is illuminated, but Jay is viewing the front of her head, and faces the sun.

As we move one step in the lunar rotation, a portion of the moon facing the earth becomes illuminated (roughly Jana's left eye). This is called the waxing crescent. Waxing indicates the illuminated view from earth is increasing (50% is still lit, but not all of that 50% faces the earth). Crescent indicated the shape as seen from earth.

Note that the earth (Jay) has also rotated to keep the moon (Jana) on the meridian. So the waxing crescent is on the meridian after noon. The new moon was on the meridian at noon. Do you know when these moons will rise? set? Notice that Jana is still facing Jay also. This is actually accurate, since the same side of the moon always faces the earth. The same side does not always face the sun, however. So there is no lunar surface always in darkness.

The next phases are:

first quarter

waxing gibbous

full moon

waning gibbous

last quarter

waning crescent

and finally we return to new moon. More explanation of these phases can be found in the text. Ponder these questions: When are these moons in the sky? When do they rise? set? The answers are not the same for any two phases. Find a partner and reproduce the motions of Jana and Jay. This will go a long way to answering the questions.

This page was last updated on 06/06/01.