World Builders™                                                                    Session One  --  Astronomy      


            The Phases of the Moon


The moon changes as we watch it night by night.  Find out here why this happens.

     The moon travels in an orbit around the earth, yet, to a viewer on the planet, the appearance of the moon changes in a regular pattern. Why is this?

     The moon shines by reflected light. When the light of the sun reaches the moon, the moon reflects the light back, much as a mirror would do. However, the surface of the moon is covered with fine grained debris, called the regolith, and the light is not reflected back very well. about 2% of the light is reflected back, which is enough to make the moon shine and shed light on the surface of the earth at night.

     The picture here is from a NASA photograph. On the lower right side of the lighted part of the moon you can see craters caused by space debris smashing into the moon. The dark areas are called mares, or seas, but there is no water in them. The mares are lunar lava flows.

     From earth we always see the same side of the moon. We see the moon change thirteen times a year, going through its phases. You can see a speeded up version of the phase changes in the picture on the right. At the very beginning of the cycle, you see the new moon like the rounded part of a P. (Remember the word Promise.) Now the moon waxes (the lighted part that we can see gets larger) until it becomes a full moon. Then the moon begins to wane, or look smaller, as the lighted part turns away from the earth. We speak of this phase as the old moon. The part of the moon that we see now begins to look like the rounded part of a d. Think of the word dying, and you will always know where the moon is in its cycle.

     The part of the moon that is not receiving sunlight is very dark. We only see the part of the moon that is receiving sunlight. When the unlighted part of the moon is facing us in the sky, we do not see it at all.

 Let's look at a few cases and see how this works.


We Do Not See the Dark Side

      Here Dr Viau is looking up at the sky. It is day time, and the moon is somewhere in the space between the earth and the sun. However, Dr Viau cannot see the moon. (You can see it if you look very closely at the picture.)

      In this position in its orbit, the moon cannot be seen from the earth. This is because the sun is shining on the other side of the moon

      The side of the moon that the sun is shining on is reflecting light, but we cannot see it. The unlighted back side of the moon is turned towards us. When people look up in the day time, the dark back side of the moon is facing them. People on the other side of the world, where it is night now, cannot see the moon either, because the moon is in the sky on the day side of the earth.

Here is a diagram of the positions of the sun, moon and earth.


     Notice that it is dark behind both the earth and the moon. The earth and the moon are like a person standing in front of a light. The side of the person toward the light is easy to see, and there is a shadow behind him/her.

Let's try another situation.


We See the Full Moon

Now Dr Viau is enjoying the light of a full moon. The moon is somewhere in space beyond the earth. The earth's shadow is small, and there is plenty of room for the moon to receive the light of the sun. At night, people on earth can see the side of the moon that is in the sunlight.

  Here is a diagram of the positions of the sun, moon and earth when the moon is full.

Notice that the bright side of the moon is now facing the night side of the earth.

Now let's try another situation:


We See The Half Moon


Here Dr Viau can see only half of the lighted portion of the moon. The lighted part of the moon that she can not see is facing out into space. Half of the dark, unlighted part of the moon also faces space, and half of it faces the earth. On earth, only part of the lighted half of the moon is visible. It can be seen in the daylight and at night.

Here is a diagram of the positions of the sun, moon and earth.

     Notice that the earth and the moon appear to be side by side, facing the sun.     When we look at the other planets through telescopes, we see that Venus appears to have phases like the moon. It is important to remember that no matter how the moon or Venus look to us, half of each one is always lighted and half is always dark. This is true of the earth also. Astronauts on the moon would see the phases of the earth as the sunlight is reflected from the earth's surface
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© 1998, 2003. Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at