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Why Does the Moon have Phases?

Instructor: Mary Ellen Ellis
When you see the moon differently each night, you are viewing the different phases of the moon. This lesson explains this changing shape and how it is related to the positions of the earth, moon, and sun.

Changes in the Moon

When you look up into the night sky, or sometimes the clear, blue sky of the middle of the day, you may see the moon. Sometimes it looks large and completely round. Other times it may seem as if a small slice has been removed or appear to be a slim crescent. What gives? How can the moon change?

The answer lies not in the changing of the moon at all! Rather, it is our view of the moon that changes as the earth, moon, and sun shift positions each day and night. This lesson explains more fully the phases of the moon and its lunar cycle.

How Do We See the Moon?

Before we get into formal definitions, let's try to visualize what is happening in the relationship between the moon, the sun, and the earth over the course of about a month. The sun is stationary; that's the easy part. The earth moves in two ways - it spins on its own axis and it moves in a circular orbit around the sun. Simultaneously, the moon moves in similar ways; the moon spins on its own axis while orbiting the earth.

Also key to our lesson is the understanding that the moon is not a light source. What we see as the illumination of the moon is actually the light of the sun reflecting off of the moon. As the earth and the moon revolve and orbit, the amount of sunlight that hits the face of the moon changes; these shifts in sunlight hitting the moon are what account for the shape of the moon changing.

You can simulate a simpler version of what's going on right at home. First, find a dark room and introduce to it a singular source of light, pointing at you. A flashlight sitting on a dresser would work well. Now, stand directly in front of this light, holding parallel to the light a ball. The flashlight is the sun, your head is the earth, and the ball is the moon.

Now, turn your body, slowly. As you turn, you'll notice that the amount of light that hits the face of the ball (the moon) changes. When you are facing your flashlight, the side of the ball facing you will be dark; none of the light can hit the side you see. As you turn, more light will hit the surface of the ball that faces you until you are turned with your back to the flashlight and the whole of the ball is illuminated. The same thing happens between the sun, the earth, and the moon every month(-ish!)!

The Different Phases of the Moon

Although the shape of the moon as we see it will change (infinitesimally) every day, there are officially four phases of the moon. A phase of the moon is defined by the shape the moon appears to be when viewed from Earth. The four phases of the moon are:

  • New moon, when the moon is not visible from Earth
  • First quarter moon, when - after a new moon - one half of the moon is visible
  • Full moon, when all of the moon is visible from Earth
  • Third quarter moon, when - after a full moon - the other half of the moon is visible

The phases of the moon are the result of the moon moving around the earth.
moon phases

Now, of course, the moon does not shift exactly from no moon to half, half to full, full to half moon. Rather, the changes are subtle with each passing night. In between each new moon and first quarter moon, there will be a few nights where the moon's light is shaped like a crescent. Similarly, after a third quarter moon, there will be another crescent, as the shape of the moon shrinks back towards another new moon.

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