What Causes the Phases of the Moon?

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Author: Beth Touchette

It’s fascinating to watch how the moon changes over a month. For a couple days, it is a full bright circle.  Then, it begins to shrink, until only a tiny crescent remains. Then, it disappears all together. Then, it appears again, growing a little bigger each night, until it is full again. What’s going on?

First of all, the changes in the amount of the moon’s surface that is lit up over the course of a month are called phases. The next thing to know is that the light we see coming from the moon isn’t moonlight at all, it is sunlight. The moon does not emit any light itself. The Sun’s light is reflected from the surface of the Moon to us. Because the positions of the Earth, Moon and Sun vary throughout the course of the month, the amount of the moon’s surface that we can see changes a little bit every night. 

When the surface of the side of the moon facing Earth is completely illuminated, we see a full moon. When none of the surface is illuminated and we can’t see the moon at all, the phase is a new moon. The phases when half the side of the moon facing Earth is lit up are called the first and third quarters. When the visible moon seems to be getting bigger, we say it is waxing, when the visible lighted surface seems to be getting smaller, we say the moon is waning. When less than half of the visible moon is lit, it’s called a crescent, and when it’s more than half, it’s called a gibbous.

Phases of the Moon Diagram

The phase of the moon we see depends on where the Earth is relative to the Moon and Sun. When the Moon is between the Earth and Sun, the surface of the moon that is lit up is not facing us, so we can’t see the moon at all. This is the new moon. When the Earth is between the Moon and Sun, the Sun’s light bounces off the surface of the moon, and the moon is completely lit up as seen from earth. This is the full moon. When the Moon is between those two positions, we see the first and third quarter.

Phases of the Moon

Astronomers have observed the phases of the moon for thousands of years and have found that it takes 29 ½ days for the moon to go through its entire phase cycle. This is called a lunar month. That means that the months on the calendar we use don’t exactly match the phases of the Moon. Have you ever heard the phrase “once in a blue moon”? You might remember that the phrase means an event that doesn’t happen very often, like cleaning your room without being asked. A blue moon occurs when there is a second full moon during the same month. For example, there could be a full moon on October 2nd and another on October 31st, just in time for Halloween. A blue moon occurs every 2 ¾ years.

A three-dimensional model can help clarify this complex topic. 

Problem: How do the positions of the Sun, and Moon determine Moon phases?


  • Cardboard box,  about 20 cm x 25 cm x 29 cm
  • Ruler
  • Pencil
  • Black marker
  • Knife
  • 3-inch Styrofoam ball
  • Large paper clip
  • 16-inch piece of string or yarn
  • Desk lamp or large flashlight
  • Tape
  • Dark room
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