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What Causes the Phases of the Moon?

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Updated on Nov 01, 2013

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 changesa 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.


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


  1. In the middle of a shorter side of the box, draw a 5-cm (2-inch) square. See diagram 3 (which shows the experimental set-up).
  2. Have a grown-up help you cut out the square.
  3. Above this opening use the marker write “Sun/Full Moon.” This is where you will always place the light.
  4. In the middle of the other shorter side of the box, make another square of the same size, but this time, cut only three sides of the square. Leave the bottom of the square uncut, so you can close it like a door. Above this opening, write “New Moon.”
  5. In the middle of the longer side of the box, make another door-hinge square.
  6. Write “First Quarter” above this opening.
  7. In the middle of the other longer side of the box, make another door-hinge square and write “Third Quarter” above it.
  8. Push the paper clip about ¾ of the way into the Styrofoam ball.
  9. Tie the string through the paper clip.
  10. Tape the top of the box shut.
  11. Use a pencil to make a tiny hole in top of box.
  12. Position the light next to the “Sun” opening.
  13. Place the Stryrofoam ball inside the box and thread its string through the hole in the top of the box.
  14. Position the lamp or flashlight about four inches away from the hole marked “Sun.” The lamp’s light should be pointed towards the hole.
  15. Turn on the lamp and darken the room.
  16. Position your eye near the opening on the other side of the box, holding the string that is attached to the Styrofoam ball.
  17. Make sure that the adjustable “doors” you made on the long sides of the box are closed.
  18. Adjust the string until the Styrofoam ball is blocking the light coming from the lamp.
  19. Keep the Styrofoam ball in this position by securing with tape.

Moon Phases Setup

  1. Observe the Styrofoam ball through the “New Moon” door. What do you see? Make a sketch in the table:

Phase of Moon

New Moon

First Quarter

Full Moon

Third Quarter

Amount of surface lit up

  1. Close the door below the “New Moon” label and open the door below the “First Quarter” label. Add this sketch to your data table.
  2. Close the door below the “First Quarter” label. Peak through the “Sun/Full Moon” opening, being careful not to get too much light in your eyes. What do you notice? Make a sketch in your data table.
  3. Open the door below the “Third Quarter” label. Look through the opening. Make your last sketch.


When you looked at the new moon, none of the surface facing you should have been lit up. When you viewed the first quarter, about half of surface you viewed was lit up. When you viewed the surface of the Styrofoam ball through the full moon opening, the whole surface should have been lit up. When you viewed the third quarter, about half of the surface you viewed was lit up. You might notice it was the opposite half from the first quarter that was lit.


None of the Moon’s surface was lit up when you viewed the new moon because the light was on its opposite side. All of the moon’s surface was lit when you viewed it through the full moon slot because the light was on the same side. You saw half moons through the first and third quarter slots because the light was coming from a 90 degree angle. Take a look at the diagram:

Going Further

Check out this cool projecton how the moon’s phases affect Earth’s tides!