Home >> Science & Nature >> Astronomy

In Science, How Do the Moon's Phases Depend on the Relative Position of the Sun, Moon & Earth?

The ancient peoples of the Earth defined their calendars by the phases of the moon. Although the Earth's cycle about the sun is more important for determining the seasons for planting and harvesting, it's also more difficult to measure. So the origins of all calendars can be traced back to the cycles of the moon --- something easy to observe. The cycles of the moon depend only upon the relative positions of the sun, moon and Earth.

A Model for the Earth-Moon-Sun System
- The phases of the moon are determined by the relative positions of Earth, moon, and the sun; all of which you can model at home.
  
  The sun is a source of light. That is, it emits its own radiation. The Earth and moon do not emit their own light; any light seen from the moon is sunlight it reflects. And, for all practical purposes, all human observers are on the Earth. So a model of the Earth-moon-sun system needs a light source, a reflective sphere and an observer's location. You can model this with an unshaded table lamp, an orange and yourself.
The New Moon
- The new moon is the lunar phase where the moon disappears. Of course, it doesn't truly disappear, it just reflects no light toward the Earth. Here's how to model this situation with your lamp, yourself, and your orange. Face the light bulb. Hold the orange at arm's length just above the line between your eyes and the light bulb. Look at the orange. You might see just a sliver of the lit side of the orange (because the model isn't perfect), but essentially you can't see any of the lit surface. So when the moon is in the same vertical plane as the Earth and the sun, on the sun side of the Earth, it's a new moon.
The Quarter Moon
- A quarter moon looks like half a pie. It's called a quarter moon because the lit portion visible from the Earth is equal to a quarter of the full surface area of the moon. (Yes, by that same logic, the full moon would be called the half moon, but it's not.) To model this, face the light bulb and extend the arm with the orange straight out to the right. Turn your head to look at the orange. You will see half the side of the orange facing you is lit up, the other half in shadow. This is what happens at a quarter moon: The line between the Earth and the moon makes a right angle to the line between the Earth and the sun.
The Full Moon
- The dramatic image of the full moon can occur only when the moon and sun are on opposite sides of the Earth.
  
  Now face directly away from the lamp. Hold the orange straight in front of you, just above the shadow of your head. You will see the entire surface of the orange lit up. This is a full moon. This happens when the Earth, the moon, and the sun are all in the same vertical plane, with the moon on the opposite side from the sun.
More From the Model
- You can tell a little more from the model system with the orange and the lamp. For example, hold the orange at a right angle to the line between your eyes and the lamp --- that is, in quarter moon position. Face your eyes to the lamp. In this position, you represent a viewer at noon. Now rotate your head to until the light from the lamp hits the side of your face. Now you represent a viewer at sunset or sunrise. The orange is directly in front of your eyes; that is, it's right overhead, just as the real moon is at the quarter moon phase. You can do similar tests with the new moon and the full moon.
Orbits
- The Earth orbits the sun once a year. The moon orbits the Earth once about every 27 days. The interaction of those movements establishes the relationship between the three bodies. To make the orange-lamp system mimic that, you would need to move the orange about your head as you walked in a large circle around the lamp. The only important thing you would notice about the phases of the moon is that they don't repeat every time the moon orbits the Earth. The Earth's motion around the sun changes the relationship between moon, Earth and sun, stretching the lunar month to about 29 days.

Astronomy