GED Science: Moon Phases
In order to understand moon phases, you have to understand the relative motions of the moon, earth, and sun relative to each other. The moon revolves around the earth in a counter-clockwise direction. The earth revolves around the sun in a counter-clockwise direction. Additionally, all three objects—the moon, earth, and sun—also rotate on their own axes. These movements are pictured in the diagram below:
Whether or not the earth faces toward or away from the sun determines whether we experience daytime or nighttime on earth. Similarly, whether or not the moon faces toward or away from the sun determines which portion of the moon is illuminated, and which part is dark. Just like at any one moment, half the earth is illuminated by the sun, at any one moment, half of the moon is illuminated by the sun. However, we don’t see the moon half-illuminated due to our positioning relative to the sun and the moon. The portion of the moon that we do see illuminated is called the phase of the moon. In the diagram below, you can see that half of the earth is illuminated, and half of the moon is illuminated, though to an observer on earth, it would only appear that half of the moon is illuminated:
The phases of the moon happen in a cycle that occurs once every 28 days, which is the same period of time it takes the moon to revolve, or orbit around the earth. As the illuminated portion of the moon gets bigger, the phase names are preceded by the word waxing, which means to get bigger. As the illuminated portion of the moon gets smaller, the phase names are preceded by the word waning, which means to get smaller. The moon phase cycle is pictured from start to finish below.
The following diagram shows the positions of the moon, earth, and sun for each phase in the cycle. Notice that the moon diagrams that appear on the circle show the illumination of half of the moon by the sun. The moon diagrams that appear outside the circle show with portion of that illumination we can see from earth.