|
|
Why does the Moon have phases?
Home » Astronomy » Learning the Night Sky » Why does the Moon have phases?The Moon has a slightly different place in the sky each night—and sometimes during the day. One week it’s on the western horizon, another it’s east, and there are times when you can’t see it at all. Why?
The Moon moves with clockwork precision around our Earth as we orbit the Sun. Our gravity causes the Moon to rotate on its axis in exactly the same time that it takes to orbit the Earth. The same “side” always faces us, but it doesn’t look the same because the angle between the Earth, the Moon and the Sun changes. This is called the Moon’s phase.
Picture the Moon’s orbit as a baseball diamond with Earth as the pitcher. Behind home plate is the catcher—the Sun—and the Moon is up to bat. If you are looking at the catcher, you are looking at the brightest light imaginable. The Moon-batter is still there, but you cannot see it because it is between you and the Sun! This phase is called New Moon.
As the Moon slowly moves away from the setting western Sun and moves towards first base, each night you will see a little bit more. When it reaches first base we see the eastern half of the Moon. This phase is called First Quarter.
When the Moon reaches second base it is directly opposite the Sun at home plate with the Earth-pitcher in between. You can now see the whole face! This is known as Full Moon. It rises in the east as the Sun sets west.
The Moon rises later each night until it reaches third base. Then we see the western half illuminated—called Third Quarter. As it gets thinner and heads toward the rising Sun, it becomes New again!
Most of the time the Moon’s orbit carries it just slightly north or south of the Earth’s position relative to the Sun. About every six months near equinox—equal times of night and day—the position changes slightly allowing Earth’s shadow to cover it while Full—a Lunar Eclipse. When the Moon is directly between the Earth and the Sun it’s a Solar Eclipse. They happen, but we might not live in a place where we can see them.
Related Learning the Night Sky Articles
- Are the stars always the same?
- What path do the planets take across the sky?
- What path do the Moon and Sun take across the sky?
- Why does the Moon have phases?
- Introduction to the Night Sky