Getting to Know: Moon Phases
It’s evening. You glance up at the sky. You notice that the moon is low on the horizon. It looks like a thin crescent. “Wow,” you say to your family. “Look at the moon!”
You don’t remember seeing the moon like this before. In fact, just a couple weeks ago you saw the moon high in the sky and it was a full circle. Sometimes you even see the moon during the day. You start to wonder why the moon changes so much. Is there a pattern to how the moon changes?
Yes, the moon’s shape changes in a predictable pattern. It goes through a series of changes called phases. The moon goes through its phases in the same pattern every month.
You may have heard people talk about the full moon. That is the phase when the moon looks like a big, bright
circle in the sky. During the new moon phase, the moon
is dark.
Sometimes the moon looks
like a thin crescent.
The other phases are in between. After the new moon, we
see a crescent. The lit part of the moon grows a little larger
each day. Soon it looks like a half circle. Then we see a circle that is three-fourths lit. After the
full moon, we see less and less of the moon each night until we see the new moon.
The moon goes through this cycle about every 28 days. This is the time it takes for the
moon to make one orbit around Earth.
Misconception 1: Wait a minute! Doesn’t the moon produce moonlight?
The moon does not make light at all. It reflects light from the Sun.
The moon’s phases happen because the moon and Earth are moving in space. Each moon
phase happens when the moon is in a certain position in its orbit. Keep reading to find
out more.
Concept: Moon Phases
Getting to Know
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Okay, what does the moon’s orbit have to
do with phases?
First, imagine Earth and the moon in space. The moon
orbits Earth. Together, the Earth–moon system orbits the
Sun.
Now, imagine how sunlight lights up the moon. It always
lights up half of the moon. Does this seem confusing?
Think of Earth. You probably know that half of Earth has
night while the other half has daytime. That is because
sunlight reaches only half of the planet at a time. The
other half is in the shadow.
Why don’t we always see the moon as a
half circle?
Just like Earth, half of the moon
is always lit by sunlight.
We don’t always see a half circle because the moon orbits Earth. As the position of the moon changes, we see different portions of its lit half. During the full moon, the entire lit
half of the moon faces Earth. During
the new moon, the shadow half of the moon faces Earth. When we see a crescent, we can only see a tiny sliver of the moon’s lit half. When the moon is three-fourths full, we see three-fourths
of the lit half.
As the moon orbits Earth, different portions of its
sunlit half are visible.
Misconception 2: Doesn’t the moon’s appearance change because it rotates?
Sometimes the lit side faces Earth. Other times a different part of the moon
faces Earth.
No, the same side of the moon always faces Earth. However, that doesn’t mean the
moon doesn’t rotate! The moon rotates once during each orbit. This keeps the same side
of the moon facing Earth. Try modeling that with two balls to figure out why that’s true.
There are some great videos and diagrams that will help you better understand these ideas.
Let’s get started!
Concept: Moon Phases
Getting to Know
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© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.