UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
How it Works
Ask a Question
Search Topics
Our Scientists
Science Links
Contact Information
Why does the moon change shapes?
Question Date: 2016-08-31
Answer 1:

The moon does not actually change shapes. However when you go out at night, at the SAME time each night and find the moon, you will note that it presents a different aspect. This is called the phase of the moon. The phase of the moon occurs because the moon rotates around the earth once a month and so the amount of sunlight that reflects off the moon varies. So, when the moon is between the earth and SUN we have a new moon . . . we can not see it because the side of the moon we face is NOT ILLUMINATED by the SUN.

phases of the moon

Then, two weeks later we have a FULL MOON because now the moon is on the opposite side of the EARTH and sunlight keeps the disc of the moon fully illuminated.

Answer 2:

It doesn't. The moon is round, just like the earth.

The phases of the moon happen because, just like the earth, half of the moon is lit by the sun (i.e. day), and the other half is shaded (i.e. night). We see the moon's day side as light because the sun is shining on it, and can't see the night side so easily because it's dark. As the moon orbits the earth, the moon can either get between the earth and sun (new moon), or on the opposite side of the Earth from the sun (full moon), or anything in-between.

Answer 3:

The moon is a sphere (like a ball) that orbits around the earth. It takes around a month for the moon to orbit (map a lap) around the earth. As the moon moves around the earth, the moon phase changes (new moon, crescent moon, half moon etc.). It's shape is always the same (a sphere), but when we see a half-moon or a crescent-moon it is because we are only seeing part of the moon and the rest is in shadow.

Think of when you're at the movie theater when the lights are out and the screen is brightly lit up. When someone in the audience stands you can't see their face because they are in silhouette -- you can see their shape but you can't see their features because they are in shadow. This is exactly the same as what happens when we are looking at the moon. Depending on how the sun, earth and moon are aligned, different parts of the moon are in silhouette.

As a science experiment, try to look at the moon every night for a month and see if you can watch the different moon phases.

Answer 4:

The Moon does not change shape, but the shadow of the Moon crosses our view of it and makes it seem to "wax and wane". A "Waxing Moon" means the Moon appears to be growing (becoming more lit-up), and a "Waning Moon" means the Moon appears to be getting less light (becoming darker). We can only see one side of the Moon's surface at a time from our home on Earth.

The Moon orbits around the Earth every 27 days. There are some days when the Moon is between the Earth and the Sun (and there are some days when the Earth is between the Moon and the Sun). When the Moon is between the Earth and the Sun, then the Moon's shadow (created by the Sun's light), falls toward the Earth's surface, and we see a "New Moon" (when the Moon is dark or completely covered in shadow). After every New Moon, the shadow will start to disappear from our view of the Moon's surface. This is because the Moon moves away from its spot between the Earth and the Sun, and the Sun begins to shine on the Moon's surface (the side we can see from Earth). As the Moon orbits around the Earth, the surface that we can see will have more and more light (from the Sun) shining on it until it becomes a "Full Moon" (which is the opposite of a New Moon).

After every Full Moon, the Moon begins to wane (or get darker) because it is moving toward the Sun and has less and less sunshine hitting the visible surface. This is the Waning Moon stage. It is during this time that we see the Moon change from Full, to half-full (called "Quarter Moon"), to a crescent-shape (called "Crescent Moon"), and finally back to a New Moon (completely covered in shadow).

Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.
UCSB Terms of Use