“Why Does the Moon Change Shape?”

By Scott Hamilton

I thought why not continue the series on the Moon one more week. I got to thinking that at least some of my readers are elementary school students and they might be interested in learning exactly how and why the moon looks different pretty much every night. We refer to this change in the Moon as the Moon’s phases. They start with the New Moon, which in reality appears to be no Moon at all, and the cycle completes with the Waning Crescent, which is the last thin slice of Moon before it starts all over again.

So let us start our discussion with the New Moon, as it is really the simplest to explain. We see, or rather do not see, the New Moon when the Moon is between the Earth and the Sun. We are only able to see the side of the Moon that has no light hitting it. The Moon does not create its own light and we only see the light of the Sun reflecting from it. We can do an experiment that simulates this if we take a basketball (the Moon) into a dark room and put a flashlight directly behind it. The basketball, like the Moon, will only reflect the light from the flashlight and if we are aligned exactly, we will not be able to see the basketball, at least not very well.

As the Moon orbits the Earth, the light reflecting off the Moon creates a sliver of light. We refer to this as a crescent moon, and since this slice of light is getting bigger, we call it a waxing crescent moon.

Eventually the moon has reached a quarter of the way around the Earth and we see half the Moon lit. This is the first quarter moon, since the moon still appears to be growing.

A few days later the shape is getting bigger still. We call this shape the gibbous moon, and since it is still growing, it is the waxing gibbous moon.

As the Moon reaches its orbit halfway around the Earth, we are then able to see the entire half of the Moon which is lit by the Sun. This phase is the full moon, and one of my favorites. On a nice clear night with a full moon, you can take a night hike without the use of a flashlight.

As the orbit continues, we see less of the lit side of the Moon until it reaches the gibbous again. This time since the Moon is getting smaller, we call this the waning gibbous moon.

Eventually we see exactly half the Moon lit, which we call the last quarter moon, which is confusing since it is half lit, but it is just beginning the last quarter of its orbit.

Finally we get another crescent moon, which is a waning crescent because it is getting smaller until we come right back to a new moon.

I have always understood that the phases of the Moon had everything to do with its orbit around the Earth, but was always really confused as to exactly how the orbit affected the light reflected from the Moon. I used to think it was because the Earth cast a shadow on the Moon, but this only happens during a Lunar eclipse, when the Earth is completely between the Sun and the Moon (which only occurs at most a few times a year.) The phases of the Moon only take 28 days to make a complete cycle, so that is not the case.

So in reality what is actually happening is the Earth and the Moon are relatively close together and the Sun only hits one side of the Moon, just like it only hits one side of the Earth. It is fairly easy to know which side of the Earth the Sun is hitting, because we easily see the difference between night and day. However, it gets a little strange when we start thinking about the Moon. You see just like only one side of the Earth gets sunlight at a time, only one side of the Moon does as well.

What we are actually seeing when we look at the Moon is not a shadow of the Earth blocking the Sun on the Moon, but the Moon blocking the Sun from reaching the side which is facing away from the Sun. As the Moon orbits the Earth the light we see from the Moon is the Horizon line. This is the area of the Moon that is in the light of the Sun. As the Moon orbits the Earth we see a different side of the Moon, but the Sun is always pointing at the same side. We can simulate this with an experiment in a dark room with a flashlight and a ball.

If you shine a stationary flashlight at a basketball and move the ball around your body, you will notice that you will see one side of the ball in the light and another in the dark, as your angle with the ball changes you will see more, or less of the ball in the light. Since it is easier to leave the ball in one place and walk around the room, we can do that and get the same effect. As we orbit the ball, we can see the phases of the moon reflected in the lit area of the ball. Guess what? This means that if you were on the Moon watching the Earth, you would see phases of the Earth. Every 28 days you would see a new Earth, two crescent Earths, two quarter Earths, two gibbous Earths and a full Earth. The only difference is because the Earth is also spinning you would see different parts of the Earth in the light each night.

Until next week, stay safe and learn something new.

Scott Hamilton is an Expert in Emerging Technologies at ATOS and can be reached with questions and comments via email to sh*******@te**********.org or through his website at https://www.techshepherd.org.