Astronomy: The Moon

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Table of Contents

Feel Free to Skip Ahead to Any of These:

Moon Composition

Phases of the Moon

For educators

Welcome to the Moon!

I hope you enjoyed your flight as we reached the Moon. The Moon(notice the capitalization, our moon is capitalized because it is special) is a 3500 kilometer(2200 miles approximately) wide rock floating about 240,000 kilometers from us.

What is the Moon made Of?

You probably have always wondered this, and no, it is not cheese 🙂 Rather, the moon has very large, smooth surfaces called maria(mare singular) which means sea in latin(they truly look like large seas but just black). These are made of basaltic rock, which is the rock that forms from dried lava(just like obsidian from volcanoes). Since it is very glassy, it cooled quickly(more of a fun fact than a major topic, but feel free to read more into rocks to understand how this works). The other part of the moon, the highlands, is also basaltic, but made of slow cooling rocks(as such it is much more bumpy and uneven).

The dark parts are the maria and light parts are the highlands

Phases of the moon

the phases of the moon in a nice diagram

Now that we know what the moon is made of, we should talk about the other important concept of the moon, which is how the phases work. This is diagrammed above but is still very confusing, so do not expect this to come easy. But let’s get started!

The moon cycles through its phases in 30 days(29.5 to be a little more accurate). There are 8 phases total so each phase goes for 3-4 days. Before we talk about them, let’s understand why these phases happen. The half of the moon we say is always the same, this is because the moon is “tidally locked” as its rate of rotation(how long it takes to spin around itself) is equal to the rate of revolution(how long it takes to circle the Earth).

Since we always see the same half, based on where the moon is in reference to the Sun is how much of it we see lit up. The moon itself is extremely dark, the only reason it is seen as bright is because of the amount of sunlight that hits it. When you imagine this, please use the photo above and just remember that the Sun is always on the inside(where the sunlight arrows come from) because of the fact that we orbit the Sun once a year(thus we get the moon to cycle 12 times a year).

We start with the moon being between the Sun and Earth. This means the half of the moon we can not see is getting all the Sunlight, so the half we see is entirely dark. This is important, half of the moon always is lit! If you remember this, the rest will make more sense. This is called a new moon and happens at the beginning of the month(this is how a month was made when a new moon occurred in the past people established that as the start of the month). Now the moon starts to revolve around us while the sun stays in the same spot(for easier reference). The moon is now not directly between the Sun and Earth but is instead at an angle and the moon turns to face us(this revolution happens clockwise). Since it still faces us, a little bit of the moon “turned” to face the Sun, causing the right side to form a crescent of light. This is known as a waxing crescent, waxing because it is “growing” to full moon. This continues to happen until the whole right side becomes lit and is called the first quarter moon. But do not let the name mislead you, half the moon is lit up but we call it a quarter because we can only see a quarter as lit(the other lit quarter faces away from us). Now, this keeps progressing towards the full moon. The stage in between is called waxing(remember, growing) gibbous. I am not aware of the meaning of gibbous, but just remember it because it’s a silly-sounding word. We finally reach the point where the Earth is between the Sun and Moon(notice the order, Sun, Earth, then Moon). This means that the side of the Moon we can see is facing the Sun directly, and is called a full moon(only half is lit, but the full half we can see). This happens in the middle of the month(around the 15th generally). Now the Moon continues to revolve past this point, and as it revolves it turns away from the Sun so to speak, so we see a crescent of darkness form. Because most of the moon is still lit, it is still a gibbous moon but is a waning(shrinking) gibbous. This continues until quadrature or when the moon is at a 90-degree angle to the Earth and Sun(forms a right triangle). This means the third quarter has arrived, as the half of the moon we can see of has half of it lit up(the sunlight is coming from the left, and we look at it straight ahead.) Now past this point, the moon continues to revolve around us, and since it is locked to have the same half keep facing us, it will further turn away from the sun to form a waning crescent(waning because it is shrinking to new moon). Finally, at the beginning of the next month, the Moon returns to its original spot to become a new moon again.

I have discussed the main facts of the moon, the moon’s composition and the phases of the moon. There is obviously a lot more to learn but that is outside the scope of my general discussion, now that you have a starting point, just think of questions and try to answer them.

For Educators

Remember to do your 5-minute presentation on the above topics. These topics are probably the most important things for a general understanding of the moon. Students will struggle with the phases so I would recommend setting up a demonstration (or click this link for a virtual lab). I would recommend doing it in person so students get the 3D spatial understanding, this youtube video does a great job showing a potential experiment and you can be creative from there.

Thank you

Thank you for reading and learning the basics of the moon. If you want to learn more, I encourage you to explore other sources like youtube channels(crash course astronomy is where I learned a lot of my astronomy knowledge) or read books about the moon. Any questions or comments please contact me at vijaypbharti01@gmail.com