Video: Describing Objects in the Solar System

Parth Gharfalkar

Which of the following is the correct definition of a moon? [A] A moon is a naturally occurring object that orbits the Sun. Moons usually have almost circular orbits. [B] A moon is a man-made object that orbits a planet. Moons usually have almost circular orbits. [C] A moon is a naturally occurring object that orbits a planet. Moons usually have almost circular orbits. [D] A moon is a naturally occurring object that orbits the Sun with a highly elliptical orbit. [E] A moon is a man-made object that orbits the Sun within the asteroid belt.

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Video Transcript

Which of the following is the correct definition of a moon?

Now we’ve been given five options here. So let’s go through them one by one and see which one sounds right.

Number one: A moon is a naturally-occurring object that orbits the Sun. Moons usually have almost circular orbits.

So, a moon is a naturally-occurring object. That much is true. However, this definition also says that a moon orbits the Sun. That part is not true. A moon is technically meant to orbit a planet, not the Sun. Now we could say that the moon orbits the planet. And the planet orbits the Sun. So the moon orbit the Sun. But even that argument doesn’t hold up because moons are not restricted to our solar system. Planets from other solar systems also have their own moons. And in that case, we can’t say that those moons orbiting the Sun. They would be orbiting the other star that’s at the centre of that solar system. So this first definition automatically is wrong. But let’s continue reading it anyway. Moons usually have almost circular orbits. That part is actually true. But simply because this definition says that a moon orbits the Sun, this one is not the right one.

Number two: A moon is a man-made object that orbits a planet. Moons usually have almost circular orbits.

Well, the last time I checked, humans didn’t make the moon. So automatically, this definition is incorrect. However, it does say that the object orbits a planet. That much is true. And moons do have almost circular orbits, as we’ve seen earlier. Because of the man-made part though, this definition is not right either.

Moving on, number three: A moon is a naturally-occurring object that orbits a planet. Moons usually have almost circular orbits.

So this definition says that a moon is naturally occurring. That’s correct. It orbits a planet. That’s also correct. And moons have usually almost circular orbits. That part is also correct. Therefore, this definition looks like the correct one. But let’s go through four and five, just to make sure.

Number four: A moon is a naturally-occurring object that orbits the Sun with a highly elliptical orbit. This one says it’s naturally occurring. That’s true. It orbits the Sun. We’ve already seen that that’s false. And it says that it has a highly elliptical orbit. That part sounds more like maybe a comet or a dwarf planet like Pluto. And that’s right. Pluto is no longer a planet. Get over it. Anyway, so this definition is also not correct, as we expected.

And finally, number five: A moon is a man-made object that orbits the Sun within the asteroid belt.

Well this one says “man-made” again. So we know it’s incorrect. It also says that it orbits the Sun and within the asteroid belt. Well, the majority of things that you find in the asteroid belt are usually asteroids. Anyway, so this one is also not true.

So our final answer is number three: A moon is a naturally-occurring object that orbits a planet. Moons usually have almost circular orbits.

By the way, if you’re struggling to remember exactly what a moon is, then think of our own moon, Earth’s Moon. It’s naturally occurring, as in humans didn’t make it. It orbits a planet, in this case Earth. And it also has almost a circular orbit. You could take the properties of our own Moon, therefore, and generalize them so that you can remember exactly what a moon is.

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