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Physical Science: Session 1

A Closer Look: Is the Moon Matter?

How Do We Know the Moon is Made of Matter?


In the video we define matter as "having weight and taking up space." Certainly the Moon seems to take up space — it appears in the sky every night, sometimes blocking our view of the stars, other planets, and even the sun during an eclipse. But how do we know that it has weight?

In the video, we define weight as the measurement of the Earth’s pull on a particular piece of matter. To be more precise, in physics the measure of the amount of stuff in matter is called its mass, which is a quantity independent of whether the matter is on Earth or not. Anything that has mass exerts a pull, or the force we call “gravity,” on other things that have mass. On Earth, we call the measurement of this pull on a particular piece of matter weight.

Since the Moon clearly isn’t on Earth, the question becomes, How do we know if the Moon has mass?

Since antiquity people have observed the moon reliably in the sky, never seeming to fly out of its orbit. This fact provides a clue that there might be some pulling going on. Newton’s third law — for every action, there is an equal and opposite reaction — describes the relationship. If the Earth is pulling the Moon, the Moon is also pulling on the Earth with an equal force and in the opposite direction. So, if the Earth is pulling on the Moon, and the Moon is also pulling back on the Earth, then they both have mass and both take up space, and so by definition are made of matter.

Now that we know that the moon is made of matter and has mass, how could we actually determine how much mass?

You may remember that during the Apollo missions in the 1970s, the Command and Service Module, with one astronaut remaining on board, was in orbit around the Moon for a couple of Earth days. By carefully recording the motion of the CSM as it passed close to the Moon, it was possible to determine the strength of the Moon's pull on the CSM, which results from its mass. This is the closest we can come to actually weighing the Moon!

It turns out that the Moon contains about 1/80 of the mass of the Earth. If we scale the weight of the Earth to that of an elephant (11,000 pounds), the Moon would weigh as much as the average person (about 140 pounds). Interestingly, the Moon’s matter is almost all solid, with just a small amount of carbon and hydrogen gases, and no liquid.

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