|
A Closer Look:
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.
|
prev:
session 1 intro
