|
I want to know why we always look at the same
face of the moon. Can you explain to me the reason?
|
Question Date: 2010-12-01 | | Answer 1:
It wasn't always this way with the moon -- the
moon _used_ to rotate relative to earth, so that
an observer on earth would have seen different
faces of the moon as it spun. But it gradually
stopped spinning due to something called "tidal
locking". Here's how it works:
You know that the moon causes the tides in the
ocean, right? The moon's gravity pulls on the
earth, and it pulls most strongly on the face of
the earth that is facing the moon. The land on
earth doesn't particularly care about this extra
tug, but the oceans do. Water is "lifted" towards
the moon, and flows to make a bulge that faces the
moon. (There is a bulge in the back of Earth too,
pointing away, which is related.) As the earth
turns, this "bulge" flows through the oceans,
always approximately facing the moon; we see it as
tides moving up and down. Well, the earth
does the same thing back to the moon -- tidal
forces from the Earth are about 80 times stronger
than the moon's tidal forces on us (because the
Earth mass is 80x that of the moon). However,
there are no oceans on the moon -- so no liquid
sloshes around like it does on earth. Believe it
or not, the earth's tidal forces _do_ deform the
moon itself, though, ever so slightly. Back when
the moon used to rotate relative to us, there was
a little "land bulge" on the moon's surface, which
"wants" to face the earth. When the moon used to
rotate, though, the rotation would tend to carry
the "bulge" along with it. This set up a tug of
war -- the moon's rotation pulls the bulge away,
and the earth pulls back on the bulge, against the
rotation. This, then, basically acted like a
bicycle brake: the earth's tidal forces
constantly acted to pull against the rotation,
hereby slowing the rotation down until it stopped.
So now -- the "bulge" just points right at us.
It's not very big -- but it is still there.
The moon is not the only heavenly body that is
tidally locked. Mercury is tidally locked to the
sun -- meaning if you lived on Mercury, it would
either always be day or always be night, depending
on which side of Mercury you lived on. A
longer description can be found at tidal-locking
They give an estimate for the time it takes
for the locking to happen -- my quick math said
something like 300,000 years for the moon. Check
my math -- I wasn't very careful about it, and may
be completely wrong! | | Answer 2:
The moon revolves around the earth AND also
ROTATES on its axis once per month. So you
can do an experiment or demonstration. Imagine the
MOON did NOT rotate on its own axis (that is that
the moon did not spin around its own axis but ONLY
REVOLVED around the Earth. If you do this using a
simple demonstration you will note that if THAT
was the case, then we would see the entire moon
after one revolution. Now do the
demonstration again, but this time allow the moon
to rotate on its axis in exactly the same amount
of time it takes the moon to revolve around the
Earth...you can get a friend plus you as earth and
actually demonstrate this to convince yourself. | | Answer 3:
What a great question! The Moon rotates about
its axis every 29.5 days(just like the Earth
rotates around its axis every day)--this is why
the moon has phases, as different parts of its
surface are exposed to the Sun over the course of
a month. It also takes the Moon 29.5 days to
complete its orbit around the Earth (this is
relative to the Earth--from the Sun's perspective,
it only takes 27.3 days for the Moon to complete
its orbit, but during this time the Earth has been
moving along in its orbit around the Sun, and the
Moon needs an extracouple of days to "catch up").
Because the time it takes for the Moonto orbit the
Earth is the same as it takes for it to rotate
about its axis, we always see the same face of the
Moon. (You can try this out for yourself--if you
walk in a circle around a friend, you will have to
rotate at the same rate as you are walking if you
want to stay facing your friend.) It's not
just a coincidence that these two things happen at
the same rate--we think that a long time ago the
Moon actually rotated more quickly than it
orbited, so if we had been around then, we would
have been able to see different faces of the Moon.
The Moon's rotations lowed down, however, due to
what are called tidal forces: the side of
the Moon that's closer to the Earth feels a
slightly stronger pull due to the Earth's gravity
than does the side of the Moon away from the
Earth, so it deforms a little bit--it gets a
little bit longer in the direction facing the
Earth. As different sides of the Moon faced the
Earth, different parts of the Moon would get
deformed, and all of this deformation produced a
lot of friction. This friction slowed down the
Moon's rotation over a long time, until eventually
the same side always faced the Earth, and the
deformed part did not have to move
anymore. The Moon's gravity does the exact
same thing to the Earth: the waters in the ocean
on the side of the Earth that faces the Moon are
more strongly attracted to the Moon than are those
part-way round the Earth, which in turn are more
strongly attracted to the Moon than those that are
on the opposite side of the Earth. This is what
produces the cycle of low and high tides that you
can see in the ocean. The friction of these tides
is slowing down the Earth's rotation as well--in
the past the Earth rotated about is axis much more
quickly, so billions of years ago, days were only
a few hours long. | | Answer 4:
Tides - the moon pulls on the side of
the Earth closest to it more strongly than it
pulls on the opposite side of the Earth, which
effectively stretches the Earth as it is being
pulled on from one side but not the other and the
Earth is ever-so-slightly flexible. This is what
causes the ocean to come up on land on the side
facing the moon and the side being (relatively)
pulled away, because water is more fluid and
flexible than land is, because it is liquid.
However, this also exerts a pull on the Earth
against the Earth's rotation, just as if you pull
on a long projection on something, it will rotate
toward the direction you are pulling (imagine
pulling on the end of a spoon sitting on the table
- the body of the spoon will rotate to be opposite
the direction you're pulling). But the Earth is
already spinning on its own axis, so the moon's
tides are actually slowing the Earth's rotation.
This is very slow, but 400 million years ago, the
year had about 400 days instead of 365.
Just as the moon exerts tides on the Earth, so
does the Earth exert tides on the moon, except the
moon has no oceans to get sloshed up and down as
this happens. Because the Earth is so much more
massive than the moon, the moon inevitably loses
more angular momentum in its own rotation. Put
simply, it has become tidally locked, with its
rotation period being the same as its orbital
period, because the Earth is basically dragging it
along as it orbits the planet. Because one side of
the moon happens to be being pulled on by the
Earth, just like the spoon on the table, that side
follows the Earth around as it is pulled, and
always stays pointed toward the Earth. Thus, we
never see the far side of the moon. | | Answer 5:
A way to visualize why we only see one side of
the moon is to walk in a circle while always
facing the middle of the circle. While it may not
feel like it, you are actually rotating your body
while revolving around the circle. The key here
is that the period of revolution is equal to the
period of rotation. When you have walked halfway
around the circle, you face the exact opposite
direction compared to when you started. You
complete 1/2 revolution in 1/2 rotation, meaning
the same rate of revolution and
rotation. The moon's revolution and rotation
periods are about 29.5 days. If you want to model
the earth and moon together, have a friend stand
in the middle of the circle and spin in place 29.5
times for every time you complete a full circle as
the moon. Just don't get too dizzy! Your friend
in the middle will see you "rising" and "setting"
each time he spins in place. This is why we see
the moon rise and set each day--the earth spins in
place once a day, a rate 29.5 faster than the
moon's rate of revolution. Cheers, | | Answer 6:
We always see the same side of the Moon
because it takes the same amount of time (about
29.5 days) for the Moon to complete one orbit
around the Earth as it does for the Moon to
complete one rotation about its axis. Now
you're probably wondering why it takes the same
amount of time for the Moon to complete one
revolution around the Earth as it does for it to
complete one rotation about its axis. This is a
phenomenon known as tidal locking. The force of
Earth's gravity pulling on the Moon causes it to
bulge slightly in the direction of the Earth. Now
imagine that the rotation of the Moon about its
axis were faster (or slower) than its orbit around
the Earth. Because the force of gravity is
inversely proportional to the square of the
distance between two bodies
(F=GMm/r2, where M and
m are the masses of the 2 objects, G
is the gravitational constant, and r is the
distance between the two objects), the force on
the bulge closer to the Earth is greater than the
force on the bulge farther from the Earth. Thus
the force of Earth's gravity pulling on the bulge
would produce a torque (a force that causes
rotation) on the Moon, causing the Moon's rotation
to slow down (or speed up), until the period
(amount of time to complete one cycle) of the
rotation of the Moon about its axis and the orbit
of the Moon around the Earth are the same. I
have also attached a drawing I made to help
illustrate this concept. the moon Click Here to return to the search form.
|
|
|
|
|
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.
UCSB Terms of Use
|
|
|