# Why do we see the near side of the moon only?

## Main Question or Discussion Point

Why do we see the near side of the moon only??

Why do we see the near side of the moon only???Why not the far side??

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russ_watters
Mentor
The moon is tidally locked to the earth. The same forces that cause our tides cause the moon to rotate once per revolution.

unlike 24 hours and 365 days for earth, for moon periods of spin and orbit are roughly the same and are in opposite directions. [think of a ball rolling on the floor and at the same time rotating in opposite direction]

rbj
The moon is tidally locked to the earth. The same forces that cause our tides cause the moon to rotate once per revolution.
Russ, could you expand on this? i also heard this on the Nova Origins series, but couldn't ask the TV (or at least expect a response).

so, i s'pose some historical accident, the angular velocity of rotation of the moon is very close to the angular velocity of revolution about the Earth (ignoring the fact that both are revolving around the Sun together). now, i would not expect these two angular velocities to be sooo exactly equal that, over thousands of years, no relative turning motion would be detected. it's like with a continuous random variable, say a spinner, the probability of the spinner pointer landing precisely at 20o is zero (as opposed to the probability of the spinner landing between 20o and 21o which is not zero).

so, somehow the angular velocity got close enough, what torque applied to the planet acts as a control system with negative feedback to keep the same side facing us? i don't understand that. it must be because of non-uniformity of the moon's surface. are there more mountains and highlands on the surface facing us than on the far side so that this extra mass is tugged toward the Earth? other than that, i don't get it.

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Wallace
Tidal Locking is an expected phenomenon in systems with satellites, such as the Earth-Moon system. Eventually the Earth will become tidally locked to the Sun for instance (actually this assumes the Sun will last forever since tidal locking would take Billions of years). The Moon didn't start out being tidally locked, but this is the lowest energy configuration and hence what the system moves towards, it doesn't require any chance agreement between the initial orbital speed and rotation.

There is some info on Wiki under 'Tidal Locking' if you are interested.

russ_watters
Mentor
unlike 24 hours and 365 days for earth, for moon periods of spin and orbit are roughly the same and are in opposite directions. [think of a ball rolling on the floor and at the same time rotating in opposite direction]
Yes, except in both cases, they rotate in the same direction.

russ_watters
Mentor
so, i s'pose some historical accident, the angular velocity of rotation of the moon is very close to the angular velocity of revolution about the Earth (ignoring the fact that both are revolving around the Sun together). now, i would not expect these two angular velocities to be sooo exactly equal that, over thousands of years, no relative turning motion would be detected. it's like with a continuous random variable, say a spinner, the probability of the spinner pointer landing precisely at 20o is zero (as opposed to the probability of the spinner landing between 20o and 21o which is not zero).
No historical accident is necessary. Tidal friction is like applying the brakes on your car. It is a very real friction force that slows the rotation.
so, somehow the angular velocity got close enough, what torque applied to the planet acts as a control system with negative feedback to keep the same side facing us?
Tidal friction is literally generated by the changing tides. It is perhaps easiest to understand with the flowing of water in and out of a bay. With any fluid flow, there is an associated viscous fluid friction that dissipates energy and generates heat. But the same force also kneads the earth and moon like a ball of dough, deforming the actual bodies themselves. This deformation is not perfectly elastic, and thus generates energy.

This, by the way, is what causes the plate techtonics and volcanism on some of Jupiter's moons. There is just that much energy being generated by this friction.

rbj
No historical accident is necessary. Tidal friction is like applying the brakes on your car. It is a very real friction force that slows the rotation. Tidal friction is literally generated by the changing tides. It is perhaps easiest to understand with the flowing of water in and out of a bay. With any fluid flow, there is an associated viscous fluid friction that dissipates energy and generates heat.
okay, so some of the rotational energy is converted to heat and eventually radiated. but, still, what torque is applied to change the angular momentum? because losing energy to heat that is radiated is a one-way street. what keeps the angular velocity from decreasing even further past the locked state?

But the same force also kneads the earth and moon like a ball of dough, deforming the actual bodies themselves. This deformation is not perfectly elastic, and thus generates energy.
could it be that this deformation elongates the moon slightly toward the Earth and that hanging blob (being a little closer to the Earth) is the handle by which this torque that i seek is applied?

i don't know, guys, i'm just guessing and trying to sew up the loose ends.

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rbj
There is some info on Wiki under 'Tidal Locking' if you are interested.
yup, thanks for the pointer. i was sorta right, what i called a "blob" is actually called a "bulge" and that is the handle used to torque the rotation of the moon back in case it spins too slowly.

Yes, except in both cases, they rotate in the same direction.
I meant if orbital motion is to your left [as seen from earth lying face up looking at the moon], moon should rotate to right. To an earth observer aren't they in opposite directions?

Chronos
Gold Member
Wallace gave the long and short of it. Read the references he gave again. You are missing the point.

Wallace gave the long and short of it.
[I assume you are talking to me] I understand that. I was saying about the 'opposite direction'.
BTW, I answered the question why moon always shows the same face, which is due to periods being roughly same and opposite from the view of the observer in earth.

[which btw, is due to moon already having its axis which minimizes the torque ("tidal force") which is the preferred ("minimal energy") state oriented towards us] - first statement would be true, even if it is not due to it being minimal state of energy, but simply is the state currently as a matter of fact.

russ_watters
Mentor
I meant if orbital motion is to your left [as seen from earth lying face up looking at the moon], moon should rotate to right. To an earth observer aren't they in opposite directions?
I'm actually not exactly sure what you mean by this, but the easiest way to see it is with a top-down view of the solar system. Most bodies both rotate and revolve counterclockwise.

Chronos
Gold Member
You injected a new meaning into the term 'missing the point', fakr.

I also learnt that Earth's rotation is slowing down due to tidal friction, this means that the orbits would have to distance themselves further.
Isn't moons attraction and the torque it provides enough to speed up the earth??

Janus
Staff Emeritus
Gold Member
I also learnt that Earth's rotation is slowing down due to tidal friction, this means that the orbits would have to distance themselves further.
Isn't moons attraction and the torque it provides enough to speed up the earth??
It is the tidal torque the Moon exerts on the Earth that is slowing the Earth's rotation down.

I also learnt that Earth's rotation is slowing down due to tidal friction, this means that the orbits would have to distance themselves further.
Isn't moons attraction and the torque it provides enough to speed up the earth??
No, while the moon distances itself from the earth, its energy is increased, so it receives energy from the earth. Part of the energy is lost due to tides on oceans, so the rotation of the earth must be reduced.
In future, the earth will rotate at the same speed of the moon orbiting around the earth. They both are tidally locked exactly like Pluto and Chadron.

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I also learnt that Earth's rotation is slowing down due to tidal friction, this means that the orbits would have to distance themselves further.
Isn't moons attraction and the torque it provides enough to speed up the earth??
They were probably refering to axial rotation, not orbital rotation.

For the experts, is it safe to say that IF the sun lasted forever, all the material in the solar system would eventually be pulled into the Sun?

Wallace
For the experts, is it safe to say that IF the sun lasted forever, all the material in the solar system would eventually be pulled into the Sun?
Not at all, there is no reason that the orbits of the planets should decay over time. Eventually you might expect that say the Earth might become tidally locked to the Sun (so that one side was always day and the other always night) but the radius of its orbit wouldn't change. Note that this is only true under the Sun lasting forever hypothetical. In reality the Red Giant phase of the Sun in several billions years time will come long before this and cause significant disruption to the inner solar system, but that is a different issue.

DaveC426913
Gold Member
Is this question satisfactorily answered for the OP?

Good.

We only see the near side of the Moon for the same reason we only see the near side of the No. 3 Hook & Ladder Engine or the near side of the Pawtucket Light and Power building:

in order to see the far side, it would have to be completely transparent.

Tidal lock implies there would be no lock if the earth had no sea?

I reckon it is the bulges of the earth, both the equatorial one and the oceanic one (pointing to the moon at all times), that make it possible for a torque to appear. But then the moon has no equatorial bulge or ocean. So how can the moon experience a torque?

Not at all, there is no reason that the orbits of the planets should decay over time.
In fact they should do the opposite. The sun has an equatorial bulge due to its rotation, therefore we should get something similar to the moon getting slowly further from the earth.

In fact they should do the opposite. The sun has an equatorial bulge due to its rotation, therefore we should get something similar to the moon getting slowly further from the earth.
Oopsa, I take it back, the bulge of the sun is aligned with the orbits of the planets and there is no equivalent to the tidal bulge of the earth-moon system.

russ_watters
Mentor