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19 Sep 2019 07:57

No, instead it would get further away faster!
Since there is a bigger difference between the rotation of Earth and the rotation of the Moon around Earth, friction will also be increasing thus the effect that is making the moon get further would be increased.
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19 Sep 2019 15:29

What evidence do we have that the moon is actually moving further away from the Earth?  Logically, shouldn't it be moving closer since gravity is an attractive force and tidal forces should be slowing down the moon's revolution and causing it to get closer with time?
 
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19 Sep 2019 17:07

A-L-E-X wrote:
Source of the post Logically, shouldn't it be moving closer since gravity is an attractive force and tidal forces should be slowing down the moon's revolution and causing it to get closer with time?

The slower the further!
A satellite that orbits the earth in 90 minutes, has a speed of about 28400 km/h. (Very roughly rounded.)
A satellite in a geostationary orbit, which orbits the center of the earth once every 24 hours, has a speed of about 11100 km/h. So it's much slower!
This means the slower the moon gets the farther it gets away from the earth! (Incidentally, the moon has about 3500 km/h. (Very roughly rounded.))
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19 Sep 2019 22:02

Wow thats interesting, I thought that the slower something moves the more prone it is to falling down to earth.  I guess I thought that way because that is my experience with airplanes :P  I thought the moon moving more quickly meant that its faster movement was counteracting the earth's gravity and it would have to move faster to reach escape velocity, like rockets have to do to escape earth.
 
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20 Sep 2019 01:42

The further two objects are away from each other, the weaker is the gravitational force between the two.
The gravity decreases with the square of the distance.
That is, when two objects are 200,000 km apart, the gravity is 4 times as weak as when they are 100,000 km apart.
In other words, a geostationary orbit is about six times as far from Earth as a 90-minute satellite orbit. That means the gravity between geostationary orbit and 90 minutes orbit is 36 times weaker!
However, the speed of an object in geostationary orbit is only about 2.6 times slower than the 90 minute orbit.
Measured by the much weaker gravity, the object on the geo-orbit has actually become much faster!
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20 Sep 2019 13:34

A-L-E-X wrote:
Source of the post What evidence do we have that the moon is actually moving further away from the Earth? 

Direct measurement!  The Apollo missions left retro-reflecting mirrors on the lunar surface, which observatories on Earth can bounce lasers off of to measure the distance to the Moon with millimeter precision. The Moon's orbit is measured to be expanding at a rate of 3.8cm per year.  (But this rate is not constant on geological timescales.  It was faster when the Moon was closer, and it also depends on the arrangement of continents and oceans, which changes how efficiently the Earth dissipates the tides, and hence the pull on the Moon.)

The physics of the tidal interaction is also quite well understood (though the details are highly complex), so it is straightforward to predict that the Moon, being tidally locked to Earth and having an orbital period slower than Earth's rotation period, must have an expanding orbit.  I will attempt to explain why in more detail below:

A-L-E-X wrote:
Source of the post  Logically, shouldn't it be moving closer since gravity is an attractive force and tidal forces should be slowing down the moon's revolution and causing it to get closer with time?

No, this is common confusion. :)  Gravity, being an attractive force, is what keeps the Moon orbiting Earth, instead of flying off in a straight line.  The reason for the expanding orbit is more complex, and involves the fact that the shapes of the Earth and Moon are deformable.  That is, the gravity distorts them, and then those distortions have an additional gravitational effect on one another!

If gravity was all there was to it, and if we treated the Earth and Moon as two perfect, undeformable spheres, then the Moon would simply keep following its same orbit forever.  (We'll also ignore the emission of gravitational waves, since that effect is so weak we may as well.)  But the Earth and Moon are deformable.   The side of the Earth facing the Moon is pulled by it more strongly than the side facing away from it.  And the gravitational force of the Moon always points toward the Moon, but this is slightly different direction for one point on Earth than another.  So the Earth's shape is distorted, elongated in the direction toward the Moon, and squeezed about its middle:

Image

This is the effect of the tidal force, and it is also exactly the same thing that causes "sphaghettification" if you fell into a black hole.  

For the Earth and Moon, the tidal force changes their shapes from oblate spheroids to triaxial ellipsoids, like this (greatly exaggerated obviously).

Image


Now then to understand the changing orbit of the Moon, all we need is to consider the gravitational pull of these shapes on one other.  Maybe that sounds really hard, but the basic idea is actually pretty simple.  First, recall that the Moon is tidally locked with the Earth -- it always shows the same side to us.  Second, realize the Earth is spinning faster than the Moon is orbiting, and in the same direction.  So the bulge raised on the Earth that would be facing the Moon is dragged by Earth's rotation, always slightly leading the Moon.  The image to hold in your mind is this:

Image


The bulge on the Earth nearest the Moon is always slightly in front of the Moon. So it is pulling the Moon slightly forwards, accelerating it constantly.  The Moon gains orbital energy, expanding its orbit!  (Paradoxically, this also makes the Moon slow down, since higher orbits are slower.  Also we might ask about the bulge on the farside.  Doesn't it pull the Moon backward?  Yes, but since it's farther away, its pull is weaker.)

Of course, such gain in energy cannot occur for free.  Where did the energy come from?  As the bulge on the Earth pulls the Moon ahead, the Moon is also pulling back on the bulge!  That's Newton's 3rd Law, or "for every force there is an equal and opposite force".  So there is a small torque on the Earth, acting opposite to its spin, and this slows down the Earth's rotation!  Earth's days grow longer over time because of it, and this too is measurable!  (However, the change in Earth's rotation is greatly complicated by several other factors, such as melting glaciers and movement of crustal plates, which change the Earth's moment of inertia!)

Probably the best physics principle and conservation law to apply to the whole problem of Earth and Moon tidally interacting with each other, is the conservation of angular momentum.  Because the Moon orbits prograde and slower than the Earth rotates, the tidal interaction transfers angular momentum from Earth's spin to the Moon's orbit, but keeps the total angular momentum the same.

Tidal interactions like these are universal.  Another great example: Mars' inner moon Phobos orbits in the same direction but faster than Mars rotates, so Phobos is always flying in front of the nearer bulge it raises on Mars, and so the bulge pulls back on Phobos and slows it down.  Phobos' orbit is shrinking and eventually it will be torn apart when it falls below the Roche limit.  This may happen within a few ten million years!  (Unless humanity prevents it...?)

I hope that helps explain the tidal interaction and changing Moon's orbit for you.  It's very complicated, but I think also one of the coolest and most interesting things in all of astrophysics.  Learning it made me look at ocean tides in a whole new way. :)
 
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20 Sep 2019 16:55

Cantra wrote:
Source of the post What if the moon revolved around Earth in the opposite direction? Getting closer each year rather than farther?
Salvo wrote:
Source of the post No, instead it would get further away faster!  Since there is a bigger difference between the rotation of Earth and the rotation of the Moon around Earth, friction will also be increasing thus the effect that is making the moon get further would be increased.


Actually, Cantra's guess was correct. :)  Salvo, your reasoning is almost right, in that the angle between the tidal bulges on Earth and the Moon would be larger, so a larger component of the force of the bulges acting on the Moon would be pulling along the Moon's orbit.  It is correct that the Moon's orbit would change faster.

But, the change is in the other direction.  With the Moon orbiting the other way, the pull would be acting opposite the orbital motion.  So instead of accelerating the Moon into a higher orbit, it would decelerate the Moon into a lower orbit!  Eventually, the Moon would crash into the Earth (or be torn apart at the Roche limit first).

A real-world example of this situation is Neptune's moon Triton, which has a retrograde (backwards) orbit, and hence the tidal interaction will shrink its orbit until eventually it is torn apart, probably in a few billion years.

Tidal evolution of the Neptune-Triton system.

By the way, this is a nice prediction of tidal theory: retrograde orbiting moons are inherently unstable to tidal evolution, so we should not expect to find many retrograde moons very close to their planets.  (Larger retrograde orbits are much more likely, since their orbits would change much more slowly -- tidal force is proportional to inverse cube of the distance, and the rate of change of the size of the orbit is inversely proportional to the 11/2 power of distance).  If we were to find a retrograde moon in a close orbit, then it must have evolved to that orbit very recently, and will not remain there for very long.
 
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20 Sep 2019 17:52

Watsisname wrote:
Cantra wrote:
Source of the post What if the moon revolved around Earth in the opposite direction? Getting closer each year rather than farther?
Salvo wrote:
Source of the post No, instead it would get further away faster!  Since there is a bigger difference between the rotation of Earth and the rotation of the Moon around Earth, friction will also be increasing thus the effect that is making the moon get further would be increased.


Actually, Cantra's guess was correct. :)  Salvo, your reasoning is almost right, in that the angle between the tidal bulges on Earth and the Moon would be larger, so a larger component of the force of the bulges acting on the Moon would be pulling along the Moon's orbit.  It is correct that the Moon's orbit would change faster.

But, the change is in the other direction.  With the Moon orbiting the other way, the pull would be acting opposite the orbital motion.  So instead of accelerating the Moon into a higher orbit, it would decelerate the Moon into a lower orbit!  Eventually, the Moon would crash into the Earth (or be torn apart at the Roche limit first).

A real-world example of this situation is Neptune's moon Triton, which has a retrograde (backwards) orbit, and hence the tidal interaction will shrink its orbit until eventually it is torn apart, probably in a few billion years.

Tidal evolution of the Neptune-Triton system.

By the way, this is a nice prediction of tidal theory: retrograde orbiting moons are inherently unstable to tidal evolution, so we should not expect to find many retrograde moons very close to their planets.  (Larger retrograde orbits are much more likely, since their orbits would change much more slowly -- tidal force is proportional to inverse cube of the distance, and the rate of change of the size of the orbit is inversely proportional to the 11/2 power of distance).  If we were to find a retrograde moon in a close orbit, then it must have evolved to that orbit very recently, and will not remain there for very long.

By what process would the moon have formed? Would it have been captured and forced into this orbit? It probably wouldn't look like our moon.
 
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20 Sep 2019 18:46

Cantra wrote:
Source of the post By what process would the moon have formed? Would it have been captured and forced into this orbit? It probably wouldn't look like our moon.

Capturing a large moon like this around the Earth seems very unlikely.  Something would have to slow it down as it was passing Earth, or else it would just fly off again.  Maybe it could happen very early on if there was still an accretion disk around the Earth for the Moon-to-be to slam into.

Otherwise, I think it could happen very similarly as under the currently accepted "Big Splash / Giant Impact" hypothesis, where roughly Mars-sized body hit the Earth and splashed a huge portion of itself and the Earth's mantle, forming a ring or Synestia around the Earth which the Moon then condensed out of.  Maybe if the impact had hit the Earth on the other side, against its rotation, it could have formed a retrograde debris disk and a retrograde Moon.  

Then again, maybe such a big impact would also change Earth's spin axis, or more than reverse it.  Maybe it's not a coincidence that from an impact like that, the Moon's orbit and Earth's rotation go about the same way?  (I don't actually know). :)
 
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20 Sep 2019 19:05

Watsisname wrote:
Cantra wrote:
Source of the post By what process would the moon have formed? Would it have been captured and forced into this orbit? It probably wouldn't look like our moon.

Capturing a large moon like this around the Earth seems very unlikely.  Something would have to slow it down as it was passing Earth, or else it would just fly off again.  Maybe it could happen very early on if there was still an accretion disk around the Earth for the Moon-to-be to slam into.

Otherwise, I think it could happen very similarly as under the currently accepted "Big Splash / Giant Impact" hypothesis, where roughly Mars-sized body hit the Earth and splashed a huge portion of itself and the Earth's mantle, forming a ring or Synestia around the Earth which the Moon then condensed out of.  Maybe if the impact had hit the Earth on the other side, against its rotation, it could have formed a retrograde debris disk and a retrograde Moon.  

Then again, maybe such a big impact would also change Earth's spin axis, or more than reverse it.  Maybe it's not a coincidence that from an impact like that, the Moon's orbit and Earth's rotation go about the same way?  (I don't actually know). :)

Unless of course the Earth already had a moon, and this moon came by and collided with it, then afterwards the new moon formed would have a retrograde orbit, though it'd be somewhat eccentric.
 
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23 Sep 2019 00:01

Watsisname wrote:
But, the change is in the other direction.

Interesting! I wasn't sure so I searched on internet, but I probably ended in the wrong site.  :)
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23 Sep 2019 15:47

Lets say that we had a planet with 0.92 Earth gravity, that had evolved humans on it, how would they be different?

With this being it's moon.

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26 Sep 2019 16:21

Watsisname wrote:
A-L-E-X wrote:
Source of the post What evidence do we have that the moon is actually moving further away from the Earth? 

Direct measurement!  The Apollo missions left retro-reflecting mirrors on the lunar surface, which observatories on Earth can bounce lasers off of to measure the distance to the Moon with millimeter precision. The Moon's orbit is measured to be expanding at a rate of 3.8cm per year.  (But this rate is not constant on geological timescales.  It was faster when the Moon was closer, and it also depends on the arrangement of continents and oceans, which changes how efficiently the Earth dissipates the tides, and hence the pull on the Moon.)

The physics of the tidal interaction is also quite well understood (though the details are highly complex), so it is straightforward to predict that the Moon, being tidally locked to Earth and having an orbital period slower than Earth's rotation period, must have an expanding orbit.  I will attempt to explain why in more detail below:

A-L-E-X wrote:
Source of the post  Logically, shouldn't it be moving closer since gravity is an attractive force and tidal forces should be slowing down the moon's revolution and causing it to get closer with time?

No, this is common confusion. :)  Gravity, being an attractive force, is what keeps the Moon orbiting Earth, instead of flying off in a straight line.  The reason for the expanding orbit is more complex, and involves the fact that the shapes of the Earth and Moon are deformable.  That is, the gravity distorts them, and then those distortions have an additional gravitational effect on one another!

If gravity was all there was to it, and if we treated the Earth and Moon as two perfect, undeformable spheres, then the Moon would simply keep following its same orbit forever.  (We'll also ignore the emission of gravitational waves, since that effect is so weak we may as well.)  But the Earth and Moon are deformable.   The side of the Earth facing the Moon is pulled by it more strongly than the side facing away from it.  And the gravitational force of the Moon always points toward the Moon, but this is slightly different direction for one point on Earth than another.  So the Earth's shape is distorted, elongated in the direction toward the Moon, and squeezed about its middle:

Image

This is the effect of the tidal force, and it is also exactly the same thing that causes "sphaghettification" if you fell into a black hole.  

For the Earth and Moon, the tidal force changes their shapes from oblate spheroids to triaxial ellipsoids, like this (greatly exaggerated obviously).

Image


Now then to understand the changing orbit of the Moon, all we need is to consider the gravitational pull of these shapes on one other.  Maybe that sounds really hard, but the basic idea is actually pretty simple.  First, recall that the Moon is tidally locked with the Earth -- it always shows the same side to us.  Second, realize the Earth is spinning faster than the Moon is orbiting, and in the same direction.  So the bulge raised on the Earth that would be facing the Moon is dragged by Earth's rotation, always slightly leading the Moon.  The image to hold in your mind is this:

Image


The bulge on the Earth nearest the Moon is always slightly in front of the Moon. So it is pulling the Moon slightly forwards, accelerating it constantly.  The Moon gains orbital energy, expanding its orbit!  (Paradoxically, this also makes the Moon slow down, since higher orbits are slower.  Also we might ask about the bulge on the farside.  Doesn't it pull the Moon backward?  Yes, but since it's farther away, its pull is weaker.)

Of course, such gain in energy cannot occur for free.  Where did the energy come from?  As the bulge on the Earth pulls the Moon ahead, the Moon is also pulling back on the bulge!  That's Newton's 3rd Law, or "for every force there is an equal and opposite force".  So there is a small torque on the Earth, acting opposite to its spin, and this slows down the Earth's rotation!  Earth's days grow longer over time because of it, and this too is measurable!  (However, the change in Earth's rotation is greatly complicated by several other factors, such as melting glaciers and movement of crustal plates, which change the Earth's moment of inertia!)

Probably the best physics principle and conservation law to apply to the whole problem of Earth and Moon tidally interacting with each other, is the conservation of angular momentum.  Because the Moon orbits prograde and slower than the Earth rotates, the tidal interaction transfers angular momentum from Earth's spin to the Moon's orbit, but keeps the total angular momentum the same.

Tidal interactions like these are universal.  Another great example: Mars' inner moon Phobos orbits in the same direction but faster than Mars rotates, so Phobos is always flying in front of the nearer bulge it raises on Mars, and so the bulge pulls back on Phobos and slows it down.  Phobos' orbit is shrinking and eventually it will be torn apart when it falls below the Roche limit.  This may happen within a few ten million years!  (Unless humanity prevents it...?)

I hope that helps explain the tidal interaction and changing Moon's orbit for you.  It's very complicated, but I think also one of the coolest and most interesting things in all of astrophysics.  Learning it made me look at ocean tides in a whole new way. :)

Wow, that explanation really helped (especially the picture!)  I can totally picture how the tidal budget offset imparts momentum to the moon and causes it to resist the earth's gravity better.  Do you think in the future, humanity would be able to prevent the moon from escaping?  We have tens of millions of years to do this, I assume?   It seems to be more practical than some ideas I've seen of moving the earth's orbit further out to avoid the sun baking us when it reaches the red giant stage!
 
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26 Sep 2019 19:40

Great!  I'm glad the explanations have helped. :)

A-L-E-X wrote:
Source of the post Do you think in the future, humanity would be able to prevent the moon from escaping?

My understanding is that the Moon will never escape.  The rate at which the Moon moves away by the tidal interaction depends very sensitively on distance.  As the Moon moves away, the tidal force weakens, and so it moves away more slowly.

A simple calculation would suggest that the Earth's rotation would slow down enough to become tidally locked with the Moon (so the length of day would equal length of lunar month).  At that point no further tidal evolution occurs, the Moon's orbit would stay fixed, and the Moon would only be visible from one side of Earth -- just like the mutual locking of Pluto and Charon.  This would happen when the length of the day and month are about 50 current days, which implies the Moon would be only about 50% farther from Earth than it is now, and still well within Earth's Hill Sphere.  So no risk of it being pulled away. 

Aside: if anyone would like to try the above calculation, it really is simple: just set the total change in angular momentum (of the Earth's rotation, moon's orbit, and moon's spin [though the last is so small it can be safely ignored]) to be zero, along with the condition that the Moon's final spin rate equals the Earth's final spin rate.

A more detailed calculation changes the conclusions a little.  It suggests the Earth will never become tidally locked with the Moon, and the Moon doesn't stop receding in the next 5 billion years.  The distance it recedes to ends up being only about 25% farther than today.  The Earth's rotation also doesn't slow down as much, and instead its obliquity increases more.

Image
 
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27 Sep 2019 00:24

^ This was actually something I was quite interested in knowing. I cannot tell you how many times I've heard people cry out that the Moon will spin off into the void someday.
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