A-L-E-X wrote:Direct measurement!
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What evidence do we have that the moon is actually moving further away from the Earth?
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:
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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:
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).
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:
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.