Newtonian Mechanics – How the Earth’s Tidal Bulge Affects the Moon’s Orbit

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I understand that the moon causes a tidal bulge on Earth, and this tidal bulge moves slightly ahead of the moon due to Earth's rotation.

When reading about why the moon is moving away from the Earth, the most common explanation given is that the tidal bulge slows Earth's rotation and the moon must speed up due to the conservation of angular momentum. This is a poorly worded explanation as it completely skips the part about what is the mechanism of angular momentum transfer. It assumes that we defined a law and the universe must follow it. No, we found that momentum is always conserved in nature, and we noted it as a law.

Upon digging further, I found the most common explanation for the mechanism of angular momentum transfer is that the tidal bulge exerts a gravitational pull on the moon and speeds it up.

The question that is driving me crazy is that higher orbits are supposed to be slower, not faster. If the moon is speeding up, it should fall closer to Earth rather than move away. The moon must be slowing down if it is moving away from Earth, gaining gravitational potential energy and losing kinetic energy. What am I missing here?

How does the transfer of angular momentum lead to reducing kinetic energy and gaining potential energy? What is the mechanism of action here to cause this?

Best Answer

To understand this, let’s start with a simpler example of orbital mechanics. Suppose we have a rocket in a circular orbit that wishes to transfer to a higher circular orbit. This proceeds in the following steps

  1. Burn the engines to accelerate forward. This increases the velocity to be greater than the circular orbital velocity. Thus the rocket is now in an elliptical orbit.

  2. Follow the elliptical orbit halfway around, to its highest point. Kinetic energy has converted to potential energy and the rocket is higher than the previous orbit and traveling slower than the circular orbit at this higher altitude.

  3. Burn the engines to accelerate forward again. This increases the velocity to be equal to the circular orbital at the new altitude. This new circular orbital velocity is smaller than the velocity for the lower circular orbit.

Note, the rocket accelerates forward both times, and yet ends up traveling slower at the higher altitude. The KE gained by the burns plus some of the original KE is changed to potential energy by gravity.

Now, with the moon, the tidal bulge leads the moon. So the moon is gravitationally attracted to a point slightly ahead of the center of the earth. This attraction can be decomposed into a component toward the center and a component forward.

This forward component acts like the rocket burn. It increases the KE, and as the moon moves up the KE is converted to potential energy. The net result being, as before, a propulsive force acting only forward, but a transition to a higher and slower orbit.

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