How can tidal friction on Earth affect the Moon?

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SUMMARY

The discussion centers on the impact of tidal friction on Earth's rotation and its consequent effects on the Moon's orbit. The law of Conservation of Angular Momentum dictates that as Earth's rotation slows due to tidal friction, the Moon's angular momentum increases, resulting in its gradual distancing from Earth. This phenomenon is facilitated by gravitational interactions between the Earth’s tidal bulges and the Moon, which lead to an increase in the Moon's angular momentum. Additionally, the discussion touches on the implications of Earth's rotational cessation and the role of torque in the exchange of angular momentum between the Earth and the Moon.

PREREQUISITES
  • Understanding of Conservation of Angular Momentum
  • Basic knowledge of gravitational forces and tidal interactions
  • Familiarity with Newton's laws of motion, particularly Newton's third law
  • Concept of torque and its role in angular momentum exchange
NEXT STEPS
  • Research the mathematical principles behind Conservation of Angular Momentum in celestial mechanics
  • Explore the effects of tidal forces on other celestial bodies, such as Phobos and Triton
  • Study the phenomenon of precession and its implications for Earth's rotational dynamics
  • Investigate the role of torque in angular momentum transfer between rotating bodies
USEFUL FOR

Astronomers, physicists, students of celestial mechanics, and anyone interested in the dynamics of Earth-Moon interactions and tidal forces.

smithpa9
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I know the following statement to be true, being proven both mathematically, and verified by astronomical observations. What I need help understanding is HOW. . .

"The law of Conservation of Angular Momentum requires that the slowing down of the rotation of the Earth around its axis caused by tidal friction must result in an equal increase of angular momentum of the Moon in its orbital motion around the Earth. . . [this] must result in the increase of its distance from the Earth and the decrease of its linear velocity." George Gammow, Gravity.

Here's the question: How is the impact of tidal friction on the Earth COMMUNICATED to the Moon? By what force is the change in angular momentum of the Earth communiated to the Moon, thus affecting it's angular momentum? Gravity? If so, how?

If by some supernatural occurrence, the Earth suddenly stopped spinning entirely, can I assume that the Moon would immediately move rapidly farther away in its orbit? If so, why? how? what would "push" it away? Would the gravitational attraction to the Earth somehow lessen ?

Thanks for any explanations in as laymen of terms as possible to a former student of first year physics only.
 
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The ocean tides also exert a gravitational force on the moon.

In the supernatural case, moon would not be affected, although the .. supernature.. would start spinning. It's basically Newton's third law.
 
Friction between the rotating Earth causes the Earth to drag the tidal bulges along with it. As a result, they do not line up exactly with the moon but lead it a little. It is the gravitational attraction between these non-aligned tidal bulges and the Moon that pulls forward on the moon, increasing its angular momentum.

If the Moon orbited the Earth faster than or in the opposite direction of the Earth's rotation the tidal bulges would lag behind the moon and you would get the opposite effect, with the moon losing angular momentum and getting closer to the Earth.
 
Last edited:
Janus said:
If the Moon orbited the Earth faster than or in the opposite direction of the Earth's rotation the tidal bulges would lag behind the moon and you would get the opposite effect, with the moon losing angular momentum and getting closer to the Earth.
Like Phobos, Triton, and the theorized lost moons of Venus.
 
Excellent. All good answers. Thank you.
 
I appreciate the answers but I guess there is more to it. The Earth spin axis follows a precession cone, to be completed every 26000 years. Newton has explained why, it's the torque exerted on the equatorial bulge by the gravitational force of the moon and a little of the sun.

Torque is able to exchange angular momentum (vector) between bodies and if you realize that every 13000 years the vector direction is at opposing ends, then it should be clear that enormous amounts of momentum (vector direction) have been exchanged. I could imagine that friction forces in this process also make that exchange imperfect. I wonder if these aspects have been studied.
 

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