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Why do moons move away from or approach their primaries?

Launcher poly Australia
Status Vaulted: Feb 02, 2015 09:20


moons ; natural satellites moon and satellite orbits


Clarification is wanted for two aspects of the orbits of moons around planets.

1) Tidal forces are said to cause moons rotating outside the synchronous distance to recede away from the planet, while closer moons lose altitude and approach the planet. The synchronous distance is that at which a moon rotating above the equator in the direction of its rotation would appear stationary.

The catcher should locate and describe formulas for the rates of recession or approach, and use these formulas to calculate the current rates of recession and approach of the two small moons of Mars, Deimos and Phobos (Deimos has an orbital period of 30.3 hours, Phobos 7.6 hours [reference given], while Mars itself rotates in 24.63 hours).

2) What forces act to bring a moon's orbit around its primary to conform with the planet's own rotation, that is, are moons not orbiting strictly in the planet's equatorial plane moved towards conforming? Are there formulas to calculate this? As background, Saturn's main rings orbit strictly in its equatorial plane, while the outer Phoebe ring is tilted at 26.7 degrees to this plane. Do the formulas in (1) still apply for a moon in a retrograde orbit, in particular for Neptune's large moon Triton [reference given]?

Expanded details are given in the attached file ZBL147X.pdf.

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In response to the zomb: Why do moons move away from or approach their primaries?

Darin Ragozzine here presents a soundly-based, detailed, yet readily understandable account of the effects of gravity and tidal forces as act between a planet and its satellites or rings. This is a notably valuable addition to the public domain.

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