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Old 10-25-2009, 03:25 PM   #2
Join Date: Jun 2006
Default Re: [Space] Orbital Mechanics of Moons

Originally Posted by Prime Evil View Post
A couple of quick questions for the astronomers out there that don't seem to be covered by GURPS Space...

If a planet has natural satellites, what would be the best way to determine the inclination of the orbits of the various moons with respect to the ecliptic plane?
Most moons orbit in the plane of the planet's equator, so it's the axial tilt of the planet. Our moon is an odd exception, its massive enough compared to the Earth it hasn't yet been tidally dragged into that situation, but for most moons the planet's equatorial bulge will force that in a few million years.

Is the rotational axis of a moon related to the axial tilt of the planet that it orbits or should you randomly determine the axial tilt of the moon with respect to its own orbital plane?
Again there are tidal forces trying to make the rotational axis perpendicular to its orbital plane, but these are weaker. A lot of non-tidelocked moons have axes pointed in all sorts of directions. Use the random system.

The table on p.118 of GURPS Space makes it possible to randomly determine the axial tilt of planets. However, the Wikipedia article on axial tilt seems to suggest that the presence of one or more large moons orbiting a planet may provide a stabilizing influence that prevents large variations in axial tilt over time. Is this true?
Stablize just means the axis doesn't move around a lot, not that there is any particular reason for it to be at a low or high angle to the orbital plane. The forces involved are the same ones that drag the moon into an equatorial orbit, acting the other way. If the equator and the orbital plane of the moon differ, they act to bring them back together. Both planes move, large moons mean the ratio of the tipping of the equatorial plane relative to the orbital plane is going to be larger.
MA Lloyd
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