View Full Version : Synchronous orbit vs. non synchronous orbit

2010-Feb-19, 02:39 AM
The majority of moons are in synchronous orbit with parent planets. However, there are several moons that are not locked in a synchronous orbit. These moons are rather far from their planet. My question is that at what distance should a moon be to escape being locked in a synchronous orbit? For example, the moon Phoebe is not in a synchronous orbit with Saturn. Why not?

2010-Feb-19, 02:51 AM
I don't think you mean synchronous orbit. You mean tidal locking, don't you? A synchronous orbit would mean that the moon went around the earth in 24 hours. I can't think of any examples of that.

grant hutchison
2010-Feb-19, 12:10 PM
If you do mean tidal locking, then it takes time for moons to spin down to tidal lock. The time depends on the tidal torque (which is reduced with increasing distance from the parent body), the mass of the parent body, the initial angular momentum of the moon, and the physical properties which influence how the moon responds to tidal forces.
If you plug in the correct values, you find that close-in moons spin down in a few million years, whereas distant moons take billions of years. You're therefore likely to find close-in moons tidally locked, but the solar system just isn't old enough for more distant moons to become tidally locked.

Grant Hutchison

2010-Feb-19, 06:18 PM
thanks for your answers :)