Rotation of solar system moons

Is there a credible source of up to date info. on the rotation periods
of planetary moons across our solar system? I want to know for
example, do the Gaililean moons all have independent rotation periods
or are they all in 'captured' orbits around Jupiter?

What about Phobos, Deimos, Titan, Triton and Charon?

Second question is: Would our own Moon have had more than 1/6th of a g
of gravity if it had a significantly faster rotation rate than its
present 'captured' rotation about Earth? (i.e. if a 'day' on the Moon
was longer than a sidereal 'month' as at present). What is the
relationship if any between the spin rate of a planetary body and the
size of its gravity field?

Thanks
Abdul Ahad
http://uk.geocities.com/aa_spaceagent/astronomy.html

Hi there. You posted:

Is there a credible source of up to date info. on the rotation periods
of planetary moons across our solar system? I want to know for
example, do the Gaililean moons all have independent rotation periods
or are they all in 'captured' orbits around Jupiter?

Most of the moons of the major planets are tidally "locked" so their rotation
period is equal to their orbital period. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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David Knisely wrote in message ...

Most of the moons of the major planets are tidally "locked" so their rotation
period is equal to their orbital period. Clear skies to you.
--

Hi,
I was after the *specific* details about each moon's rotation vs
revolution. While *most* of the moons in our solar system are in
captured orbits (tidally "locked") around their planets, I am thinking
there has to be a few that have independent rotations? I am probably
asking for the impossible as many moons are too small to bounce radar
off them and get reliable estimates of rotation based on radar echoes,
doppler shifts, etc. but if I could *know* the score with the bigger
ones like Titan, Triton, Charon then that's sufficient.

I know, for example, Triton's revolution around Neptune is in a
*retrograde* orbit i.e. its orbital motion is in a direction opposite
to the planet's rotation. But does Triton have an independent
rotation of its own or is it also *locked* to Neptune in the same way
that our Moon is locked with the Earth?

The second question is more dynamical... I would have thought that if
a planetary object spins faster on its axis, then it has a stronger
graviatational field...is this a fair assumption? Hence if our Moon
had an independent rotation, would it have had more than the present
1/6th of a g?

Abdul Ahad

Abdul Ahad wrote:

Is there a credible source of up to date info. on the rotation periods
of planetary moons across our solar system? I want to know for
example, do the Gaililean moons all have independent rotation periods
or are they all in 'captured' orbits around Jupiter?

What about Phobos, Deimos, Titan, Triton and Charon?

Have you tried NASA's "Planetary Fact Sheets" at

http://nssdc.gsfc.nasa.gov/planetary/planetfact.html ? There's a
fair bit of data on at least the larger satellites. I would think
NASA to be at least reasonably credible, but I don't know how
up-to-date the pages are.

Second question is: Would our own Moon have had more than 1/6th of a g
of gravity if it had a significantly faster rotation rate than its
present 'captured' rotation about Earth? (i.e. if a 'day' on the Moon
was longer than a sidereal 'month' as at present). What is the
relationship if any between the spin rate of a planetary body and the
size of its gravity field?

No; an object's gravitational attraction is completely unaffected by
its spin, being a function of its mass alone. For an observer on the
surface of a rotating planet or moon (anywhere but at a pole) the
body's spin actually makes its gravity seem weaker because of
"centrifugal force". This effect is pretty small, though: for example
on the earth's equator objects seem only about 0.2% lighter than they
would if the planet didn't rotate, and of course there's even less
difference at higher latitudes. The Fact Sheets at the above link
give both the "surface gravity" and "surface acceleration" for each
body, showing the effect of its spin on an object's weight at the equator.

--
Odysseus

In message , Abdul Ahad
writes
David Knisely wrote in message
...

Most of the moons of the major planets are tidally "locked" so their
rotation
period is equal to their orbital period. Clear skies to you.
--

Hi,
I was after the *specific* details about each moon's rotation vs
revolution. While *most* of the moons in our solar system are in
captured orbits (tidally "locked") around their planets, I am thinking
there has to be a few that have independent rotations? I am probably
asking for the impossible as many moons are too small to bounce radar
off them and get reliable estimates of rotation based on radar echoes,
doppler shifts, etc. but if I could *know* the score with the bigger
ones like Titan, Triton, Charon then that's sufficient.

It's known by direct observation that nearly all the moons are tidally
locked - AFAIK the only exception is Hyperion, which is in a chaotic
rotation. Some of the small moons haven't been observed directly -
Phoebe comes to mind (cancel that - I just found that Phoebe has a 4 day
rotation period. That's interesting, because most of the rocks in the
solar system rotate in a few hours. I'd guess it's feeling _some_ tidal
effect).
Pluto and Charon are _mutually_ locked. Triton is locked.
--
Rabbit arithmetic - 1 plus 1 equals 10
Remove spam and invalid from address to reply.

Have you tried NASA's "Planetary Fact Sheets" at

http://nssdc.gsfc.nasa.gov/planetary/planetfact.html ? There's a
fair bit of data on at least the larger satellites. I would think
NASA to be at least reasonably credible, but I don't know how
up-to-date the pages are.


Thanks! yes, the NASA factsheets are a rich source of info, last
updated 2001, so takes account of many recent spacecraft visits.

In summary, ALL the major moons from Mars' tiny Phobos and Deimos all
the way up to Titan and Ganymede are in synchronous orbits. Only
exceptions are two moons of Saturn: Hyperion (*chaotic* rotation) and
Phoebe - the latter has all the hallmarks of a captured asteroid!

Abdul Ahad
http://uk.geocities.com/aa_spaceagent/astronomy.html