why do some moons spin and others dont?

some moons in our solar system keep the same face always to the planet like
ours, and plutos
and others spin, why is that? is that a clue as to how that moon was formed.
to my knowledge most moons in our solar system, are either captured
astroids, formed out of the
material that formed the planets at the same time, or are results of
colisions, like the theory of the
creation of our moon.

On Tue, 6 Jan 2004 16:55:38 -0000, "Simon"
wrote:

some moons in our solar system keep the same face always to the planet like
ours, and plutos
and others spin, why is that? is that a clue as to how that moon was formed.
to my knowledge most moons in our solar system, are either captured
astroids, formed out of the
material that formed the planets at the same time, or are results of
colisions, like the theory of the
creation of our moon.

Do a search on "captured rotation".

Basically the gravitational forces of a primary body (e.g. the Earth)
tug away at a secondary body (e.g. the Moon). Eventually tidal
friction causes the the spin rate of the secondary to synchronise with
it's orbital period around the primary. The same face of the secondary
is therefore presented to the primary.

Earth -- Moon, Pluto -- Charon and the five inner satellites of
Jupiter are some of the bodies that exhibit this phemomenon.

Other phrases you might like to search on are "synchronous rotation"
and "gravitational lock".


--
Pete Lawrence
http://www.pbl33.co.uk
Home of the Lunar Parallax Demonstration Project

The moon IS spinning. Its just that its spin is coincident with its
rotation round the earth so that it always keeps the same face to us.

David
"Simon" wrote in message
...
some moons in our solar system keep the same face always to the planet
like
ours, and plutos
and others spin, why is that? is that a clue as to how that moon was
formed.
to my knowledge most moons in our solar system, are either captured
astroids, formed out of the
material that formed the planets at the same time, or are results of
colisions, like the theory of the
creation of our moon.

"Pete Lawrence" wrote in message
...
On Tue, 6 Jan 2004 16:55:38 -0000, "Simon"
wrote:

some moons in our solar system keep the same face always to the planet
like
ours, and plutos
and others spin, why is that? is that a clue as to how that moon was
formed.
to my knowledge most moons in our solar system, are either captured
astroids, formed out of the
material that formed the planets at the same time, or are results of
colisions, like the theory of the
creation of our moon.

Do a search on "captured rotation".

Basically the gravitational forces of a primary body (e.g. the Earth)
tug away at a secondary body (e.g. the Moon). Eventually tidal
friction causes the the spin rate of the secondary to synchronise with
it's orbital period around the primary. The same face of the secondary
is therefore presented to the primary.

Earth -- Moon, Pluto -- Charon and the five inner satellites of
Jupiter are some of the bodies that exhibit this phemomenon.

Other phrases you might like to search on are "synchronous rotation"
and "gravitational lock".


--
Pete Lawrence
http://www.pbl33.co.uk
Home of the Lunar Parallax Demonstration Project

Could I throw in my query?
The solid angle at the Earth's surface subtended by the moon and the sun are
nearly the same so possible to have total solar eclipse with little margin
for overlap.
Is this just coincidence ?
Do all other planets and moons combinations not have this phenomenon ?

Nigel Cook wrote:
Is this just coincidence ?

Yes. The other part of the coincidence is that we happen to be alive
whilst it is so -- Moon is gradually receding so, some time in the
future, the only "totals" will be annular.

Best,
Stephen

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"Pete Lawrence" wrote in message
...
On Tue, 6 Jan 2004 16:55:38 -0000, "Simon"
wrote:

some moons in our solar system keep the same face always to the planet
like
ours, and plutos
and others spin, why is that? is that a clue as to how that moon was
formed.
to my knowledge most moons in our solar system, are either captured
astroids, formed out of the
material that formed the planets at the same time, or are results of
colisions, like the theory of the
creation of our moon.

Do a search on "captured rotation".

Basically the gravitational forces of a primary body (e.g. the Earth)
tug away at a secondary body (e.g. the Moon). Eventually tidal
friction causes the the spin rate of the secondary to synchronise with
it's orbital period around the primary. The same face of the secondary
is therefore presented to the primary.

Earth -- Moon, Pluto -- Charon and the five inner satellites of
Jupiter are some of the bodies that exhibit this phemomenon.

Other phrases you might like to search on are "synchronous rotation"
and "gravitational lock".


Could I throw in my query?

Mecury's rotation period is not the same as its orbital period, although one
would possibly expect this.

Could this be due to Mercury's eccentric orbit? It seems (after running a
simulation on "Redshift 4" of Mercury as seen from the Sun) that Mercury's
rotation seems to be "locked" to its angular movement at perihelion, and for
the rest of the orbit it rotates as seen from the sun, resulting in the
observed rotation rate greater than the orbital period.

Does anyone know if this is the case?
--
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OO 0 OO
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"Yokel" now posts via a spam-trap account.
Replace my alias with stevejudd to reply.

"Yokel" wrote in message
...


Earth -- Moon, Pluto -- Charon and the five inner satellites of
Jupiter are some of the bodies that exhibit this phemomenon.

Other phrases you might like to search on are "synchronous rotation"
and "gravitational lock".


Could I throw in my query?

Mecury's rotation period is not the same as its orbital period, although
one
would possibly expect this.

Could this be due to Mercury's eccentric orbit? It seems (after running a
simulation on "Redshift 4" of Mercury as seen from the Sun) that Mercury's
rotation seems to be "locked" to its angular movement at perihelion, and
for
the rest of the orbit it rotates as seen from the sun, resulting in the
observed rotation rate greater than the orbital period.

Does anyone know if this is the case?
--


Not quite, but if you consider that Mercury's 3:2 rotationrbit lock is
stable, then if the planet started with faster rotation, and gradually
slowed down, it would stop in the 3:2 mode and never get to 1:1 rotation
rate.

In binary stars with eccentric orbits, and periods more than a few days,
there is an expectation that the rotations will be "pseudosynchronous",
i.e., at the angular rate near periastron rather than synchronised to the
period. But the expectation is not always borne out.

--
Mike Dworetsky

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