Seasons on gas giant moons

[Hephaestus]

Here's something I got to thinking about while considering Earth-like
moons around gas giants in the habitable zones around stars. What
would seasons be like? (I will use the term "year" for one orbit of
the gas giant, "month" for the time it takes for the gas giant to go
from "full" to "new" and back to "full", and "day" for the time
between sunrises on the moon). There are a number of effects to
consider:

1) the tilt of the moon's spin relative to the orbit around the gas
giant
2) the tilt of the moon's orbit relative to the gas giant's orbit

and less importantly, the eccentricities of the two orbits. Here are
a few things I would like to get a handle on:

what is the likely ranges of 1) and 2) ?

For a habitable moon around a sun-like star, the "year" will be ~1
Earth year; Titan is tidally locked with a 16 day "day/month".
Jupiter's major satellites range from a 2-16 days, with the largest
one having a 7 day "day/month". These are all significantly smaller
than Earth, though; would something Earth-sized also likely be
tidally locked?

Just think: somewhere out there, there could be a planet with normal
year long seasons, but where it's usually cold on Wednesday but warm
for the weekends :)

Though the fact that day and night last three and a half days could
be somewhat annoying ...

[Pat Flannery]

Hephaestus wrote:

>
>For a habitable moon around a sun-like star, the "year" will be ~1
>Earth year; Titan is tidally locked with a 16 day "day/month".
>Jupiter's major satellites range from a 2-16 days, with the largest
>one having a 7 day "day/month". These are all significantly smaller
>than Earth, though; would something Earth-sized also likely be
>tidally locked?
>
There is a peculiar resonance going on between Earth and Venus that
leads to Venus always having the same face displayed toward us at
closest approach; considering the great distance between the two
planets, to me this suggests that objects tend to get tidally locked
sooner or later despite great distances.
A quick check of my "Atlas Of The Solar System" shows that _all_ the
known moons of Jupiter and Saturn are presumed to be tidally locked.
Saturn's "shepherd moons" might make an interesting case in this regard,
it will be fun to see if Cassini finds them locked also.
One of the big problems in regard to habitable planets around a gas
giant is going to be the radiation fields; on the side facing the star
it orbits they will probably come out a fair distance, but they really
extend out behind the planet- if Jupiter is anything to go by- and might
sterilize the surface of any habitable moon as it passes through them in
its orbit: http://blueox.uoregon.edu/~courses/BrauImages/Chap11/FG11_011.jpg

Pat

[Doug...]

In article >,
says...
>
> <snip>
>
> One of the big problems in regard to habitable planets around a gas
> giant is going to be the radiation fields; on the side facing the star
> it orbits they will probably come out a fair distance, but they really
> extend out behind the planet- if Jupiter is anything to go by- and might
> sterilize the surface of any habitable moon as it passes through them in
> its orbit: http://blueox.uoregon.edu/~courses/BrauImages/Chap11/FG11_011.jpg

Well... that depends on whether or not such a moon had its own strong
magnetic field, wouldn't it? If you speculate that a gas giant is
within a star's habitable zone and that it has a moon that has similar
composition to Earth, then it would become a question of whether or not
it would retain enough rotation to generate a dynamo effect from its
molten core that would create a magnetic field.

If you *did* have a magnetic field comparable to Earth's, wouldn't that
serve to protect said moon from the radiation effects of the gas giant's
own magnetic field?

Doug

[Joe Strout]

In article >,
Pat Flannery > wrote:

> One of the big problems in regard to habitable planets around a gas
> giant is going to be the radiation fields; on the side facing the star
> it orbits they will probably come out a fair distance, but they really
> extend out behind the planet- if Jupiter is anything to go by- and might
> sterilize the surface of any habitable moon as it passes through them in
> its orbit: http://blueox.uoregon.edu/~courses/BrauImages/Chap11/FG11_011.jpg

Sterilization is relative. What might be lethal doses of radiation to
Earth life may well be a handy source of energy to life forms that had
evolved in it. I can see no a priori reason why evolution couldn't cope
with quite high radiation levels.

(It reminds me of this other planet I heard of, where the whole
atmosphere and even the oceans were polluted with an extremely reactive,
toxic molecule, but in the end this just kick-started a whole new phase
of evolution.)

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

[Pat Flannery]

Doug... wrote:

>Well... that depends on whether or not such a moon had its own strong
>magnetic field, wouldn't it? If you speculate that a gas giant is
>within a star's habitable zone and that it has a moon that has similar
>composition to Earth, then it would become a question of whether or not
>it would retain enough rotation to generate a dynamo effect from its
>molten core that would create a magnetic field.
>
>If you *did* have a magnetic field comparable to Earth's, wouldn't that
>serve to protect said moon from the radiation effects of the gas giant's
>own magnetic field?
>

That's a very interesting thought; but it implies a molten iron core in
the Earth-sized moon to generate the magnetic field.
Assuming we are dealing with a gas giant planet in a sun's habitable
zone, would such a planet/moon evolve? It's thought that all the major
satellites of the gas giants consist primarily of ices and the lighter
types of rock such as silicates, not the heavier elements, such as iron.
So to get this to work, you have to have a big and hot star whose
habitable zone was out at the distance where you would find gas giants,
and I would think that most of the ices and gases that would have made a
proto-planet into a gas giant would have boiled off during the planet's
accretion, so that you would end up with a huge rocky planet instead of
a gas giant. (between Earth to Neptune sized?)
Alternately you could have the proposed predator gas giant that spirals
in toward the sun- eating all the other planets in the system as it
goes- until it arrives at a Sun-type star's habitable zone- I imagine
it _might_ be possible (though I think highly unlikely) for such a
planet to grab an Earth-type world as a satellite on it inexorable way
in, but the whole situation sounds unstable and short lived in
biological evolutionary terms; I doubt the captured planet's orbit would
be anywhere near circular, and that could lead to some pretty
devastating tidal effects as it orbits the gas giant. besides that,
interaction with the gas giant's own moons could lead to either
collision with them, or a perturbation of the captured planet's orbit to
where it either collided with the gas giant or was hurled out of its orbit.
Frankly, I wouldn't like to look up in the sky and see something like
Jupiter hanging there, far larger than the full Moon.

Pat

[Doug...]

In article >,
says...
> Doug... wrote:
>
> >Well... that depends on whether or not such a moon had its own strong
> >magnetic field, wouldn't it? If you speculate that a gas giant is
> >within a star's habitable zone and that it has a moon that has similar
> >composition to Earth, then it would become a question of whether or not
> >it would retain enough rotation to generate a dynamo effect from its
> >molten core that would create a magnetic field.
> >
> >If you *did* have a magnetic field comparable to Earth's, wouldn't that
> >serve to protect said moon from the radiation effects of the gas giant's
> >own magnetic field?
> >
>
> That's a very interesting thought; but it implies a molten iron core in
> the Earth-sized moon to generate the magnetic field.
> Assuming we are dealing with a gas giant planet in a sun's habitable
> zone, would such a planet/moon evolve? It's thought that all the major
> satellites of the gas giants consist primarily of ices and the lighter
> types of rock such as silicates, not the heavier elements, such as iron.
> So to get this to work, you have to have a big and hot star whose
> habitable zone was out at the distance where you would find gas giants,
> and I would think that most of the ices and gases that would have made a
> proto-planet into a gas giant would have boiled off during the planet's
> accretion, so that you would end up with a huge rocky planet instead of
> a gas giant. (between Earth to Neptune sized?)
> Alternately you could have the proposed predator gas giant that spirals
> in toward the sun- eating all the other planets in the system as it
> goes- until it arrives at a Sun-type star's habitable zone- I imagine
> it _might_ be possible (though I think highly unlikely) for such a
> planet to grab an Earth-type world as a satellite on it inexorable way
> in, but the whole situation sounds unstable and short lived in
> biological evolutionary terms; I doubt the captured planet's orbit would
> be anywhere near circular, and that could lead to some pretty
> devastating tidal effects as it orbits the gas giant. besides that,
> interaction with the gas giant's own moons could lead to either
> collision with them, or a perturbation of the captured planet's orbit to
> where it either collided with the gas giant or was hurled out of its orbit.
> Frankly, I wouldn't like to look up in the sky and see something like
> Jupiter hanging there, far larger than the full Moon.

Actually, several of the extrasolar planets that have been discovered in
the past few years are gas giants that orbit their stars either as close
as one AU, or closer. They're called "hot Jupiters" in some of the
literature.

Now, the question is, could a hot Jupiter form in a second-generation
star's planetary system? The planetary nebula/disk of a second-
generation star incorporates the heavy elements formed in a massive star
that went kablooey, right? Perhaps the systems with hot Jupiters are
all first-generation stars, and the lack of heavier elements caused a
single gas giant to form in the close-in range.

But if a hot Jupiter *could* form in a second-generation star's system,
then it might make sense that some or all of its moons would be formed
of rock, not ice. That would mean that they could have roughly
terrestrial compositions.

The bigger problem in generating a strong magnetic field, as I see it,
would be the rotation issue. Every sizable moon of the gas giants in
our own solar system are tidally locked to their primary. I wonder just
how fast a moon with a molten nickel-iron core would have to rotate in
order to generate a magnetic field? That would define the maximum
distance at which such a moon could orbit its primary, since distance
determines orbital period, and with a tidally locked moon, rotation
would equal orbital period.

Can you imagine living on such a moon, though? On one side of the
thing, the primary would hang like a huge, baleful presence, dominating
the sky and everything under it. On the other side, there would just be
the alternation of the sun, stars and other moons. Such a dichotomy
would have profound effects on any civilization that arose there.

Doug

[Matthew Funke]

"Doug..." > wrote:
> But if a hot Jupiter *could* form in a second-generation star's
system,
> then it might make sense that some or all of its moons would be
formed
> of rock, not ice. That would mean that they could have roughly
> terrestrial compositions.
>
> The bigger problem in generating a strong magnetic field, as I see
it,
> would be the rotation issue. Every sizable moon of the gas giants
in
> our own solar system are tidally locked to their primary. I
wonder just
> how fast a moon with a molten nickel-iron core would have to
rotate in
> order to generate a magnetic field? That would define the maximum
> distance at which such a moon could orbit its primary, since
distance
> determines orbital period, and with a tidally locked moon,
rotation
> would equal orbital period.
>
> Can you imagine living on such a moon, though? On one side of the
> thing, the primary would hang like a huge, baleful presence,
dominating
> the sky and everything under it. On the other side, there would
just be
> the alternation of the sun, stars and other moons. Such a
dichotomy
> would have profound effects on any civilization that arose there.

Is there a mechanism that suggests that large terrestrial moons
would be more likely to form *close to* a gas giant? The Earth's
distance to the Sun varies by about five million km, and besides
that, the seasons have more to do with Earth's axial tilt than with
actual Earth-Sun distance.

I have no tools with which to analyze the likelihood of an
Earth-sized planet forming "far away" from a gas giant, though.
Still, if possible, it would remove some of the difficulties
associated with radiation from the gas giant itself and tidal
locking.

I could imagine a civilization working things out with geometry for
the first time, and shortly after their Eratosthenes-equivalent
figures out the size of their *own* planet, discovering with a
certain amount of shock how big that small disk in the sky *really
is*...

-- Best regards,
Matt Funke

[Doug...]

In article >,
says...
>
> <snip>
>
> Is there a mechanism that suggests that large terrestrial moons
> would be more likely to form *close to* a gas giant?

The only examples we can look at closely (at least comparatively
closely) are the gas giants in our own solar system, and their moons.
It's really impossible to say if the moons of a hot Jupiter would form
in the same manner as they have here, around the relatively cold gas
giants in Sol System, but it *is* true that the Jovian moon of any real
size closest to its primary, Io, is entirely made of rock. The rest of
the Galilean moons are also made mostly of rock -- they just have a
whole lot of ice (and maybe liquid water) on top of the rock.

Also, consider that Io does seem to have a molten core. Now, the Jovian
system is probably so depleted in heavy elements (relative to the inner
system) that Io's core is mostly silicate rocks and not primarily
metals, like iron or nickel, so it doesn't have a strong intrinsic
magnetic field. But if Jupiter was located one AU from Sol and it and
its moons collected most of the heavy metals out of the original solar
nebula, it might be possible that Io and the other large moons would
have been blessed with molten metal cores.

Of course, there is the problem that a hot Jupiter might just gather all
of the heaviest elements into itself, leaving only the lighter elements
for aggregation into its moons -- in which case, achieving gas giant
moons of terrestrial composition might be impossible. I'm sure you can
come up with models for both cases -- it will probably have to wait
until we can investigate other planetary systems far more closely before
we can know anything for certain.

And that isn't likely to happen in our lifetimes, unless we develop an
FTL drive in the next couple of years.

Doug

[Hop David]

Pat Flannery wrote:
> Hephaestus wrote:
>
>>
>> For a habitable moon around a sun-like star, the "year" will be ~1
>> Earth year; Titan is tidally locked with a 16 day "day/month".
>> Jupiter's major satellites range from a 2-16 days, with the largest
>> one having a 7 day "day/month". These are all significantly smaller
>> than Earth, though; would something Earth-sized also likely be tidally
>> locked?
>>
> There is a peculiar resonance going on between Earth and Venus that
> leads to Venus always having the same face displayed toward us at
> closest approach; considering the great distance between the two
> planets, to me this suggests that objects tend to get tidally locked
> sooner or later despite great distances.
> A quick check of my "Atlas Of The Solar System" shows that _all_ the
> known moons of Jupiter and Saturn are presumed to be tidally locked.
> Saturn's "shepherd moons" might make an interesting case in this regard,
> it will be fun to see if Cassini finds them locked also.
> One of the big problems in regard to habitable planets around a gas
> giant is going to be the radiation fields; on the side facing the star
> it orbits they will probably come out a fair distance, but they really
> extend out behind the planet- if Jupiter is anything to go by- and might
> sterilize the surface of any habitable moon as it passes through them in
> its orbit: http://blueox.uoregon.edu/~courses/BrauImages/Chap11/FG11_011.jpg
>
> Pat
>

SFAIK Saturn doesn't have a powerful magnetic field like Jupiter's.
Which mystifies me as it would seem the Saturn also would have a rapidly
rotating ball of metallic hydrogen at it's core.


--
Hop David
http://clowder.net/hop/index.html

[Pat Flannery]

Doug... wrote:

>Actually, several of the extrasolar planets that have been discovered in
>the past few years are gas giants that orbit their stars either as close
>as one AU, or closer. They're called "hot Jupiters" in some of the
>literature.
>

From what I've read these are assumed to be the predatory gas giants in
the last stages of their descent into their suns.

>
>Now, the question is, could a hot Jupiter form in a second-generation
>star's planetary system? The planetary nebula/disk of a second-
>generation star incorporates the heavy elements formed in a massive star
>that went kablooey, right?
>

That would put the planet in orbit around a white dwarf, wouldn't it?

> Perhaps the systems with hot Jupiters are
>all first-generation stars, and the lack of heavier elements caused a
>single gas giant to form in the close-in range.
>

There is obviously something different about what happened in these
systems and what our own is presently like

>
>But if a hot Jupiter *could* form in a second-generation star's system,
>then it might make sense that some or all of its moons would be formed
>of rock, not ice. That would mean that they could have roughly
>terrestrial compositions.
>
But now you have the problem of again having heavy elements to work with
for the moons, but a lack of light gases for the formation of the
proposed Jupiter-like planet; they would have been striped away from the
area near the star by the nova explosion as it entered old age.

>
>The bigger problem in generating a strong magnetic field, as I see it,
>would be the rotation issue. Every sizable moon of the gas giants in
>our own solar system are tidally locked to their primary. I wonder just
>how fast a moon with a molten nickel-iron core would have to rotate in
>order to generate a magnetic field? That would define the maximum
>distance at which such a moon could orbit its primary, since distance
>determines orbital period, and with a tidally locked moon, rotation
>would equal orbital period.
>

On something as big as a gas giant, that would be a very good distance
indeed; and I'm fairly sure that large bodies get tidally locked sooner
than small ones (maybe even during formation) so a Earth-sized one would
have to be pretty far out to escape the effect. The Venus/Earth
resonance is a good case in point- either it is one hell of a
coincidence, or there is some sort of a tidal lock caused by the gravity
fields of the two planets even at the great distance in comparison to
their masses that is involved.

>
>Can you imagine living on such a moon, though? On one side of the
>thing, the primary would hang like a huge, baleful presence, dominating
>the sky and everything under it. On the other side, there would just be
>the alternation of the sun, stars and other moons. Such a dichotomy
>would have profound effects on any civilization that arose there.
>

But on the other hand, what an invitation to space travel, assuming that
the primary has other moons!
.....and the obvious orbiting of such moons around the primary as
indicated by its eclipsing of them would be an easily seen lesson in
celestial mechanics, and mean that there would probably be no rise of a
geocentric cosmological theory, but rather a early recognition of the
true nature of a solar system.

Pat

[Pat Flannery]

Hop David wrote:

>
> SFAIK Saturn doesn't have a powerful magnetic field like Jupiter's.
> Which mystifies me as it would seem the Saturn also would have a
> rapidly rotating ball of metallic hydrogen at it's core.

http://www-ssc.igpp.ucla.edu/personnel/russell/papers/sat_mag.html
The core is a lot smaller, and the field lined up better with the axis
of rotation.

Pat

[Matthew Funke]

"Doug..." > wrote:
> says...
> >
> > <snip>
> >
> > Is there a mechanism that suggests that large terrestrial moons
> > would be more likely to form *close to* a gas giant?
>
> The only examples we can look at closely (at least comparatively
> closely) are the gas giants in our own solar system, and their
moons.

Yeah. I've seen theorizing, though, about what kind of formations
we could expect in different star systems -- broad generalizations
that tried to explain why we see what we do in the Solar System.

Some of those generalizations turned out to be wrong, of course,
such as the idea that gas giants forming close to their parent star
would be exceedingly rare; the notion was that the parent star would
tend to accrete most of the available material before a gas giant
could really form nearby. Studies are just beginning to show that
that theory may have been wrong. Of course, it's awfully hard to
make solid predictions with the limited observational data we have
available, so one must take them with a grain of salt... but I was
wondering if anyone had heard of reasons why terrestrial planets
forming far away from a gas giant might be expected to be unlikely.

It will be enlightening to see what theories arise and develop as
our understanding of extrasolar planets increases (though, as you
say, we may *still* not arrive at a definitive answer). We live in
an interesting time.

> Of course, there is the problem that a hot Jupiter might just
gather all
> of the heaviest elements into itself, leaving only the lighter
elements
> for aggregation into its moons -- in which case, achieving gas
giant
> moons of terrestrial composition might be impossible. I'm sure
you can
> come up with models for both cases -- it will probably have to
wait
> until we can investigate other planetary systems far more closely
before
> we can know anything for certain.

Of course. Still, speculation is interesting.

> And that isn't likely to happen in our lifetimes, unless we
develop an
> FTL drive in the next couple of years.

Well, barring some kind of revolutionary theory that reshapes our
understanding of physics, I don't see that happening; we don't even
have firm evidence that FTL travel is *possible*, let alone
developing the wherewithal to put it to use. More's the pity,
though.

-- Best regards,
Matt Funke

[Hop David]

Doug... wrote:

> Can you imagine living on such a moon, though? On one side of the
> thing, the primary would hang like a huge, baleful presence, dominating
> the sky and everything under it. On the other side, there would just be
> the alternation of the sun, stars and other moons. Such a dichotomy
> would have profound effects on any civilization that arose there.

Some non-arbitary marks on our globe: North and South poles and the equator.

The moon would also have a north and south pole & equator. And there'd
be four non arbitrary points on its equator: Gas giant pole (point
closest to the primary), anti gas giant pole, & two points on the
equator 90 degrees from these. The six points would define 3 great
circles (of which the equator is one). Their globe would be marked like
an octahedron.

The planet side would have frequent and long solar eclipses during the
spring and fall equinox and be eclipse free the summer and winter
solstice if the orbit plane is tilted enough.

Midnight during the spring and fall equinox would be of special
significance: The moon's shadow would be cast onto the center of the gas
giant as viewed from the moon's inhabitants.



--
Hop David
http://clowder.net/hop/index.html

[Hop David]

Matthew Funke wrote:

> Some of those generalizations turned out to be wrong, of course,
> such as the idea that gas giants forming close to their parent star
> would be exceedingly rare;

I remember being delighted by that news. A favorite fantasy of mine is a
Gas giant In an Earth Like Orbit (Gielo) with an Earth Like Moon (Elm).

http://clowder.net/hop/gofix/artifact.html



--
Hop David
http://clowder.net/hop/index.html

[Pat Flannery]

Hop David wrote:

>
> The planet side would have frequent and long solar eclipses during the
> spring and fall equinox and be eclipse free the summer and winter
> solstice if the orbit plane is tilted enough.
>
> Midnight during the spring and fall equinox would be of special
> significance: The moon's shadow would be cast onto the center of the
> gas giant as viewed from the moon's inhabitants.


This could lead to some very interesting breeding and migration cycles
in regard to such a planet's wildlife; assuming more than one moon (with
a gas giant that seems a very reasonable assumption, based on Jupiter
and Saturn) and the night sky would be a very interesting place
indeed...think of the strange religious beliefs that might evolve
regarding just what was going on overhead; our star-like appearing
planets generated a great deal of mythology just by their movement;
imagine what ones that become visible disks would do in this regard- are
they moving closer and further away, or are they shrinking and growing
in size? In fairly short order the first would be suspected to be the
case, based on the apparent reduction in size of people seen at a distance.
A hypothetical civilization on such a planet might have a fairly
accurate idea of true cosmology at a far earlier stage of its evolution
than any Earth civilization did. Imagine nomadic hunter-gatherers with
a understanding of a sun-centered universe with multiple worlds... is
one of the other moons where you go when you die?
Io would make a pretty good hell.

Pat

[Sander Vesik]

Pat Flannery > wrote:
> indeed...think of the strange religious beliefs that might evolve
> regarding just what was going on overhead; our star-like appearing
> planets generated a great deal of mythology just by their movement;
> imagine what ones that become visible disks would do in this regard- are
> they moving closer and further away, or are they shrinking and growing
> in size? In fairly short order the first would be suspected to be the

Well.. would it really be all that different from our moon (which is much
more active in beliefs than mere planets) except there would be more of
them and on different schedules?

> case, based on the apparent reduction in size of people seen at a distance.
> A hypothetical civilization on such a planet might have a fairly
> accurate idea of true cosmology at a far earlier stage of its evolution
> than any Earth civilization did. Imagine nomadic hunter-gatherers with
> a understanding of a sun-centered universe with multiple worlds... is
> one of the other moons where you go when you die?

I'm not certain that would happen, or at least happen considerably earlier
than on Earth (where it didn't really happen all that late either). But
yes, going to another Moon when you die sounds liek a resonable guess.

> Io would make a pretty good hell.
>
> Pat
>

--
Sander

+++ Out of cheese error +++

[Pat Flannery]

Sander Vesik wrote:

>Well.. would it really be all that different from our moon (which is much
>more active in beliefs than mere planets) except there would be more of
>them and on different schedules?
>
Assuming we are both talking about other moons viewed from an inhabited
one in orbit around a gas giant, there would be a major difference- our
Moon undergoes phases and an occasional eclipse, but it stays the same
size in the sky (although it still looks bigger to me when it's on the
horizon...I know it's not... but it looks bigger anyway); but the moons
in orbit near the inhabited one would appear in the sky as bright stars
when they are far away, but become visible discs like the Moon as they
drew close to the inhabited one in their orbits.
The planets in our solar system vary in position and brightness to the
naked eye, but the only two things that can be seen as discs in our sky
(the Sun and Moon) stay the same apparent size all the time (the Moon
varies a bit, but probably not enough to be very noticeable).
Now put yourself in the position of an observer on our hypothetical
inhabited moon- the sky is a very complex place; assuming that your
planet is tidally locked, then on one side of the moon, you've got the
big gas giant hanging pretty much motionless in the sky, but changing
its phases as you orbit around it; given it's size, you probably get
very frequent eclipses of your system's sun; but there are two other
groups of odd objects in the sky- the other moons of your gas giant, and
the other planets in your solar system. The moons inboard of you
probably cross in front of the gas giant quite often, and their shadows
can be seen falling on the gas giant; in fact _your_ moon's shadow can
probably be seen falling on the gas giant- and once you make the
connection between _that_ shadow and the ones the other moons are
casting, the realization sinks in that those discs in the sky are worlds
like the one you are living on. Then there are the other star-like
moving objects to contend with- the star's other planet; their motion is
noticeably different from the moons, and this soon would also start
speculation as to what they were.
Remember the contortions that philosophers went through in an attempt to
make a geocentric cosmos work? Those would look simple compared to what
one would have to go through to make a moon-centered one work! You'd end
up with something with as many layers as a onion hooked together in
incredibly convoluted ways- the whole thing would be so complex to even
try that I'd bet that it never would be even seriously considered before
the correct sun-centered view arose.

>
>
>>case, based on the apparent reduction in size of people seen at a distance.
>>A hypothetical civilization on such a planet might have a fairly
>>accurate idea of true cosmology at a far earlier stage of its evolution
>>than any Earth civilization did. Imagine nomadic hunter-gatherers with
>>a understanding of a sun-centered universe with multiple worlds... is
>>one of the other moons where you go when you die?
>>
>>
>
>I'm not certain that would happen, or at least happen considerably earlier
>than on Earth (where it didn't really happen all that late either). But
>yes, going to another Moon when you die sounds liek a reasonable guess.
>
I'll bet they would probably think that all the other moons had life on
them of one sort or another.

Pat

[Christopher M. Jones]

Pat Flannery > wrote in message >...
[snip]
> Remember the contortions that philosophers went through in an attempt to
> make a geocentric cosmos work? Those would look simple compared to what
> one would have to go through to make a moon-centered one work! You'd end
> up with something with as many layers as a onion hooked together in
> incredibly convoluted ways- the whole thing would be so complex to even
> try that I'd bet that it never would be even seriously considered before
> the correct sun-centered view arose.

Not only that, but don't forget the planet looming over
half the moon. It would be many times larger than the
full moon and would hang nearly motionless in the sky
(due to tidal locking). That sorta thing leads away
from a "my local spot is the center of the universe"
concept. And once you've demoted your own planetary
body from the center it's much easier to continue from
there, so even if you start thinking the planet is the
center of the universe you can more easily and
naturally move to a Sun centric model once you have
basic data. Compare, for example, the shift from
geocentric to heliocentric to the shift from
heliocentric to galactocentric etc. Plus, you can do
the kind of astronomy Kepler and Tycho did with your
bare eyes.

[Earl Colby Pottinger]

(Christopher M. Jones) :

> Pat Flannery > wrote in message
> >...
> [snip]
> > Remember the contortions that philosophers went through in an attempt to
> > make a geocentric cosmos work? Those would look simple compared to what
> > one would have to go through to make a moon-centered one work! You'd end
> > up with something with as many layers as a onion hooked together in
> > incredibly convoluted ways- the whole thing would be so complex to even
> > try that I'd bet that it never would be even seriously considered before
> > the correct sun-centered view arose.
>
> Not only that, but don't forget the planet looming over
> half the moon. It would be many times larger than the
> full moon and would hang nearly motionless in the sky
> (due to tidal locking). That sorta thing leads away
> from a "my local spot is the center of the universe"
> concept. And once you've demoted your own planetary
> body from the center it's much easier to continue from
> there, so even if you start thinking the planet is the
> center of the universe you can more easily and
> naturally move to a Sun centric model once you have
> basic data. Compare, for example, the shift from
> geocentric to heliocentric to the shift from
> heliocentric to galactocentric etc. Plus, you can do
> the kind of astronomy Kepler and Tycho did with your
> bare eyes.

With the ability to ensure world-wide timing of measurements (IE the
terminator on the Gas planet crossing certain long term features on the gas
planet) you can take multiple measurements (long baseline) of the other moons
and very accurately calculate thier speed and positions in a short time frame
and with very early tech.

Earl Colby Pottinger

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