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BDaniels
2014-Sep-06, 05:58 AM
If a settlement existed on a moon orbiting a planet and was built on the side facing the planet around which the moon orbits, an inhabitant of that colony would always see the planet in the same position in the sky. Is that correct assuming that the moon is tidally locked and one side always faces the planet? If correct, then the position of the planet in the sky would depend on where a settlement was constructed on the surface of the moon, and if a settlement built on the opposite side of the moon in this scenario, the planet would never appear in the sky, correct?

I am writing a science fiction story and want to ensure my description from a imagined settlement in another solar system is at least astronomically accurate. Hopefully this is an appropriate question for this forum.

Additionally, periods of daylight and darkness would likely be lengthy wouldn't they or could the moon complete an entire orbit in approximately 24 hours? If days are lengthy, then I assume weather on the moon would likely be extreme as the side facing the sun would become very hot while the side away from the sun would become very cold. Is this realistic?

Lastly, assuming the presence of liquid water on a moon, how would it be affected by the larger planet? I assume there would be extreme tides and currents. If the length of days was extensive then water might be absent from the side facing the sun and frozen on the opposite side, only remaining as a liquid near the poles and in then briefly in the zone transitioning from darkness to starlight.

I would appreciate any fact checking of my assumption or the suggestion of an appropriate astronomical resource where someone has explored these possibilities. As a biologist, stargazer and someone interested in scientific accuracy I would appreciate any help I can to make sure everything is as accurate as possible.

Thank you.

antoniseb
2014-Sep-06, 09:59 AM
I'm guessing you're thinking of a Moon like Io around Jupiter, except larger and closer to the star so it is habitable.
So here are a few things to consider:
With a 24 hour period, such a moon would be close to the Roche limit, and so would be highly volcanic and seismically very active.
These same influences would affect tides if you had oceans on the Moon. Those tides would be colossal... except that the tidal locking would mean that the tidal bulges wouldn't move much.
If the Moon's orbit was at all elliptical, or influenced by other bodies, the host planet would vary as to where it appeared in the sky, even if it was tidally locked.
The diurnal extremes of the weather could be softened quite a bit if the atmosphere of the Moon were relatively thick compared to ours, if that's what you need.

Nowhere Man
2014-Sep-06, 11:59 PM
Not to mention libration, the otherwise-locked moon rocking back and forth, making the primary appear to move in the sky.

Fred

chornedsnorkack
2014-Sep-07, 12:25 PM
I'm guessing you're thinking of a Moon like Io around Jupiter, except larger and closer to the star so it is habitable.
So here are a few things to consider:
With a 24 hour period, such a moon would be close to the Roche limit, and so would be highly volcanic and seismically very active.
Io is volcanically very active, but Amalthea is not. Of the inner satellites of Saturn, only Enceladus is active.


These same influences would affect tides if you had oceans on the Moon. Those tides would be colossal... except that the tidal locking would mean that the tidal bulges wouldn't move much.
If the Moon's orbit was at all elliptical, or influenced by other bodies, the host planet would vary as to where it appeared in the sky, even if it was tidally locked.
The diurnal extremes of the weather could be softened quite a bit if the atmosphere of the Moon were relatively thick compared to ours, if that's what you need.
Also note that slower rotation also means weaker Coriolis effect, allowing for more efficient heat exchange (winds would blow more directly into depressions, not so much around cyclones).

Moon librates with amplitude of about 8 degrees of longitude. If you live near the edge of Moon then Earth (just 2 degrees in diametre) rises and sets. If you, however, live near the centre of near side then small changes of Earth being high overhead all along are not conspicuous unless you are following markers like exactly where fixed high objects cast shadow on ground.

Moon has conspicuously big eccentricity - 0,055. And Moon should experience tides about 16 times stronger than Earth in terms of tidal acceleration gradient. A nearby moon which has modest even though nonzero tides due to low eccentricity should experience very small range of libration.

antoniseb
2014-Sep-07, 12:42 PM
Io is volcanically very active, but Amalthea is not. Of the inner satellites of Saturn, only Enceladus is active. ...
Amalthea is a small hunk of solid rock. The OP was about a Moon large enough to support a breathable atmosphere with weather. Io and Enceladus are both smaller than that, but both are large enough to be round by reasons of self-gravity. Saturn's other close moons are not that big.

Moon has conspicuously big eccentricity - 0,055. And Moon should experience tides about 16 times stronger than Earth in terms of tidal acceleration gradient. A nearby moon which has modest even though nonzero tides due to low eccentricity should experience very small range of libration.
Io and Europa also have eccentric orbits largely because of the perturbing influence of other large moons (Ganymede especially) If, for some reason there were only one moon around a gas giant, and it was near the roche limit, I'm sure it would be tidally locked in a very nearly circular orbit, but how likely is that solitary moon situation?

chornedsnorkack
2014-Sep-07, 02:43 PM
Io and Europa also have eccentric orbits largely because of the perturbing influence of other large moons (Ganymede especially)
And Ganymede itself has much lower eccentricity than Io and Europa.

If, for some reason there were only one moon around a gas giant, and it was near the roche limit, I'm sure it would be tidally locked in a very nearly circular orbit, but how likely is that solitary moon situation?

Triton is noted for conspicuously low eccentricity.

cjameshuff
2014-Sep-07, 04:13 PM
Triton is noted for conspicuously low eccentricity.

And in 3-4 billion years it will pass through the Roche limit due to tidal drag in its retrograde orbit.

chornedsnorkack
2014-Sep-07, 09:23 PM
Periods and eccentricities of major satellites, with expression for comparative tidal strength which is chosen to be 1 million times the eccentricity divided by square of period:
Moon 27,32 0,0549 73,5

Io 1,769 0,0041 1310
Europa 3,551 0,0094 745
Ganymede 7,155 0,0011 21,5
Callisto 16,69 0,0074 26,6

Mimas 0,9424 0,0202 22700
Enceladus 1,370 0,0047 2500
Tethys 1,888 0,0001 28
Dione 2,737 0,0022 294
Rhea 4,518 0,00126 61,7
Titan 15,95 0,0288 113
Iapetus 79,32 0,0286 4,55

Miranda 1,413 0,0013 651
Ariel 2,520 0,0012 189
Umbriel 4,144 0,0039 227
Titania 8,706 0,0011 14,5
Oberon 13,46 0,0046 25,4

Triton 5,877 0,000016 0,46

Earth has about 4,5, for comparison with these last listed numbers.

DaveC426913
2014-Sep-07, 10:44 PM
Not really what you asked about, but something interesting in binary systems:

The tidal force operates not just on water, but on atmo as well - and to a correspondingly greater degree. If the two bodies are close enough, the atmo can rise enough that the two bodies actually share some atmo.

This is expected to be the case with the Pluto-Charon binary.

http://news.discovery.com/space/pluto-and-moon-charon-may-share-same-atmosphere-140609.htm
http://themeridianijournal.com/2014/06/pluto-charon-share-atmosphere/

Jens
2014-Sep-08, 08:31 AM
Lastly, assuming the presence of liquid water on a moon, how would it be affected by the larger planet? I assume there would be extreme tides and currents. If the length of days was extensive then water might be absent from the side facing the sun and frozen on the opposite side, only remaining as a liquid near the poles and in then briefly in the zone transitioning from darkness to starlight.


Well, if it's tidally locked to the planet then you wouldn't have tides, well except ones created by the libration.

Jens
2014-Sep-08, 08:32 AM
Not to mention libration, the otherwise-locked moon rocking back and forth, making the primary appear to move in the sky.


Just generally speaking, how fast of a process is libration usually? Is it something that you would immediately notice, a rocking back and forth motion, or is it something that would be difficult to detect?

chornedsnorkack
2014-Sep-08, 08:54 AM
Well, if it's tidally locked to the planet then you wouldn't have tides, well except ones created by the libration.

You WILL have the ordinary tidal forces from Sun, plus any tides due to other satellites. Also, since the period of the tides due to libration does not match the period of the tides due to Sun (solar tides have 2 high tides per month, libration tides 1 high tide per month), they cannot cancel out.

As stated, the period of libration matches month (anomalistic I think). So an observer on the edge of Moon would see Earth rise and set - once per month.

Jeff Root
2014-Sep-08, 09:17 AM
Earth's Moon's libration would be easy to detect by someone
lying in a lawnchair and a spacesuit on the Moon for a few days.
Of course, a few days on the Moon is a pretty long time to lie
in a lawn chair, but it wouldn't be too difficult to detect in the
course of a couple of Earth days, either, though the periodic
nature of it wouldn't become apparent in that time. Watching
for a few months would allow one to notice that the east-west
libration was synchronized with perigee and apogee. Earth
would be moving to the east around the time of perigee, and
to the west around the time of apogee.

-- Jeff, in Minneapolis

chornedsnorkack
2014-Sep-08, 10:37 AM
Earth's Moon's libration would be easy to detect by someone
lying in a lawnchair and a spacesuit on the Moon for a few days.
Of course, a few days on the Moon is a pretty long time to lie
in a lawn chair, but it wouldn't be too difficult to detect in the
course of a couple of Earth days, either, though the periodic
nature of it wouldn't become apparent in that time. Watching
for a few months would allow one to notice that the east-west
libration was synchronized with perigee and apogee. Earth
would be moving to the east around the time of perigee, and
to the west around the time of apogee.

-- Jeff, in Minneapolis

Provided you detect the phenomena of perigee and apogee in the first place. The perigee and apogee of Moon are not so easy to detect, especially if you do not know they are there to look for. Earth is 4x bigger, but still.

Since Moon also has libration of latitude, I suspect Earth does not rise and set at the same spot on horizon. What is the figuree in the sky that Earth follows?

antoniseb
2014-Sep-08, 10:52 AM
... What is the figuree in the sky that Earth follows?
That would be pretty cool to see. Nice question. I expect it would look like a squashed Spirograph(tm) drawing with an 18 year cycle (roughly), but that's a guess, I don't now.

AGN Fuel
2014-Sep-11, 10:57 PM
Earth's Moon's libration would be easy to detect by someone
lying in a lawnchair and a spacesuit on the Moon for a few days.


Or William Hawkins, perhaps. ;)

I have idly wondered in the past about a primitive civilisation that arose in a region of a librating tidally locked moon, where the parent body was near the horizon and so periodically rose above and then fell below the local horizon. It would make for some interesting myths & legends in that culture!

LookingSkyward
2014-Sep-12, 10:26 PM
I don't have anything to contribute, but I really wanted my name in this thread...

SixDegrees
2014-Sep-13, 06:01 PM
Thanks to libration, the planet orbited will describe a figure-eight path, or analemma. The precise shape will vary depending on the particulars of the moon's orbit, which will cause it to be relatively stretched or squat, and the observer's position on the moon, which will cause the figure to rotate relative to the local horizon. The analemma will be traced completely in the course of a single orbit.

Libration is normally small, so without bright, nearby stars to compare positions with, it would be difficult to perceive this motions, which is typically on the order of just a few degrees in both latitude and longitude.

BDaniels
2014-Sep-15, 07:06 AM
Thanks to everyone in the thread. I appreciate the responses very much. I believe my original questions are answered completely and I have some additional ideas to think about as well.

CaptainToonces
2014-Sep-17, 06:37 AM
So I guess the moon is stretched away from a purely spherical shape to a shape like an egg, pointing at the Earth? (As opposed to the Earth and the Sun whose rotation gives them a slightly gourd-like shape.)

Jeff Root
2014-Sep-17, 09:20 AM
Yes. The oblate ellipsoid shape of Earth was observed before
the advent of space satellites. The Sun, on the other hand, has
a low enough rotation rate that as far as I've heard, any out-of-
roundness to its shape has yet to be detected. It looks exactly
spherical. The Moon is quite lopsided. There was something in
the news about it here not long ago, regarding how the crust
got to be so much thicker on the side facing away from Earth,
but I don't remember what they said... Having opposite faces
so different like that would speed up tidal locking of a moon.

-- Jeff, in Minneapolis

chornedsnorkack
2014-Sep-17, 11:33 AM
Libration is normally small, so without bright, nearby stars to compare positions with, it would be difficult to perceive this motions, which is typically on the order of just a few degrees in both latitude and longitude.
Libration is not relative to stars - these undergo uniform rotation of heavens. Libration is relative to ground.

For an observer near the edge of Moon, Earth undergoing 8 degrees libration and being itself a disc 2 degrees across would be conspicuous, rising and setting against horizon. But 8 degrees movement high in sky is not so obvious, unless you can watch the shadows of some fixed vertical object.