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Living on the far side of a tide-locked moon

  1. Jun 20, 2017 #1
    You're the Galileo for a developing society on a remote archipelago on the far side of a tide-locked moon orbiting a gas giant. How do you figure out your place in that solar system? How do you convince others, who believe your world to be the center of the universe, of the truth? Will a road trip be involved, or can this be shown otherwise?

    I figure watching other moons in your planetary system fall into the shadow of the gas giant will play a big part in it, but knowing how well it worked out for Galileo (and others) in our history, how can my hero(ine) convince the masses they orbit a planet they can't see?
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  3. Jun 20, 2017 #2
    Am I correct in assuming the orbital periods of the sister moons would be way too long if they were only orbiting the world? I imagine they would disappear for long stretches of time when they were on the other side of the gas giant.

    If the gas giant had rings, is it possible you could see part of the ring from the moon? Is it possible the moon was very close/in the ring? I imagine that would cause a lot of meteoric activity.

    Note to self: Use this hypothetical system to teach self how to build planets, etc. in Space Engine. Land on moon, see the skies from that perspective.
  4. Jun 20, 2017 #3
    The far side does not see the gas giant.

    On earth Eratosthenes calculated the radius in 2nd century BC. A similar method would work on a moon.

    I was trying to think through Aristotle's spheres from this perspective. I think they would have each moon on a sphere and the archipelago above center. The other moons spend a long time on the horizon as small bodies that grow to large moon sizes and zip across. Phases of the moons are crazy. The could still be orbit us, the bottom goes much further than the tip of the island mountains .

    when the other moons are eclipsed by the planet it is because the sun is taking a break. the gas giant lunar eclipses only happen at night. The other moons can eclipse each other and also eclipse the sun within view.
  5. Jun 21, 2017 #4


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    Epicycles of moons further out from the gas giant would add a clue. As your moon caught up to them and passed them, they would appear to make little loops against the background of the Stars
  6. Jun 21, 2017 #5


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    Interesting idea. Observing the other moons would certainly help, by tracking their motions it should be possible to figure out that you're all orbiting...something. I'd love to read a book about an expedition to the planet-facing side :)
  7. Jun 21, 2017 #6
    They only get moons further away. Mercury and Venus are not visible at midnight.
  8. Jun 21, 2017 #7
    Regarding tides on a tide-locked oceanic moon: Would the tides simply follow the gravitational pull of the sun, thereby making them fairly straightforward to predict?
  9. Jun 21, 2017 #8


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    Since I was only referring to moons further away, I'm not sure what the relevance of this statement is.
    Here's an example of what I mean. It is Mars' apparent path as seen from Earth as Earth catches up to and passes Mars
  10. Jun 21, 2017 #9
    Was not trying to argue. Seams like a better picture is emerging.

    If the orbit is a 2:1 resonance you see a full moon every other night. Is there a reason the full moon is also when they pass? If the pass happens during the day you can not use the constellations to measure the motion.

    Would they pass on the full moon once per year? When does the gas giant cause an eclipse?

    You still get tides from the moons too. If the orbits are in a resonance pattern the tides should be more regular/predictable than here.
  11. Jun 21, 2017 #10


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    If the other moon are massive/close enough to produce measurable tides, then there is another effect they will cause. The perturbing effect they have on your moon (especially so if there is a resonance) will keep it from settling into a circular orbit. Your changing orbital distance from the gas giant will then produce a variance in the tide produced by it. It will undergo a small libration and rise and lower. (The internal heat of the Jovian moon Io arises from just this type of tidal flexing precipitated by the perturbing effects other moons have on its orbit.)
  12. Jun 23, 2017 #11
    I'm not quite sure that this is the case, but I have a very strong feeling that the ratio between the orbits of the moon around the planet and the planet around the star should produce a detectable pattern in the time of sunrise and the length of the day if you tracked it over a long enough time.
  13. Jun 23, 2017 #12
    If they are using a sundial to measure time they will not measure a change in the time of sunrise. A water clock will run faster when it warms up. The star may slightly slow/speed the orbit but it would be hard to confirm. A fancy verge and foliot clocks could get within 15 minutes a day. Galileo wanted something like a pendulum clock but they were not invented until after he died.
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