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Moon and tidal forces

  1. May 15, 2013 #1
    Not homework here but merely curiosity.

    I was reading that on Titan there are rivers of methane, and that at least one is 400km long. Now I was thinking about the earth and moon; imagine that the moon becomes a copy of earth exactly, keeps all of it's features and becomes the diameter of the moon, so it has the same mountains/ocean trenches in proportion to the diameter in the moon, water to the same depth. Now the moon rotates like earth and is tilted on its axis, and the earth moves around the moon like the moon around earth. Would the water stay in the duplicate oceans and rivers/lakes with more pronounced tides, or would it get pulled to one pole of the planet? :confused:

    And on the Titan's methane rivers, when rotating around such a big planet as Saturn, how do the rivers form a river bed with such strong tidal forces? I'm asking this because my assumptions about tidal forces are possibly, completely wrong. Thanks!
  2. jcsd
  3. May 15, 2013 #2

    Simon Bridge

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    Welcome to PF;
    You want to know what?

    Do you know how tides are formed?

    Anyway - the secret with thinking about things scientifically is to not get distracted by details.
    So you can just put a shallow swimming pool, in a habitat, on the Moon and measure the tides - and see if the water wants to stay in the pool the same as it does on the Earth or if it is more wont to slop over the edges and assume a different shape.

    Note: everything has tides - not just water and not just liquids.
    The shapes the surface liquids depends on a lot of things - the holes they get to fill being the major contributor (though billions of years in contact means the water affects the holes as well.)
    Lower gravity allows greater movement across the surface - less energy to slop a wave farther up the beach for eg.
    Rapid rotations affect currents - etc.

    Titan's situation is rather special...

    The rivers on Titan have had a long time to interact with the surface to form depressions.
    I don't see why you'd expect tides would prevent it - remember there are low tides as well as high tides and that the liquid is still in contact with the surface so it can still erode it.
  4. May 16, 2013 #3
    I understand most of what you said already, may question was odd and vague. Thanks for the reply. I suppose I was wondering how strong the earths tidal forces are on the moon, and the best way I could think to visualize it was with water.

    My thinking was that the moon is fairly small relative to earth and it still affects water very noticeably. I understand the earths tidal forces squeeze the moon and vice versa, but that is harder for me to understand the forces at work. Water on the moon seemed like a good way to visualize it. From what you've said I think I'm overestimating the strength of these forces. Their ability to create tides has to do with the vast quantity of water being acted on right?

    So in your shallow swimming pool would it be mild but measurable changes or striking visual shifting?

    I get what you're saying about the river. Next time I ask a question I'll try and shave it up with occam's razor first.
  5. May 17, 2013 #4

    Simon Bridge

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    It is a common misconception that the moon has a strong effect on the Earth's water.

    The only reason you see the tide as a large change is because the shape of coastlines channels the tidal "surge" to make the water bunch up... i.e. the highest tides occur in long narrow inlets like in the Bay of Fundy (Nova Scotia) where the incoming tide is "funnelled" and the longest tides occur over shallow beaches. Probably why you wanted the scaled down geometry.

    Also, "dramatic" is a relative thing: what about the tidal effect in a glass of water?

    You can calculate the difference in the Earth's gravity on opposite sides of the Moon and compare them with the difference in the Moon's gravity on opposite sides of the Earth to see which is the bigger effect - also compare the effect of the Sun - the Moon moves closer to and farther from the Sun than the Earth, and more quickly.
    It's a good exercise for you which is why I don't just tell you ;)
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