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I'm writing a story about a sailor in an alien world, the world is exactly the same as Earth, except for one difference: this planet has nine moons. 5 moons go clockwise, while 4 go anti-clockwise All of these moons are similar in size of our own moon, and have similar orbits around the planet. However, my moons orbit at different altitudes, so they don't collide. What obstacles would the sailor face, if there were more moons. Will there also be any other gravity-made challenges, such as whirlpools or bigger waves. Also, would there possibly be more then two tides per day. This question has been identified as a possible duplicate of another question concerning tides, How would having multiple moons affect tides?, however my moons travel different way, so would this give different answers?

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marked as duplicate by Frostfyre, cobaltduck, L.Dutch, Hohmannfan, HDE 226868 Mar 30 '17 at 13:14

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    $\begingroup$ I don't think that's scientifically possible. $\endgroup$ – Mormacil Mar 30 '17 at 11:59
  • $\begingroup$ do you have a plausible mechanism to explain why certain moons move differently? $\endgroup$ – L.Dutch Mar 30 '17 at 12:29
  • $\begingroup$ @L.Dutch : Gravitationnal capture could be an explanation, as this happened in our solar system. See en.wikipedia.org/wiki/Triton_(moon) as an exemple $\endgroup$ – Tryss Mar 30 '17 at 12:37
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    $\begingroup$ The math given in the top ranked answer for How would having multiple moons affect tides? works regardless of the direction of rotation. $\endgroup$ – sphennings Mar 30 '17 at 13:08
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    $\begingroup$ several moons, some going clockwise and some other counterclockwise on very similar orbits around the planet tell me only "BANG meteor shower" $\endgroup$ – L.Dutch Mar 30 '17 at 13:20
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To answer your question only about how tides would be affected, the way the moon affects the tides right now is the same way that the sun does. The tides are pulled towards where the other object is, towards the opposite direction, and away from the 90 degree angles. That means the orientation of the moons is very important. Any "moons" at 90 degrees to each other will effectively cancel out each other's affect on the tides. If say, there were 4 in a + formation, with Earth at the center, they'd all do nothing.

So the question of how the tides would work is dependent on how you want the moons to be oriented. Regardless, just looking at two moons going in opposite directions (assuming they can't collide) then you can simplify it to be that they will go in a cycle of lining up - being at 90 degrees - being opposite - being at 90 degrees, in the course of half a month. This would make tides stronger whenever they're lined up or opposite, and tides weaker when they're at 90 degrees. But the question about how the moons are oriented throws a bit of a wrench into this rough calculation.

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The Moon's gravity is what causes tides and other side effects on Earth because Luna is relatively close, but most of all, because it's exceedingly large for a satellite - if we look at the rest of the solar system, all satellites have a mass that's a tiny fraction of the planets they orbit.

If you want nine satellites orbiting an Earth-like planet, then they must be rather small and have orbits from the fairly close to the far, far away. Otherwise, their gravity interactions would render the orbits unstable. In fact, the presence of the Moon it's what makes sure that Earth is not going to have any other satellite but the tiny pieces of metal we put in LEO with our rockets.

As a result, you would have no tides at all. Any satellite big enough to cause noticeable effects on an Earth-like planet without disintegrating due to crossing the Roche limit or falling down onto the planet it's going to clear the rest of the available orbits out there.

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  • $\begingroup$ I disagree with "no tides at all" part, the moon is not only thing that causes tides. Even planet without any moon will have tides, maybe with smaller amplitude, but they still will be here. From rotation, fron central sun and some more less important factors... $\endgroup$ – Antoine Hejlík Mar 30 '17 at 13:31
  • $\begingroup$ True. I was refering only about the moon's effect on tides. There would be tides, as you say, but about the same intensity as if there weren't any moons. $\endgroup$ – Rekesoft Mar 30 '17 at 13:41

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