Assuming a planet would have Earth's surface gravity and about the same share of it's surface covered by water, how great could you make tides if you could make the moon bigger, smaller, denser, less dense, closer or farther away as you wished? And would a planet subjected to a bigger pull by it's moon necessarilly be more geologically active or not?

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    $\begingroup$ You're going to need to narrow this down a bit to a specific answerable question, at the moment it's not asking a specific question, more of a bunch really. If you told us what you were trying to achieve, then we maybe can help you get there. $\endgroup$ May 1, 2020 at 5:13
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    $\begingroup$ In your future attempt to simplify the question, you can forget the density of the Moon. At least as long as the distance between the Moon and the Earth is much larger than the spatial dimensions of both. $\endgroup$ May 1, 2020 at 5:25
  • $\begingroup$ I liked Slarty's answer. I would like the question to fit that answer. Is there a suggestion for that? Maybe "How great could tides be on Earth if you could make the moon closer or farther away?" Thank you. $\endgroup$
    – aadv
    May 1, 2020 at 15:21
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    $\begingroup$ According to calculations, backed up by measurements of laser beams bounced off of reflectors left on the Moon by Apollo missions, the moon is slowly receding from the Earth. .The Moon used to be much closer to Earth billions of years ago, and the tides used to be gigantic. skeptoid.com/episodes/4683 Tidal heating of moons of giant planets can cause vulcanic activity,as on Io. Heller, René; Rory Barnes (2012). "Exomoon habitability constrained by illumination and tidal heating". Astrobiology. 13 (1): 18–46. liebertpub.com/doi/10.1089/ast.2012.0859 $\endgroup$ May 2, 2020 at 17:11
  • $\begingroup$ Thank you, Mr. Golding. I think I got everything I will need. $\endgroup$
    – aadv
    May 6, 2020 at 13:39

1 Answer 1


That’s a bit of a general question to which I will attempt a general answer. The closer the Moon gets to Earth the stronger the tidal effects would be. Limit around 11,500 miles – the Roche limit at which point tidal forces would destroy the Moon and it would break up. In reality bad things would start to happen before then but that’s an absolute limit. The greater the mass of the Moon the greater the tidal effects would be, but at some point the Earth Moon system would be better described as a double planet rather than a planet moon system.

Would the planet be more geologically active? Yes It probably would as tidal forces could exert a lot of force in the crust as we see with Jupiter’s moon Io.

How big could the tides be? Well firstly I suggest any huge tidal swings would not be stable in the long run as they would create a lot of drag dissipating energy and probably leading to a collision. However assuming the orbit can be varied and the mass increased the tides could be very large indeed.

In the extreme case of a moon the size of the Earth (Roche limit not an issue for similar sized bodies) orbiting just above the atmosphere the surface of the oceans would become almost weightless (perhaps 1/10 or less the weight) and normal meteorological effects could easily turn huge swathes of the ocean surface into an unstable foam of air debris and water droplets. A massive tide operating in that environment might transfer water high up into the atmosphere and given the proximity of the “moon” beyond.


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