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I want to create a world for a story: Charybdis - a world of high tides. Very high tides. Tides that drag whole ocean around the world - or most of it - in a long cycle. It must lasts at least several months, if not longer. I want to make regular cycle of deluges and dry seasons to be dominant force on the planet.

Basically I want this to look like this: there is dry cycle, where things hunker down in torpor, underground or sheltered by thick shells. Then ocean comes, tidal waves smash everything that is not secured and oceanic cycle begins. Fish comes, kelp forest sprout, everything frantically eats and reproduces. Then, after several months ocean leaves, leaving behind small remnant seas that slowly shrink. Meanwhile salt resistant savanna grows and land animals migrate from not submerged areas to graze. Everything slowly dries, awaiting new high tide.

Charybdis is flatter than Earth, so there is less water to be dragged around but it covers much larger surface. The same source that creates tides fuels tectonic activity and it’s a good thing, because something has to raise new land as it is under constant erosion by tides. It also makes new volcanoes for the tidal deluge to pour in.

Now there is question. Can I have this? Is there some way that Charybdis can orbit some other celestial body (no Black Holes, please), that can raise such high tides in a long cycle?

I tried to imagine some orbital configurations but as I see there are problems with everything:

Moon-planet configuration - both Charybdis and its partner body would have to be tide locked to each other. Stationary tide bulge stays forever in one place on the “visible to each other” hemisphere.

Star and planet - that would mean very long day and night cycle, that would climatically and thematically dominate the Charybdis - not tidal deluges. Also star would have to be very big or very close.

So, the question is - is there some reasonable way for my world to have long tidal cycles?

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  • $\begingroup$ It takes time to become tidally locked. So early in the solar system formation. $\endgroup$
    – Jon Custer
    Jul 9 at 15:21
  • $\begingroup$ Same as this? worldbuilding.stackexchange.com/questions/230268/… $\endgroup$
    – Willk
    Jul 9 at 17:09
  • $\begingroup$ @Wilk - thanks, I must have missed this question somehow. I will check it. $\endgroup$
    – Zjerzy
    Jul 10 at 1:26
  • $\begingroup$ @Jon Custer - Tidal locking is rather fast. It wont give enough time for life of enough complexity to menace heroes. $\endgroup$
    – Zjerzy
    Jul 10 at 1:28

2 Answers 2

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Good choice on avoiding neutron stars and black holes. I'm pretty sure the planet in Interstellar would have been roasted by x-rays given the time dilation they specified.

The short answer is "no." You have conflicting priorities. If the two bodies are tide-locked to each other, then the wall of water wouldn't move. It would always be right under the other body. That's the definition of tide locking.

If we loosen up that requirement, we could theoretically give it a month-long day with a 45 degree tilt, so that the object it orbited would be above a shifting patch of planet. Now we run into problem #2, which is that the wall of water would exist both on the near and far side.

Which is where we run into our third problem, the thing it's orbiting. There's no way you could do this with a moon. As you run up the scale of sizes, the near parter couldn't both have a month-long orbital period AND be close enough to cause those tides.

Think about it. The current Earth/Moon configuration has an orbital period of a month, and tidal variation of around 16 meters. My guess is that you're looking for a couple of thousand meters. Only the peaks visible at high tide. If I understand my math right, that would require a secondary mass close to that of the Sun. The orbital range would increase with the mass, but you'd be inside the Sun.

There is a solution. Not a black hole, not a neutron star, but a trillion year old white dwarf. Actually, that would make it a black dwarf. A reject from the heat death of the universe, thrown back in time by some cataclysmic event. Basically a lump of diamond and ice, held by gravity at just above the electron degeneracy pressure, with nothing left to fuse.

It would be dense enough that you wouldn't be inside of it, and cool enough that it wouldn't make the existence of water impossible. It would make the planet very, very tectonic. The planet would actually have a one-month day, orbiting the dwarf at whatever period you chose. The dwarf would need to be a binary system, orbiting an actual star so that the planet wouldn't suffer Mercury's fate, with one side being fried while the other freezes.

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  • $\begingroup$ I was thinking rather about several hundred meters of ocean - that's why I want Charybdis to be flatter and drier than Earth. $\endgroup$
    – Zjerzy
    Jul 10 at 21:41
  • $\begingroup$ Ok, now you're talking about something close to a twin planet. I don't know that you'd be able to do super-flat with those tidal forces. Look up Jupiter's moon Io to see what strong tides do to a rigid body. The surface would either be rugged or molten. Granite is basically a chemical froth that builds up in basalt subduction zones and floats on the basalt like bubbles, and that makes continents. Maybe you have smaller continents? You're the author, though, so apply artistic license as necessary. $\endgroup$ Jul 11 at 15:33
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Higher Order Spin Orbit Resonance

Tidal locking is inevitable if you want that much tidal forces involved. However, tidal locking doesn't automatically mean a 1:1 spin-orbit resonance. 1:2, 3:2 (Mercury), 2:1, 5:2, 3:1, 7:2, and so on for every half-integer ratio of rotation period to orbital period are possible. In fact, as the eccentricity of the orbit increases, as 1:1 resonance becomes impossible. Highly eccentric orbits work best if there are few planets in the system and if there is a bigger planet in mean-motion resonance that stabilized the eccentric arrangements (think Neptune and Pluto).

Probably best to put this around a small star. Note that your days will be wild, but habitable (check the link for further information) additionally, you will have eccentricity induced seasonal variation.this is actually a good thing, since the world's axial tilt will tend towards zero due to tidal effects.

If you want a more complex setup, this would also work if the central body were a huge gas giant or brown dwarf orbiting the star in turn.

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