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I'm writing a short story that takes place on an imaginary planet that is 88% ocean. The planet has two moons and one continent that is regularly flooded under 500 to 1500 feet of water. The tides are only extreme once or twice a month. I'm trying to work out the parameters for a two-moon system that produces such an effect. I'm working from the bottom up. This regularly flooding continent is essential to the story, so what is the best way to make this work?

I'm thinking one moon orbits at a very high velocity, while another moon is very large and further away, but when they align the flooding gets to be bad. Does this make sense?

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    $\begingroup$ I do not know why someone would give a down vote. I think we have a troll. $\endgroup$ – Kavi Vaidya Jun 26 '18 at 7:42
  • $\begingroup$ As mentioned in a similar question, for such extreme tide, it would rather be a moon with an ocean and tide cause by a planet. $\endgroup$ – Madlozoz Jun 26 '18 at 11:26
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    $\begingroup$ Related: worldbuilding.stackexchange.com/q/71/28 $\endgroup$ – Monica Cellio Jun 26 '18 at 12:54
  • $\begingroup$ How often does this extreme flooding occur? Every moon cycle, every couple moon cycles, every year, couple of years, decades/s? $\endgroup$ – EveryBitHelps Jun 26 '18 at 22:51
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Look at planet Earth: also there tides are extreme only when the Moon and the Sun (the main attractors when it comes to the tides) are aligned with the Earth.

Approximately twice a month, around new moon and full moon when the Sun, Moon, and Earth form a line (a configuration known as a syzygy), the tidal force due to the sun reinforces that due to the Moon. The tide's range is then at its maximum; this is called the spring tide. When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth, and the solar tidal force partially cancels the Moon's tidal force. At these points in the lunar cycle, the tide's range is at its minimum; this is called the neap tide, or neaps.

If you want to change the height of the tides, just work with the distance between the moon and the planet, or the mass of the moon.

A second moon added to this system would introduce a further pulsation and would likely not make possible the effect you want

The tides are only extreme once or twice a month.

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  • $\begingroup$ Why? It would mean increased tides when 1 moon is eclipsing the other because it would be like sine waves with different frequencies and amplitudes adding up. So for example, if you have a moon which is like our moon in both orbital speed and mass and you have another moon with half the orbital speed and twice the mass, the tide, to simplify things would be like: $sin(x)+2sin(1/2x)$ $\endgroup$ – Caters Jun 26 '18 at 4:04
  • $\begingroup$ @Caters, plus the sun... $\endgroup$ – L.Dutch Jun 26 '18 at 4:59
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    $\begingroup$ Also the extreme height he's wanting are a bit far fetched, the highest spring tides has more to do with the water depth "feeding" it and channeling around the continent in question. London has a tidal range of 7 metres which is "fed" by the north sea, however the river Severn, almost identical location (on a planetary scale) which is "fed" by the Atlantic has the second largest in the world, 15 metres... whereas a fair distance away, Miami has just 1.2 metres... so it would require more than just the moons to create the flooding he wants $\endgroup$ – Blade Wraith Jun 26 '18 at 6:33
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    $\begingroup$ @BladeWraith Well, he doesn't make a statement about the depth, what if the planet has a rather low gravity and strong tectonic movements creating a gigantic underwater mountain range all in one place. That might make a continent possible with a really deep ocean. Now you have one big moon creating a pull in a rather wide area, and with the planet's low gravity it does quite fairly. Additionally (when the two moons are aligned) the smaller moon creates more pull in parts of the area. Someone in the matter would have to do the math but I as a layman wouldn't consider it impossible. $\endgroup$ – Otto Abnormalverbraucher Jun 26 '18 at 13:39
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    $\begingroup$ @OttoAbnormalverbraucher: but 500 to 1500 feet? as i said before, a part of the extreme range is channeling the water between two landmasses so for the entire continnet to be flooded... is unlikely, even if we assume he meant that much of a rise above normal sea level, and not the entire continent flooded by that much it is a very extreme rise and at once a month or so? the speed at which it would have to rise would destroy absolutely everything in its path and mean that the continent itself would be a complete barren wasteland. $\endgroup$ – Blade Wraith Jun 26 '18 at 14:13
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As mentioned in other answers, for the purposes of tides, the earth already has two 'moons', i.e. the moon and the sun. The difference in their gravity pull between the near side and the far side of Earth determines their influence on the tides. For nearby bodies this will be a much bigger difference.

The height of the tides however is much more determined by the shapes of the oceans and continents. There are places in the middle of oceans where there is nearly no tide at all. The strongest tides are at coasts where tides are funneled into a bay or similar body that has the right size to resonate with the pull of the moon. Tides in the oceans are more comparable to a bathtub on wheels that is being shaken and in which standing waves form, than with water flowing around the world following the moon's pull. So for a planet that is mostly water, with small continents that do not block the east to west flow of oceans like earth's continents do, the tides would be lower than on earth.

Another thing to note is that not just the oceans experience tides, the earth mantle does too. On the scale of the entire planet, the crust and mantle rock is flexible enough that you don't notice it. Earth tide is about a meter high. A closer moon will also increase the earth tide, which reduces the relative increase in ocean tide. Since water is lighter and much less viscous than rock I would still expect the ocean tide to increase if you put the moon closer, but I'm not sure about the details.

The best way I can think of for creating once-a-month very large tides would be to have a large moon in a very elliptic orbit. When it comes close to the planet you will have a single high tide (depending on how close the moon comes and how fast the planet rotates), when the moon is far away for a long part of its orbit, tides will be much smaller. One gotcha with such an orbit is that it is probably not stable over astronomic time scales, the tidal forces will act to circularize the orbit over time. So you can't have this configuration for the age of your planet if the planet is billions of years old as the earth is. But the moon could have been slung in this orbit by an encounter with another body a few 100000 or million of years ago. Given the previous paragraphs I'm not sure how high a tide this could create. My guess would be that you could still have higher tides, but you still need to make sure the continents are rather flat. My guess would be tides of a few hundred meters would be possible, but not kilometers. And your continents will need to be small and with big ocean inlets. Water still takes a lot of time to flood the land, and a big continent where the closest ocean is thousands of km away from the center would not have enough time to flood in a day.

Another note: a planet system with multiple large moons would also be unstable on astronomic time scales. One moon is fine, multiple small moons is also fine, but multiple large moons only works for a long period of time if the central planet is much larger, e.g. gas giants, so that the moons are small compared to their host planet.

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  • $\begingroup$ This is extremely helpful, thank you. I imagine the continent as a giant, intertidal biome, so I'm thinking I need the billions-year old history for some species to have evolved. An elliptical orbit caused by an encounter seems like a good planetary history. Since geography seems to play an essential role, the continent will probably be flat, longitudinally long with not too much latitudinal width (Like Chile if it were isolated from South America), with an inlet-laden coastline. The moons can be smaller, perhaps like Mars's moons. $\endgroup$ – Anthony Worman Jun 27 '18 at 16:19
  • $\begingroup$ @AnthonyWorman Small moons like Mars (12 and 22 km in diameter) work nice for having a stable system, but for large tides you will need a large moon. I don't know how quickly or slowly an elliptical moon's orbit actually circularises, you might want to calculate it if you care a lot about physical accuracy. Did you take into account that at least on earth, the continents moved significantly during evolutionary history? Though of course you could have a single continent stay at the same latitude by chance. $\endgroup$ – JanKanis Jun 28 '18 at 17:02
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As L.Dutch pointed out, the alignment of the Moon and Sun cause the biggest variation in tidal range but the largest of this on Earth is about 17m (50feet).

Interestingly, in a large open body of water the actual difference in level is minimal (usually less than a metre) and is instead caused by the geography of the land.

Even if you were to have two moons on opposite sides of the planet to cause as much spring tide as possible, 500 feet is way too high. The coast may suffer from high tide but as mentioned before, the shift in a body of water is too small to account for flooding an entire continent.

A possible solution around this might be to have the continent abnormally flat and close to that of sea level. Then even a slight change in ocean height could cause serious issues, but not enough to cause 500 feet worth of flooding.

Bring the Moon closer! Accounting for the Roche Limit, what if you moved the Moon closer to Earth? After a bit of reading it looks like roughly 6000 km surface to surface would be a good ballpark figure for how close we can place the Moon in a "consistent" orbit. But at this distance its orbit speed would be much higher so now you have the pull on the Earth (and thus tides) would occur more often.

Using this calculator the difference in gravitational pull of a Moon at this distance would increase by over 750% its current value. So maybe 500 feet isn't so unlikely as I first thought...

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    $\begingroup$ There's a secondary calculation you haven't done, you've only calculated the water tidal change, not the land tidal change with the moon at that distance. $\endgroup$ – Separatrix Jun 26 '18 at 9:39
  • $\begingroup$ @Separatrix That was the difference in gravity between a normal Moon and a close Moon, rather than the tidal force. But good spot. This was more guesswork as including the difference in planet bulging etc is far to complex for me :P $\endgroup$ – Jonnyboy Jun 26 '18 at 13:24
  • $\begingroup$ Thanks for the input. I'm thinking two small moons, one closer and with an eccentric orbit and the other further away (300,00 km) and tidelocked. The geography of the continent probably should be flat at the coast with a sudden rise at a longitudinal mountain range caused by a faultline, not unlike Chile if it were an isolated continent. $\endgroup$ – Anthony Worman Jun 27 '18 at 17:05
  • $\begingroup$ It's an interesting question. The problem is balance: If you have two smaller moons orbiting an Earth sized planet both with different orbit types, it will be difficult for those 3 objects to remain in equilibrium. You can change this by increasing the size of the planet, but then its gravity will be stronger so the effect of the moons is less $\endgroup$ – Jonnyboy Jun 28 '18 at 6:55
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Just have one moon (about the mass of ours; call it Moon A) in a highly eccentric orbit around the planet (perhaps it got flung into this orbit due to a collision with a Pluto-mass object, since the orbit, as mentioned in other answers, is unstable over billions of years). Then one can add a smaller one (around the mass of asteroid/planetoid Ceres which is about 0.03x that of our Moon) in a close in inclined orbit.

Then when the moon A reaches perigee at about 6x closer than our Moon is to Earth, the tidal range will be (IIRC) 6^3 ~ 200x higher than on Earth over that one small time period (less than a day). These tides can easily get over 1500 feet (and depending on the time of day, its maximum can go as low as 500, especially considering the geography of the land).

The purpose of the second moon, Moon B, is to be in roughly a geosynchronous orbit (closer in than A's perigee) so the planet will get tidelocked over geologic time with it, not with Moon A's much longer orbit on the order of a month (ensuring a comfortable day length).

Of course, one major side effect is that, during that day every month that the tides swell to 1000+ feet, the tidal forces will create massive volcanic eruptions and earthquakes.

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  • $\begingroup$ cool stuff, thanks for your input. how massive will the eruptions be? Would they cause mass extinctions? I am hoping to have a history of some evolved intertidal lifeforms existing on this planet. $\endgroup$ – Anthony Worman Jun 27 '18 at 16:22
  • $\begingroup$ Depends. Probably life will adapt to them. Occasionally they could cause mass extinctions, but the areas of the continents that get flooded would be less affected. I'm thinking of some land life in the mountains that needs to worry. $\endgroup$ – Majestas 32 Jun 27 '18 at 16:40
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An entirely different way of creating your occasionally flooding land, you could also consider the land being made of floating mats of plants. You'll need to invent a story on why they sink occasionally, but since we're now talking biology there could be lot of reasons. Perhaps the plants evolved this behavior to get rid of the land creatures that feed on them, and the land creatures adapted to this leading to a kind of biological arms race.

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Does the continent have to flood just by the moons affecting the tides?

You could always make the continent that floods be below sea level. This would introduce a different problem on what drains it so it wouldn't just hold the water and be a lake.

If it was bowl-shaped with a "hole" in the bottom it could effectively drain back out.

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  • $\begingroup$ Hello, Deathonus, and welcome to Worldbuilding. While this is a possible answer, could you explain where the "drain" would lead to, and what would make the flooding cycle cyclic? Please take our tour and visit the help center to learn more about the site. Have a nice day! $\endgroup$ – Gryphon Jun 26 '18 at 20:46

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