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Like the title, my world has drastic tides that complete a cycle in roughly 25 days. is this possible? My limited knowledge of astronomy leaves me pretty limited. I understand that there would have to be a fairly large cosmic body acting on my planet to cause such drastic tides, but that the body would have to have an irregular orbit (if even possible) to extend the cycle to 25 days.

anything would be appreciated.

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  • $\begingroup$ "A mile" doesn't mean much as it really depends on how sloped the surface is. A relatively small tide on a large flat plain will advance extremely quickly. A large tide against a cliff will advance very slowly. $\endgroup$ – Tim B Sep 25 '18 at 15:32
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Yes it is possible. Take Mercury, for example.

'It takes Mercury about 59 Earth days to spin once on its axis (the rotation period), and about 88 Earth days to complete one orbit about the Sun. However, the length of the day on Mercury (sunrise to sunrise) is 176 Earth days.'

from https://www.windows2universe.org/mercury/News_and_Discovery/Merc_orbit_reson.html

Tides are not just created by moons, but by the sun. The closer to the sun, the stronger the tidal effects. The shallower the water, the greater the tidal effects. The slower the planet spins, the longer the 'day' and the solar tide. The lower the gravity of the planet, the greater the gravitational effect of the sun on the water. The length of the solar day determines the length of a solar tidal cycle.

However, it depends on what you mean by a 'day'. A solar day and a solar tide would have to coincide.

Or your world could be a moon around a massive planet, which creates a 'moon tide' on the moon itself. Thus, the moon tide and the solar day do not have to coincide.

The further away the moon is from the planet, the longer it takes to make one revolution. The length of the moon tidal period would be a combination of the rotation of the moon around itself, and the rotation of the moon around the planet. The length of a solar day on the moon would be determined by the rotation of the moon around its axis, and the rotation of the moon-planet combination around the sun. Eclipses would be a factor.

On Earth, the time of a moon tide and its strength is determined by the Earth's rotation around its own axis, and the period of the rotation of the Moon around the Earth. See the referenced article for an example of how this can be calculated for the Sun-Mercury combination.

You can do all of the calculations if your readership is absolutely demanding, or you can just assume the parameters are correct on your world for a non-discriminating readership. For a short story, assume. For a space opera, calculating it is perhaps the preferred method.

However to be science-based, your calculations would have to include an analysis of the tendency of the moon-planet combination to be tidally locked.

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Have a moon in an eccentric orbit. Generally it will be not much closer than our Moon is, but on one day of the 25, it will be very close. If it is a tenth the distance of our Moon, tides on that day will stretch hundreds of miles inland and recede similarly far. To be less extreme, just change the distance (it varies as 1/distance cubed) so having it be 1/2 or 1/3 the distance of our Moon would probably do the trick.

Of course, this might not exactly be what you are asking for. If you truly want a 25-day cycle (i.e. one high tide and one low tide every 25 days), then have the moon in an orbit of 1.02 days; however, if it were the size of our Moon, then the tides would be about 800x the size of Earth's. This should be made less extreme by making the moon 100x smaller than Earth's (it would also be at about 43000~44000 km semimajor axis; if it is similar density to our Moon, the apparent size would be roughly the same).

As an aside, radius of the planet increases tidal force proportional to it, but I am not sure whether that would increase the range as well.

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Tidal effects have a synchronizing effect on the planet and its moon. Given enough time, the moon's orbit and your daily rotation can get arbitrarily close.

As for the strength of the effect, the easiest way to do this is to simply have a very flat tidal basin. If the land is very flat, your tides can move as far inland as you please. Indeed, this is made even easier by your long tidal cycle, giving the water all the time it needs to move into place.

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