Image from Steve Bowers

A tide is the alternate rise and fall of the surface of an open body of water or other fluid on a rotating hab, moon, or planet. Measurable tides arise from the periodic gravitational tug from some body that is sufficiently large and close to produce a significant tidal force if the period of that influence is close to the natural frequency (seiche period) of the lake, sea or ocean in question. Details of the shore and bottom profile of the body of liquid can further amplify that effect. On Old Earth, tides caused by the influence of Sol and Luna were important all along the shores of the major oceans, and were a factor for baseline humans and all the other organisms found there. They are just as significant today for many Terragens who live on or visit the shoreline, whether that shore is in a rotating hab or is on a natural or terraformed world.

When an object is in the gravity well of a nearby body, items on the surface experience a small net acceleration towards that body when they are facing it, and a small net acceleration away from the nearby body when they are facing away from it, and effect known as tidal force. Though this effect is typically very small the repeated tug of this effect can set a basin of fluid (typically water) rocking back and forth, in the same way that a small basin of water can be made to spill over its rim if it is shaken at its own resonant frequency, known as the seiche period. The typical result is two high tides and two low tides per rotation along the shores of that basin, though of course the details and timing vary according to shoreline shapes and other factors. Somewhere in the middle of that body of fluid, since it is sloshing back and forth, is a point that doesn’t see a rise and fall in level, called the nodal point.

Whether tides are actually important on a given body of fluid depends on three things. The first is the mass of the external influence. The larger it is, the greater the influence, in direct proportion. The second factor how near the body is, and therefore how steep the gradient of the “acceleration” from its tidal effect. This varies as the inverse cube of distance. An object that is twice as close for instance has eight times the influence. The last factor that influences the strength of the tides is the size of the body of fluid. If it is the right size to be in resonance with the regular tidal pulls, there will be significant tides, and if it is not then actual tidal effects will be insignificant. For instance, on a typical Terragen planet with a rotation period measured in hours, such as Old Earth, only the major oceans are the right size to have tides that may be measured on the scale of metres, while isolated seas may have ranges of less than a metre and very large lakes have tides that are measured in centimetres or fractions of a centimetre.

If there are two bodies close enough and large enough to cause tides, they will sometimes work together and sometimes against one another. In the case of Old Earth, Sol was able to raise a tidal effect equal to about one third that of Luna. When they were aligned to work together the Lunar tides were thus higher, and when they worked in opposite directions the Lunar tides were significantly smaller. On a hab (or on the rare rotating moon) that is in orbit around a large gas giant there are similarly times in its orbit when the local star and the gas giant are aligned and tides are larger and times when they are opposed and the tides are smaller.

Planets that have been made habitable by Terragens rarely have lunar tides as large as those of Old Earth, since very large and close moons are relatively rare. Ridgewell is an exception, being rocked by two moons, one with 15 times Luna’s influence and another with 50 times Lunar strength. However since most of the rocky worlds that have been terraformed are around stars dimmer than Sol, many of them have much stronger solar tides. For instance, a terraformed planet in the habitable zone around a K type star may have tides between 4 and 40 times as great as those experienced on Old Earth, and worlds near an M type star have tides greater yet, if they do not have captured rotation. Natural moons of large gas giants usually have captured rotation too, but if they are spun up before they are worldhoused or terraformed they can have spectacularly large tidal effects on their surfaces: hundreds of times the tidal range that the original Terragens were accustomed to.

Habs large enough to have significant bodies of water may also experience tides, if they are orbiting near a star or planet. However with the exception of extremely large objects like Banks Orbitals most rotating habitats spin much more quickly than a planet if they hold Terragens, and so the kinds of body of water that are significantly affected by tides is different. Though the details vary according to the acceleration the inhabitants have chosen to experience to simulate gravity, as a rule of thumb those who live on objects like a Stanford Torus or Bernal Sphere can expect tidal effects on bodies of water that are about 100 metres across, those using O'Neill Cylinders need to take that into account for smallish lakes in the 500 metre range, and ecosystem engineers for Bishop Rings or McKendree cylinders need to think of tides on bodies of water that are about 1 to 3 kilometres across. Some arrange the hab and its topography to avoid tides altogether, or orient the spin of the hab to make tidal effects less significant, but others have been known to make it part of the ecosystem, to the extent of gengineering organisms that actually depend on a high tide and a low tide every few minutes.

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Development Notes
Text by Stephen InnissInitially published on 27 February 2014.

Thanks to members of the Worldbuilding Group, particularly Mike Miller, Steve Bowers, Ian Campbell, Westcott P. Smith and 'xilman' for comments and suggestions. Special thanks to the late Poul Anderson for his essay The Creation of Imaginary Worlds: The World Builder's Handbook and Pocket Companion. It an invaluable resource. A review of by Robert Silverberg it may be found here.