# Ocean tides on a Banks Orbital

Consider a Banks Orbital, a space station three million kilometers in diameter, rotating once per day for 1g artificial gravity, intermediate in size between a Bishop Ring and a Niven Ring: https://www.orionsarm.com/eg-article/4845ef5c4ca7c

I'm trying to figure out what conditions would be like if you were living on such a structure. That is, what the conditions would tend to be like, generated by the physics of the system, in absence of further deliberate modification.

The main factors driving weather on a planet are uneven solar heating by latitude, and Coriolis force. On an orbital, these factors are absent and much weaker respectively.

The accepted answer to this question points out that there will be uneven solar heating between a flat floor and steeply climbing sides: Prevailing winds on a rotating space habitat

And the answer to this question discusses the effect of solar tides: Weather on a mini-ringworld/Banks Orbital

In summary, apparently the solar tide would drive a high-altitude wind to match the apparent motion of the sun.

I'm now wondering about the tidal effect on the ocean. Suppose there is a single connected ocean running the full circumference of the Orbital. My first guess would be by analogy with a planet, a tidal bulge would be created in the directions toward and away from the sun, producing two high tides per day, just like on a planet.

But is that correct? This object is much larger than a planet. Would six hours be enough time for water to slosh a significant fraction of the circumference?

Conversely, if solar tide produces a steady wind following the apparent motion of the Sun – I don't know what mechanism produces that – but if so, would the same mechanism apply to the ocean, and produce a steady current? If not, why not?

Would six hours be enough time for water to slosh a significant fraction of the circumference?

Waves travel faster than the individual particles in the medium which the wave is travelling through. This is why you can hear people when they talk to you, but don't get blasted by a jet of air moving at the speed of sound.

Here's a nice visualization of water particle movement in a deep ocean wave:

Cropped from a larger animation in this article. Though the article is about wind waves, they're just a specific kind of gravity wave, and tides are another kind.

In deep water, the motion of the particles is fairly negligible. It is only when you reach shallower water and coastlines and the wave is disrupted that you get interesting effects... wind-driven waves get surf, tides generate strong currents, whirlpools, bores, all the rest.

So: 6 hours is fine. The tide will roll around the world in that time, but the water does not have to.

by analogy with a planet, a tidal bulge would be created in the directions toward and away from the sun, producing two high tides per day, just like on a planet.

With an orbital, the water is on the inside of the curve. This means you don't get tidal bulges where you would on a planet: instead you get tidal "dimples" because gravity and centrifugal effects are pushing water away from the hub of the orbital. I think this will have an effect on the "shape" of the tide (the rates and change of rates of rise and fall) it'll be basically the same as a planetary tide.

would the same mechanism apply to the ocean, and produce a steady current? If not, why not?

Coriolis effects can be neglected at the scale of humans in an orbital, but the plates themselves and the structure as a whole is big. Movement of the sea in the direction of rotation of the orbital will tend to push particles outward, towards the sea-bed. Movement against the direction will tend to push particles upwards, towards the hub.

Large currents in a circumferential direction therefore seem likely to break up into horizontal vortices... not necessarily like vast, deadly whirlpools of doom, but present nonetheless, like the horizontal cousin of an oceanic gyre on a planet. This will inhibit large scale current forming.

There's a larger and more complex question here, related to the problem of whether big rotating habitats inevitably form giant rotary winds which destroy everything, or whether you get steady-state smaller scale winds, etc. The exact nature of the coriolis effects can be simulated if you were familiar with computational fluid dynamics, but they're not entirely intuitive. My guess probably isn't much better than yours, and CFD is hard (and the tools are expensive). One day someone might answer these questions properly, but probably not today!

• Thanks, that is actually good news! I was disappointed by the other answers suggesting convective airflow exactly the wrong way around, warm air rising over the central ocean dumping rain where it's not needed, then descending over the edges as hot dry air turning the highlands into desert. But from what you're saying, it sounds like more chaotic airflow that makes interesting weather and at least sometimes brings rain where it's needed, is at least consistent with our limited knowledge. Jan 29 at 0:02
• @rwallace one thing that most authors of works with big rotating habitats don't consider is careful engineering of the landscape to help break up large wind patterns and ensure that rain falls where you want it to fall. Howard Taylor does consider this in his Schlock Mercenary webcomic with Eina Afa, which has baffles and swiss-roll clouds of the sort you might get in a big spun habitat. Clever stuff, though unclear how accurate it might be. Jan 29 at 10:05