In chapter 21, how did the waterfall work with the Coriolis effect inside Rama?

Here's the description of the waterfall

So there was the origin of the sound they had heard. Descending from some hidden source in the clouds three or four kilometers away was a waterfall, and for long minutes they stared at it silently, almost unable to believe their eyes. Logic told them that on this spinning world no falling object could move in a straight line, but there was something horribly unnatural about a curving waterfall that curved sideways, to end many kilometers away from the point directly below its source. “If Galileo had been born in this world,” said Mercer finally, “he’d have gone crazy working out the laws of dynamics.”


2 Answers 2


The waterfall is following the laws of dynamics for the spinning Rama: water is falling away from the centerline of the cylindrical world, but is twisted by the Coriolis effect so that the water lands in an unexpected place - not directly below its source above, but off to one side. A similar effect occurs if someone near the center of a carousel tries to throw a ball to someone further out - relative to objects fixed to the carousel, the ball's flight is curved.

This illustration taken from Wikipedia shows the carousel example with someone one the outer portion of the carousel throwing the ball towards the axis of rotation but a very similar effect would occur with the ball going outward not inward.

As to the source of the water, the characters noted high humidity a few paragraphs earlier due to the melting of the Cylindrical Sea; I think it should be concluded that rainfall/condensation is the ultimate source of the waterfall, perhaps deliberately channeled to one location (or more likely three) by the Raman designers.

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    I'm gonna need a diagram
    – Valorum
    Commented Jul 16, 2023 at 17:23
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    I guess the real question is, why did the waterfall start from the Cylindrical Sea? Did the thawing of the ice cause some sudden "spurt" of water to suddenly project from the surface with sufficient force to over come the centrifugal force of Rama's spin?
    – Mwalston
    Commented Jul 16, 2023 at 18:27
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    @Mwalston I'd guess something sticking out that gathers condensation. Imagine spider web lattice of metal designed to trap moisture, that funnels it down into a single exit point.
    – Yakk
    Commented Jul 17, 2023 at 2:57
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    Personally, I think the animated gif at the top of the Wikipedia article (including its description) is even better than the diagram you linked to.
    – Heinzi
    Commented Jul 17, 2023 at 11:07
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    These are the Ramans. They'll channel the water to three locations.
    – Mark
    Commented Jul 17, 2023 at 22:52

Let's look at this from a different direction to see if it's any clearer:

The water is at the top of the waterfall. At that point it has a horizontal velocity due to the spin of Rama. Once free of the ground there are no other forces acting on it, as Newton says objects in motion stay in motion. It proceeds on the same path it was on.

Note that this path is completely horizontal by the frame of reference of the point it fell, not vertical as one would imagine. However, Rama itself is spinning, the farther from the center the faster it's spinning. Thus the ground is moving faster than the water, it appears to be deflecting in the direction opposite from it's actual movement. It's like going down the highway at 100kph, seeing the car next to you going 110kph and feeling like you're going backwards.

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