[40four]

I don’t know the earth is pretty big :) It is a near perfect circle, as are all the other planets & moons we observe. Maybe the rings of Saturn?

Just kidding around really, but I have to imagine that pockets of circular water currents like this, are more common than we would think.

We just can’t easily observe the dynamics of water flow until something like this happens, where a bunch of ice builds up and makes it obvious.

[danwills]

A near-perfect sphere? sure! Only looks-like-a-circle from some distant projection.

Vortices in a near-2d-flow like a river will absolutely be circular, vortices need to be coherent in some plane in order to sustain themselves.

The specific circumstances here must be that that the water is on the edge of freezing during downstream flow and there is a naturally-very-long-lived vortex in one region for it to initially form the ice-disc. Must also be followed by some very-gently-bounceing-off-the-edges, and the river never getting too thin for it to get stuck.

[ergothus]

The earth is pretty lumpy, what with mountains and oceans, and the earth is where I learned the term "oblate spheroid".

[Talanes]

Less lumpy than a billiard ball. The mountains and trenches only seem significant because the whole thing is so big. Totally not a perfect sphere though, yeah.

[scbrg]

> Less lumpy than a billiard ball.

Well, no: https://ourplnt.com/earth-smooth-billiard-ball/

The myth is springs from the fact that if you had a smooth earth with a radius of center to Mariana Trench, and another smooth earth with a radius of center to Mt. Everest, you could shrink both of them by the same factor and fall within allowed billiard ball specifications. That says nothing about smoothness, only about the allowed size of ball.

If you actually shrunk the earth we're living on, the ball would still be rejected as quite too rough.

[djmips]

If you shrunk the earth down to the size of a billiard ball and the imperfections where only as rough as the bumps in medium sandpaper then that's still pretty darn round even if it wouldn't be acceptable under official billiard ball specifications. I never could get behind the idea that it would be smooth as a billiard ball but to know you could feel the ridges is pretty neat and yet it's very very round.

[fctorial]

Earth is a disk?

[imron]

On the back of 4 elephants, then it's turtles all the way down.

[saalweachter]

One turtle, and everyone is very worried about the order a second would be added in.