Live data from Hacker News

Walking on a cube-shaped planet

straightdope.com

11–20 of 79 posts

Re: Walking on a cube-shaped planet

#11

> On spherical earth the horizon on average is a little over three miles away. I read (long time ago) that the horizon is 29km on a seashore. In other words if you see a ship disappear on a horizon, it was 29km away. And so his 3mi vs my 29km is a bit of a discrepancy. Can anyone set things straight here?

http://en.wikipedia.org/wiki/Horizon#Distance_to_the_horizon

Wikipedia says 3.1 miles.

Re: Walking on a cube-shaped planet

#12

> On spherical earth the horizon on average is a little over three miles away. I read (long time ago) that the horizon is 29km on a seashore. In other words if you see a ship disappear on a horizon, it was 29km away. And so his 3mi vs my 29km is a bit of a discrepancy. Can anyone set things straight here?

The "distance to the horizon" is strongly dependant on how high the viewer is and how tall the object being observed is. If you put your eyes pretty much down to the ground the horizon will be pretty darn close. If you are standing 6 feet tall the horizon will be 3 miles away. If you're standing a bit higher up on the shore and looking at the top of a tall ship then 29 kilometers is easy to achieve. I'm providing a link to a handy calculator. You have to add the results from both the height of the viewer and the viewed object. http://www.ringbell.co.uk/info/hdist.htm

Re: Walking on a cube-shaped planet

#13
post #4

Earlier quoted context omitted.

Interesting point, but I'm not so sure. We take the shortcut of assuming that the earth's gravity is drawing everything toward a point at the center of the earth. But that's not really correct; every bit of matter is drawing everything else toward it. So those giant mountains that comprise the vertexes of the cube would be pulling the oceans toward them. That works against your point. I don't know where the equilibri…

A sphere with some gravitational influence from the points, but with most of the mass towards the center of the cube, it would still be mostly sphere-like, I think.

For a cube, what portion of its volume lies within an inscribed sphere? In other words, given a cube of edge=X and a sphere of diameter=X, what's the difference in volume?

Of course, this doesn't begin to account for the vectors, and it assumes that the density is uniform. But it should give us a sense of how much of the gravitational pull differs from our simplistic model.

Re: Walking on a cube-shaped planet

#14

I imagine walking on a cube-shaped planet will be not different that doing it on a sphere shaped one.

Why not? The most important thing about a round planet in this case (and, in fact, the reason planets tend towards roundness) is that it means gravity is (roughly) equal at every point on the surface (and, in fact, at every point on any planet-shaped shell inside it, as well). On a cube, this is entirely gone. The centers of the faces would have the strongest gravities; the corners would have the weakest. For localised walks we wouldn't notice the gravitational changes (just as we don't notice them when we walk up a staircase), but if you made the trek from the center of a face to the edge, you will definitely notice.

Re: Walking on a cube-shaped planet

#15
post #4

Earlier quoted context omitted.

A sphere with some gravitational influence from the points, but with most of the mass towards the center of the cube, it would still be mostly sphere-like, I think.

For a cube, what portion of its volume lies within an inscribed sphere? In other words, given a cube of edge=X and a sphere of diameter=X, what's the difference in volume? Of course, this doesn't begin to account for the vectors, and it assumes that the density is uniform. But it should give us a sense of how much of the gravitational pull differs from our simplistic model.

Easier to figure using the radius rather than the diameter. Volume of the sphere is 4pi/3 * r^3, volume of the cube is (2r)^3. Factor out r^3 and you get (4pi/3)/8, or 52% of the cube's volume is within the sphere.

Re: Walking on a cube-shaped planet

#16
post #4

Earlier quoted context omitted.

A sphere with some gravitational influence from the points, but with most of the mass towards the center of the cube, it would still be mostly sphere-like, I think.

For a cube, what portion of its volume lies within an inscribed sphere? In other words, given a cube of edge=X and a sphere of diameter=X, what's the difference in volume? Of course, this doesn't begin to account for the vectors, and it assumes that the density is uniform. But it should give us a sense of how much of the gravitational pull differs from our simplistic model.

The math on that is easy. The volume of a cube is X^3, the volume of a sphere is 1/6 * pi * X^3, so the difference is a little under 1/2 X^3. Put another way, the cube is (roughly) twice the volume of the sphere.

Re: Walking on a cube-shaped planet

#17

I imagine walking on a cube-shaped planet will be not different that doing it on a sphere shaped one.

Why not? The most important thing about a round planet in this case (and, in fact, the reason planets tend towards roundness) is that it means gravity is (roughly) equal at every point on the surface (and, in fact, at every point on any planet-shaped shell inside it, as well). On a cube, this is entirely gone. The centers of the faces would have the strongest gravities; the corners would have the weakest. For localis…

the trek from the centre to the edge or a corner would be rather difficult too....make a mistake and you will fall back to the centre

Re: Walking on a cube-shaped planet

#18
post #11

> On spherical earth the horizon on average is a little over three miles away. I read (long time ago) that the horizon is 29km on a seashore. In other words if you see a ship disappear on a horizon, it was 29km away. And so his 3mi vs my 29km is a bit of a discrepancy. Can anyone set things straight here?

http://en.wikipedia.org/wiki/Horizon#Distance_to_the_horizon Wikipedia says 3.1 miles.

Hmm, indeed. Even with the atmospheric refraction included.

Re: Walking on a cube-shaped planet

#19

I imagine walking on a cube-shaped planet will be not different that doing it on a sphere shaped one.

Why not? The most important thing about a round planet in this case (and, in fact, the reason planets tend towards roundness) is that it means gravity is (roughly) equal at every point on the surface (and, in fact, at every point on any planet-shaped shell inside it, as well). On a cube, this is entirely gone. The centers of the faces would have the strongest gravities; the corners would have the weakest. For localis…

I took the question (which I thought was rather absurd) and rationalized (and imagined it) this way: this cubic planet will not rotate, or it will not be cubic then. I took out gravity as we know it, and imagined I could just "walk" in it.

We walk on a sphere shaped planet, yet we do not "slide down" if we stand on the North Pole, yes? Same with a cubic one, as I imagine, the 90 degrees angle will be crossed without even realizing is there. I imagine we would perceive it as a flat planet.

As you can see, I did not take the whole thing too seriously. And that, of course, diminishes karma. So it goes. Cheers!

Post reply on HN