# If the earth was flat...? **URL:** \ **Category:** Factual Questions\ **Created:** [September 9, 2005, 1:03am UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755 "2005-09-09T01:03:05Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [September 9, 2005, 11:54pm UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/21 "2005-09-09T23:54:51Z")
> [@Chronos](#): > > Please note that this does not work on a Dyson sphere (at least not on the inside) since the gravitational field anywhere on the inside of a uniform spherical shell is zero (the opposite side of the shell is pulling you up as much as the near side is pulling you down). Ah. So that Dyson Sphere in the now-classic Star Trek:TNG episode had artificial gravity on its interior surface? > [@](#): > > And on a ringworld, it’d be irrelevant, since one would spin a ringworld to generate whatever gravity is desired, independant of the mass of the ring. Ah! Good to know. I’m not up on my Niven. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 10, 2005, 1:34am UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/22 "2005-09-10T01:34:36Z")
> [@toadspittle](#): > > > [@Chronos](#): > > > > Please note that this does not work on a Dyson sphere (at least not on the inside) since the gravitational field anywhere on the inside of a uniform spherical shell is zero (the opposite side of the shell is pulling you up as much as the near side is pulling you down). > > Ah. So that Dyson Sphere in the now-classic Star Trek:TNG episode had artificial gravity on its interior surface? It had to. This is a big flaw with the so-called “Dyson Sphere”–without some kind of gravity drawing down (away from the star it contains) the atmosphere would just disperse, and in fact a failure of a gravity generator (assuming they are distributed across the surface of the shell) would result in atmosphere jetting out away from the surface. Let it be noted that Freeman Dyson’s original proposal had multitudes of individual habitats orbiting the star at a variety of orbits creating a virtual shell, rather than the less practical single monolithical construct that has come to be associated with the term. The point of either type is to create a surface that absorbs essentially all of the radiation from the star. It was Dyson’s proposition that an extremely advanced civilization would have to build such constructs in order to provide sufficient energy and would therefore be invisible to our astronomical efforts to locate extrasolar civilizations. > [@toadspittle](#): > > > [@Chronos](#): > > > > And on a ringworld, it’d be irrelevant, since one would spin a ringworld to generate whatever gravity is desired, independant of the mass of the ring. > > Ah! Good to know. I’m not up on my Niven. Of course, as we all know, _the Ringworld is unstable!_😃 A ringworld is not gravitationally centered on the star it ostensibly orbits; in fact, as a rigid structure, it isn’t in orbit at all, and any planar displacement relative to the star will cause it to fall inward with positive feedback. Any lateral movement (along the axis of its rotation) will result in oscollatory movement with the ring hoping from one side of the star to the other (from the star’s point of view), and attendant precessive and nutational modes also result in progressive imbalance. In short, a ringworld (about a star) is really not a good idea. The ringworld concept has other problems as well, not the least of which is that the tensile strength of the material for a stellar-size ringworld is many orders of magnitude stronger than any concievable material. Niven circumvented the problem by creating _scrith_, a material that blocks 40% of neutrinos and has a tensile strength similar to that of the residual strong force but acting over atomic, rather than subnuclear, distances, which is a neat trick you can do if you happen to be a science fiction author who can make up physical laws as you go along. Back to the flat Earth; you could actually make the material rather thin (conservatively, about .25μm) if you could make it out of neutronium…provided, of course, that you could find some way to make it stable, rather than have it explode via degeneracy pressure or collapse into a quark-gluon quagma. Og help you if you get a ripple, though. Stranger ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [September 10, 2005, 2:36am UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/23 "2005-09-10T02:36:14Z")
> [@](#): > > ``` > * The shadow of the Earth on the Moon during lunar eclipses always has the same curvature. > * When travelling, you see the tops of distant tall objects first, then gradually, you can see more and more of them. > * No matter where you go on the planet, the horizon always appears roughly the same distance away. > > ``` I once read that World War I was the first conflict which had to take account of the shape of the planet. Gunners early in the war found themselves overshooting their targets. What was happening was that the range of the guns had become long enough that the curvature of the Earth had become a factor that the gunners needed to compensate for. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [September 10, 2005, 4:40am UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/24 "2005-09-10T04:40:12Z")
How can YOU tell if the earth is round? Say you’re in Germany at noon: call someone in the USA and they’ll tell you the sun is rising, call someone in Japan, and They’ll tell you the sun is setting. Time Zones on a sphere…how can that be reconciled with a flat disc? ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [September 10, 2005, 5:33am UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/25 "2005-09-10T05:33:19Z")
> [@Q.E.D.](#): > > Of course there would be: > [ul] > [li]The shadow of the Earth on the Moon during lunar eclipses always has the same curvature.[/li][li]When travelling, you see the tops of distant tall objects first, then gradually, you can see more and more of them.[/li][li]No matter where you go on the planet, the horizon always appears roughly the same distance away.[/li][/ul] > All of these things were noticed long before space travel. Since this thread is still open I’ll add a little to this. Actually we have no immediate visual signals that the earth is anything but flat. The items on the list above are clues that have to be interpreted in light of our knowledge of geometry and astronomy. The only immediate visual sign of a spherical earth is from space. So I’ll take back about 30% of my hasty and craven admission of error but leave 70% of it intact. ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [September 10, 2005, 11:24am UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/26 "2005-09-10T11:24:32Z")
> [@Enola Straight](#): > > How can YOU tell if the earth is round?Say you’re in Germany at noon: call someone in the USA and they’ll tell you the sun is rising, call someone in Japan, and They’ll tell you the sun is setting.Time Zones on a sphere…how can that be reconciled with a flat disc? Don’t bother me with facts. My mind is made up! ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [September 10, 2005, 6:23pm UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/27 "2005-09-10T18:23:40Z")
Since the subject of Ringworld and flat worlds has come up, I thought I’d mention the Alderson Disk. > [@Wikipedia](#): > > An Alderson disk (named after Dan Alderson, its originator) is an artificial astronomical megastructure, like Niven’s Ringworld or a Dyson sphere. The disk is a giant platter, like a CD or phonograph record. The sun rests in the hole at the center of the disk. The radius of an Alderson disk would be roughly equivalent to the orbit of Mars or Jupiter, with a thickness of several thousand miles. According to the proposal, a sufficiently massive disk would have greater gravity than its sun (though the fact that the pull of the disk as a whole would still be toward its center causes problems with this idea). > > Assuming the gravitational issues were overcome, however, life could exist on either side of the disc, although the closer to the sun one would get, the hotter it would get until life was impossible. Conversely, the further away, the colder it would become, until living beings would freeze. > > One drawback to a disc is that the sun remains in the same place. There is no day/night cycle, only a perpetual twilight. This could be solved by forcing the sun to bob up and down within the disc, lighting first one side then another. (As the disc far outmasses the sun, this is a plausible option.) ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 10, 2005, 10:17pm UTC](https://boards.straightdope.com/t/if-the-earth-was-flat/320755/28 "2005-09-10T22:17:20Z")
> [@Little Nemo](#): > > I once read that World War I was the first conflict which had to take account of the shape of the planet. Warships had to navigate properly in many conflict prior to WWI. To do so when out of sight of land, they often used celestial navigation which certainly takes account of the generally spherical shape of our planet. [Previous page](https://boards.straightdope.com/t/if-the-earth-was-flat/320755.md?page=1)