# How can one prove that the earth is a sphere (or close to it)? **URL:** \ **Category:** Factual Questions\ **Created:** [October 15, 2018, 11:50am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973 "2018-10-15T11:50:12Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Sloe\_Moe](https://avatars.discourse-cdn.com/v4/letter/s/8797f3/32.png) [@Sloe\_Moe](https://boards.straightdope.com/u/Sloe_Moe)\ **Post date:** [October 16, 2018, 1:37am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/41 "2018-10-16T01:37:19Z")
Beside the Erathosthenes work, an ordinary straight line traverse 500 to 1000 kilometers long should show that your points, adjusted to sea level, will no longer be aligned in a cross section. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [October 16, 2018, 2:11am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/42 "2018-10-16T02:11:01Z")
> [@cormac262](#): > > My first thought was “if the earth were flat, why would there be a need for time zones ?” That is, a flat earth would all experience the same “day” and “night”. But the flat-earthers have an explanation for that: the sun is a LOT closer than you think, and in being so close, the “daylight” is focused on only parts of the (flat) earth at a time. See #24. In what flat-earth model can you _lose an entire day_ just by traveling? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 16, 2018, 2:12am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/43 "2018-10-16T02:12:09Z")
> [@cormac262](#): > > My girlfriend’s brother works with a guy who is a flat-earther. So much so that his daughter caused a stir in school by confronting her teacher who was showing a spherical earth. > This guy has been dying to “discuss” his view with me - with the challenge for me to prove him wrong. > > I’ve given this a lot of thought, and even checked out the flat earth website (there are SO many questions about that model, and you need to have some “basis” or foundation from which to start any counter-arguments). > I decided two things: > > 1. that to prove that the earth is round might be “too much”. So I decided to focus on dis-proving the flat earth model. I figured if I could poke holes in his view, he would then be open to the truth and see how none of the failings are valid with the true model. > 2. it would be best to dis-prove his view with observations that he could see/confirm personally. He would not have to trust anything other than his own experience. (no trusting those “faked” NASA photos, etc.) > > My first thought was “if the earth were flat, why would there be a need for time zones ?” That is, a flat earth would all experience the same “day” and “night”. But the flat-earthers have an explanation for that: the sun is a LOT closer than you think, and in being so close, the “daylight” is focused on only parts of the (flat) earth at a time. > > I considered the eclipse idea (with the flat earth model, it is unclear “what” causes lunar eclipses). But this would require having him observe a lunar eclipse, and this could take a while. > > So my next idea was kind of an inverse of Eratosthenes’ approach. Being that the flat-earther is in the northern hemisphere, I was going to have him measure the angle from the horizon to the North Star (I even made a crude sextant for him) at different locations (one where he lives, one several hundred miles south). IF he believed the North Star to be “far enough” away, then the angles (his latitude) would be different. This could not be with a flat earth. > Again using the North Star, if I could get him to travel south of the equator, I was going to have him confirm that he could no longer see the North Star. But this was a bit trickier in that it might be tougher to find “reference stars” to “point to” the North Star (or where it should be) as well. I thought this might be too prone to error to be conclusive. Really, the only “proof” you need is a cell phone with GPS. If he is reasonably intelligent, ask him how the GPS timing would work for anything but a spherical Earth. ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [October 16, 2018, 2:23am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/44 "2018-10-16T02:23:41Z")
[UNDENIABLE! One Simple & Fatal Flaw of the Flat Earth “Model”](https://www.youtube.com/watch?v=m0x8CEp58uE) ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [October 16, 2018, 2:25am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/45 "2018-10-16T02:25:37Z")
> [@Napier](#): > > Have you actually tried this? While in the Navy, I spent a decent amount of time on a ship at sea. The phenomenon of seeing a distant ship “hull down” (i.e. with only its superstructure visible above the horizon), then watching the whole ship become visible as it gets closer is something encountered dozens of times a day. As distance increases, the opposite effect is noted, until all that’s visible is the very highest point on the ship, then nothing. As you’d expect, lookouts stationed well above the water routinely see further. From a lower place, you’ll reliably see less of a distant ship (or island, or building on the shore, or anything else). To be sure, on some days poor visibility (rain, fog, etc.) interferes with this. But in general it’s clear, apparent, obvious and unmistakable - not in any way subtle, obscure or mysterious. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [October 16, 2018, 2:55am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/46 "2018-10-16T02:55:26Z")
Perhaps the simplest demonstration of the failure of all flat-earth models is a video of the night sky looking south in the southern hemisphere (see 1:32 in [this video](https://www.youtube.com/watch?v=huysYcz-AiQ)). You clearly see all the stars rotating around the southern celestial pole - which flat-earth models say is impossible. ---
**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:** [October 16, 2018, 3:27am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/47 "2018-10-16T03:27:28Z")
The simplest demonstration that the Earth is a sphere is to get yourself a good pair of shoes and a swimsuit and start traveling in a straight line. No matter which direction you choose, you will end up back at your starting point without ever having encountered an edge. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 16, 2018, 3:33am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/48 "2018-10-16T03:33:46Z")
> [@Xema](#): > > Perhaps the simplest demonstration of the failure of all flat-earth models is a video of the night sky looking south in the southern hemisphere (see 1:32 in [this video](https://www.youtube.com/watch?v=huysYcz-AiQ)). You clearly see all the stars rotating around the southern celestial pole - which flat-earth models say is impossible. Nah, it’s all due to refraction. Refraction explains _everything_. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 16, 2018, 3:39am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/49 "2018-10-16T03:39:09Z")
If you are going to rely on geometric arguments, no need to navigate all the way around the Earth. Just mark off a reasonably big triangle and sum up its angles… ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 16, 2018, 3:40am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/50 "2018-10-16T03:40:59Z")
> [@septimus](#): > > See #24. In what flat-earth model can you _lose an entire day_ just by traveling? In the [standard flat-earth model](https://www.youtube.com/watch?v=hiOhsKAR6OY). ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [October 16, 2018, 3:44am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/51 "2018-10-16T03:44:10Z")
> [@Xema](#): > > Perhaps the simplest demonstration of the failure of all flat-earth models is a video of the night sky looking south in the southern hemisphere (see 1:32 in [this video](https://www.youtube.com/watch?v=huysYcz-AiQ)). You clearly see all the stars rotating around the southern celestial pole - which flat-earth models say is impossible. Oh yeah?: [![]( " - YouTube") ](https://www.youtube.com/watch?v=kWsvHOyHMc8) ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 16, 2018, 3:58am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/52 "2018-10-16T03:58:58Z")
> [@mcgato](#): > > The circumference of the earth was estimated in about 240 BC by [Eratosthenes.](https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_the_Earth's_circumference) **That pretty much implies that the earth is spheroid.** Nope. I can get the same results with a flat Earth and a Sun at an altitude of 40722 stadia (around 5000 miles) ---
**Author:** ![Isamu](https://avatars.discourse-cdn.com/v4/letter/i/7c8e57/32.png) [@Isamu](https://boards.straightdope.com/u/Isamu)\ **Post date:** [October 16, 2018, 4:06am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/53 "2018-10-16T04:06:58Z")
> [@Alpha\_Twit](#): > > Lunar eclipses > > As the Earth passes between the Sun and Moon, it always produces a circular shadow. It’s not possible for a flat, disk like Earth to always produce such a shadow. Only a nearly spherical world can do that. “Always” is problematic. No one has been around to check the “always” results, and it is possible for a flat round disc to repeatedly cast a round shadow. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [October 16, 2018, 5:15am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/54 "2018-10-16T05:15:49Z")
> [@scr4](#): > > In the [standard flat-earth model](https://www.youtube.com/watch?v=hiOhsKAR6OY). So in the standard flat-earth model, the earth isn’t flat but instead has a mound peaking at the North pole, to separate night from day? And to reach the edge of the Earth you just sail to Antarctica and start walking “South”? OK. Of course when you start calculating the actual curvature parameters of this “mound”, e.g. by sailing completely around the Southern Ocean to measure the circumference there, you’ll discover … the shape is a sphere! ---
**Author:** ![Bullitt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bullitt/32/5725_2.png) [@Bullitt](https://boards.straightdope.com/u/Bullitt)\ **Post date:** [October 16, 2018, 5:21am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/55 "2018-10-16T05:21:38Z")
> [@Chronos](#): > > **ftg** , those shapes don’t cast circular shadows. [Here’s a picture of some](https://www.thingiverse.com/thing:649488). > > **Bullitt** , want to lay odds that the argument, in full, consists of “the details of the eclipse prove that the Earth is flat!”, with no actual calculations? Just like their other arguments. At least they’re consistent. And consistently wrong. ---
**Author:** ![Alpha\_Twit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alpha_twit/32/265_2.png) [@Alpha\_Twit](https://boards.straightdope.com/u/Alpha_Twit)\ **Post date:** [October 16, 2018, 5:26am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/56 "2018-10-16T05:26:19Z")
> [@Isamu](#): > > “Always” is problematic. No one has been around to check the “always” results, and it is possible for a flat round disc to repeatedly cast a round shadow. Do you have any documented observations of a non-round Earth shadow falling across the moon? We do have centuries of well documented observations of round Earth shadows creating lunar eclipses on predictable schedules from hundreds of sources. I am content with my use of “always” in this context. If you want to present theories regarding what actually has been reliably and repeatedly observed, wonderful. Bring it on. ---
**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:** [October 16, 2018, 5:27am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/57 "2018-10-16T05:27:06Z")
> [@DPRK](#): > > If you are going to rely on geometric arguments, no need to navigate all the way around the Earth. Just mark off a reasonably big triangle and sum up its angles… If things like facts and reason were going to work, these people wouldn’t be flat earthers in the first place. They’ll just invent some nonsense to explain any discrepancies in their geometry and keep on believing. That’s why you need something like taking a trip around the world. It has to be something so overwhelmingly obvious it shakes their faith and forces them to start thinking. ---
**Author:** ![Bullitt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bullitt/32/5725_2.png) [@Bullitt](https://boards.straightdope.com/u/Bullitt)\ **Post date:** [October 16, 2018, 5:31am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/58 "2018-10-16T05:31:19Z")
I wonder how they’d explain that, a trip around the world? ---
**Author:** ![crowmanyclouds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crowmanyclouds/32/19884_2.png) [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)\ **Post date:** [October 16, 2018, 6:22am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/59 "2018-10-16T06:22:19Z")
> [@beowulff](#): > > Nah, it’s all due to refraction. > Refraction explains _everything_. No, as someone that watches way too many flat Urf videos on the YouTubes, it’s perspective! CMC fnord! ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [October 16, 2018, 6:45am UTC](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973/60 "2018-10-16T06:45:22Z")
> [@Xema](#): > > While in the Navy, I spent a decent amount of time on a ship at sea. The phenomenon of seeing a distant ship “hull down” (i.e. with only its superstructure visible above the horizon), then watching the whole ship become visible as it gets closer is something encountered dozens of times a day. As distance increases, the opposite effect is noted, until all that’s visible is the very highest point on the ship, then nothing. > > As you’d expect, lookouts stationed well above the water routinely see further. From a lower place, you’ll reliably see less of a distant ship (or island, or building on the shore, or anything else). You don’t even need a ship; a tall building with a decent overlook of flatlands or of the sea is enough. Both the Mediterranean coasts and the flatlands of Castille can provide a few such; there has to be others available in the American flatlands. [Previous page](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973.md?page=2) [Next page](https://boards.straightdope.com/t/how-can-one-prove-that-the-earth-is-a-sphere-or-close-to-it/822973.md?page=4)