# How can you prove the earth is round?

How can you prove the earth is round and not flat using physics

if anyone can help, thanks!

arildno
Homework Helper
Gold Member
Dearly Missed
How can you prove the earth is round and not flat using physics
1. You can't "prove" anything in physics.

2. The negation of "being flat" is not "being round"; rather, the negation is "being curved".

First off, the facts that, aside from bumps, valleys, mountains, etc. we generally have the impression that the Earth is "flat", we would have exactly the same impression as long as the curvature of the Earth was sufficiently small, but, crucially, non-zero.

Thus, we have no evidence whatsoever in favour of the hypothesis that the Earth is "flat".

Furthermore, we DO have lots of evidence that is far easier to read as evidence for the "curved" proposition, the one already known in antiquity is that of the horizon, how ships appear/disappear there.

On a flat Earth, the image of the ship would gradually decrease in size as its distance grows, but NOT that the lower parts of it disappeared from view.

How ships actually disappear or appear at the horizon is, however, fully consonant with the "curved"
hypothesis.

russ_watters
Mentor
Is taking a photograph of it physics?

1. You can't "prove" anything in physics.

2. The negation of "being flat" is not "being round"; rather, the negation is "being curved".

First off, the facts that, aside from bumps, valleys, mountains, etc. we generally have the impression that the Earth is "flat", we would have exactly the same impression as long as the curvature of the Earth was sufficiently small, but, crucially, non-zero.

Thus, we have no evidence whatsoever in favour of the hypothesis that the Earth is "flat".

Furthermore, we DO have lots of evidence that is far easier to read as evidence for the "curved" proposition, the one already known in antiquity is that of the horizon, how ships appear/disappear there.

On a flat Earth, the image of the ship would gradually decrease in size as its distance grows, but NOT that the lower parts of it disappeared from view.

How ships actually disappear or appear at the horizon is, however, fully consonant with the "curved"
hypothesis.

This could be a combination of perspective effect and atmospheric refraction causing objects to appear or disappear from the bottom up at the vanishing point.

I have been told this is purely perceptual. a good telescope with sufficient zoom will change the observer's perspective and restore the ship's hull back in full view. This would not be possible if the shop was beind a "hill of water". The fact that a telescope can restore a half-sunken ship demonstrates that the ship is not traveling behind a convex sea.

- Oh and photos are not sufficent evidence. anyone can manipulate a photo.

arildno
Homework Helper
Gold Member
Dearly Missed
Is taking a photograph of it physics?

That wasn't an available option prior to the 20th century, so I chose to post the option that has been available since ancient times.

Not that I disagree with photographic evidence..

Staff Emeritus
2021 Award
I have been told...

Oh and photos are not sufficent evidence. anyone can manipulate a photo.

But nobody can manipulate what they tell you?

You have a pretty unusual idea of what constitutes good evidence and what does not.

arildno
Homework Helper
Gold Member
Dearly Missed
- Oh and photos are not sufficent evidence. anyone can manipulate a photo.
And you might just be deluded and brainwashed into thinking you are speaking English, whereas in reality, you are speaking Fulfulde.

Alfi

I seem to remember that this was one of the first experiments for answering the question.

And you might just be deluded and brainwashed into thinking you are speaking English, whereas in reality, you are speaking Fulfulde.

hmmm...nope, im pretty sure i'm speaking english. sorry...:uhh:

siddharth
Homework Helper
Gold Member
This could be a combination of perspective effect and atmospheric refraction causing objects to appear or disappear from the bottom up at the vanishing point.

Knowing the distance to the ship, and the temperature of the air at different heights, one can calculate how refraction occurs. Similarly, if you know the height of the mast, and the velocity of the ship, and observe the rate at which the mast drops, you can verify the radius of the earth.

- Oh and photos are not sufficent evidence. anyone can manipulate a photo.

Right, there's an international conspiracy among different goverments which have launched satellites or sent astronauts into space :uhh:

Look at the moon during a lunar eclipse, when there are no clouds. Or, take a jet to a different latitude and look at the night sky. Then, there's also Newton's law of gravity, looking at other planets through telescopes, foucalt's pendulum, etc.

Alfi said:

I seem to remember that this was one of the first experiments for answering the question.

Absolutely. That was Eratosthenes's brilliant experiment.

Last edited:
arildno
Homework Helper
Gold Member
Dearly Missed
hmmm...nope, im pretty sure i'm speaking english. sorry...:uhh:
That doesn't count, your confidence might just be the result of an EFFECTIVE brainwashing.

russ_watters
Mentor
This could be a combination of perspective effect and atmospheric refraction causing objects to appear or disappear from the bottom up at the vanishing point.

I have been told this is purely perceptual. a good telescope with sufficient zoom will change the observer's perspective and restore the ship's hull back in full view. This would not be possible if the shop was beind a "hill of water". The fact that a telescope can restore a half-sunken ship demonstrates that the ship is not traveling behind a convex sea.
That's just plain wrong.
- Oh and photos are not sufficent evidence. anyone can manipulate a photo.
Fair enough. If this is just plain a matter of belief, then there is no scientific content here and nothing to discuss.

I find it odd that you would post on a physics site that you want a question answered without physics. You should see that that doesn't fit with the purpose of this site.