In 200 B.C. Eratosthenes showed that the earth is round and calculated it's circumference by means of an experiment. However:
How did Eratosthenes know that b) was the case and not a)?
I tried looking it up, the two answers I found were basically:
>they already suspected that the earth was round
or
>it was common knowledge at the time that the sun's rays that hit earth are (roughly) parallel
If the first, how could they argue against a), if the second, how did they know that?
>if the second, how did they know that?
That can be observed from anything that emits light. But you are right at some level. Technically they could not have been 100% sure because they had shit lighting like fires unlike the controlled lighting we have today.
This is simply a case of a genius' intuition saving the day. They had no way of 100% knowing that the rays were parallel but as Erastosthenes wasn't a fucking brainlet he knew exactly what was happening.
If your IQ is less than 160 you may not comprehend this notion of intuition but trust me, it works.
>trust me, it works
I thought this was Veeky Forums desu
>That can be observed from anything that emits light
... but the rays aren't parallel, no? They only appear that way because of how far away the sun is. So basically the question is, how did he know the sun was so far away?
>Science has no place for intuition
>I thought this was Veeky Forums desu
Yeah and you are asking a historic question which begs the question why the fuck aren't you on Veeky Forums?
>how did he know the sun was so far away?
INTUITION
NTUITIONI
TUITIONIN
UITIONINT
ITIONINTU
TIONINTUI
IONINTUIT
ONINTUITI
NINTUITIO
Brainlets would never understand.
That's not what I'm saying. "Just trust me on this" is how magic works, in science you should be able to explain it, no? Intuition might give you a hunch, but it's not a proof
>geometry isn't science if it happened in the past
>INTUITION
Calm down on the autism. Intuition isn't proof. So you're saying he really didn't know and was just making an assumption that turned out to be correct, though he couldn't justify it to anyone else at the time?
>So you're saying he really didn't know
No, I was clear on what I said. You can't observe this property of light by simply looking at anything that emits it. But I said that objectively, there was no way of 100% being sure because they only had natural lighting so they couldn't really do an experiment that confirmed it.
We can today but back then they had to take their inferior observations and their intuition.
>was just making an assumption that turned out to be correct, though he couldn't justify it to anyone else at the time?
Almost. But as you outline in your diagram, there are only two sensible possibilities. But as Erathostenes had the goal of finding the shape of the earth with this experiment, it is clear why he would choose the round-earth option. But after that he had to convince other people, otherwise his experiment would not be famous. He probably did justify to everyone, but it is just that his justifications are not to our standards today.
The only no-bullshit explanation is that he was really smart and his intuition guided him well.
A wouldn't give consistent results throughout the day like B. In A, the rays would become more divergent when it was closer to mid day, whereas they would stay equally divergent in B.
This is based on my own intuition, I'm not sure if this would be observable or was the case.
Camera obscura was known about then & Euclid knew light travelled in straight lines in 300BCE so it's fairly easy to suggest that far away light sources have rays that appear parallel
>You can't observe this property of light by simply looking at anything that emits it.
>That can be observed from anything that emits light.
I assume you mean can and not can't there?
If so, again: which property? That the rays are basically parallel if the light source is far enough away? If so, that's not the problem, it's clear to me that you can easily observe this.
The question is then simply, how did he know that the sun was far enough away that he could consider the rays hitting Alexandria to be parallel to those hitting Syene?
>He probably did justify to everyone, but it is just that his justifications are not to our standards today.
Right, but what I get from your post is "He was just really smart and knew, you brainlet", which wouldn't hold up at the time either.
Could you specify why, or what you mean by more divergent? In which direction, that is?
>far away light sources have rays that appear parallel
Right, in both models the light is traveling in straight lines. Like I said above, the question is, I guess, on what basis did he assume that the sun was so far away that he could consider the rays hitting Alexandria to be parallel to those in Syene?
Of course ancient people knew all along that the earth is round. Even back then they could observe ships disappearing in the horizon
he only measured earth's circumference, a) was known to him.
sun being very far away was also a known fact, as archimedes knew about it.
Okay, fair enough in that case. I guess at this point it does really become Veeky Forums
The ancients weren't retards, they had a model of our solar system even if it was wrong (they placed earth at the center)
'The ancients' is a long period to talk about, it includes, for example, Anaximander, who definitely didn't yet know that the earth was round.
But fair enough, like I said, if it was common knowledge at the time of Eratosthenes, then my question wasn't justified.
>what basis did he assume that the sun was so far away
The Sun was in the heavens, put there by the gods. It's pretty much the furthest conceivable distance at the time. it's actually sensible to think at the time that it is a great distance away
Sure, "ancients" as a term is pretty ambiguous but you still get the meaning
Flat Earthers literally think it's more reasonable to assume Earth's water magically doesn't flow off the sides of the Earth in a flat gravitational potential than to assume there's a spherical gravitational potential holding everything together.
They knew rays go parallel because from a point source they extend beyond obstacles parallel and along the line from their edges to the point source.
>how did he know the sun was so far away?
if the earth was some sort of infinite flat plane, the sun couldn't set. it's simple as that
>Be dumbfuck fisherman in 200bc
>See tall boat going away
>It drops under the horizon
Really mullers the gulliver
>How did Eratosthenes know that b) was the case and not a)?
Have you ever seen a ship departing from a port?
It wasn't just suspected, it was known.
They knew rays of light traveled in straight lines because they had mirrors. have you ever played with a mirror trying to get the light into someone's eyes? They did too back then, ad notices that the light reflected by the mirror always went in a straight line. As for why the earth would be round, there's plenty of observational data for that, like noticing that a ship traveling away from shore disappears under the horizon from bottom to top, which is consistent with traveling on a curved surface (same shit is observed when going over a hill for example) but not a flat one (think about carriages traveling over flat terrain).
It was only a matter of putting 2 and 2 together.
>In 200 B.C. Eratosthenes showed that the earth is round
Wrong. The experiment did nothing of the sort. The world was already known to be a globe. The sole purpose of the experiment was to determine its size.
This experiment on a flat Earth could produce the same result. Using three points or more (properly placed) would reveal the spheroidal geometry of the Earth.
sneaky flat earth thread?
>How did Eratosthenes know that b) was the case and not a)?
the proof that earth is a sphere was known and published by aristotle. ships disappearing over the horizon, earth's shadow on the moon etc. are his arguments. only trolls on Veeky Forums pretend that those aren't decisive.
There is one source and shadows are uniform, so i guess he decided to assume b.
>earth's shadow on the moon
note: not the normal monthly lunar cycle
>Like I said above, the question is, I guess, on what basis did he assume that the sun was so far away that he could consider the rays hitting Alexandria to be parallel to those in Syene?
look into a well when the sun is at its height. there are no shadows... as the day goes on, the shadow is a straight line from the sun changing slowly
im not even sure what you're asking at this point, or what people are discussing. his approach was pretty intuitional
>>earth's shadow on the moon
>note: not the normal monthly lunar cycle
Lunar phases have nothing to do with the Earth's shadow.
I thought this was sci and a hint would be sufficient because everybody read aristotle anyway, and knows that he talked about eclipses.
He didn't prove that the earth is round. He calculated it's radius with knowledge of the spherical shape of the earth.
Ancient Greeks actually left their basement, looked up in the sky and there was it, the MOON. And it was ROUND. And it had PHASES Meaning it was a SPHERE. They also observed solar eclipses. So they found out the sun had to be further away then the moon. Now they used their knowledge in maths and calculated sizes and distances.
> if the earth was some sort of infinite flat plane, the sun couldn't set
Something is infinite flat plane the sun couldn't set.
The Sun never sets on the British Empire.
Therefore, British Empire is some sort of infinite flat plane.
>the earth is round
Fake news.
>How did Eratosthenes know that rays
>from the Sun are parallel and not divergent?
Solar parallax is not hard to measure, user.