AbbyNormal

Warning, strange question approaching!!
I have recently read about Eratosthenes, the man credited with being the first to put forth the notion that the Earth is round. He also was the first to calculate the circumference of a round Earth, to surprising accuracy, given the tools available to him. For those not familiar with his experiment, Eratosthenes noted that, on the summer solstice, a stick driven into the ground in Alexandria cast no shadow while a similar stick 800 km south in Syrene did cast a shadow. He concluded that this was only possible on a curved Earth.
So, there's the background, here's my question. The big underlying assumption in his experiment was that the Sun was far enough way that its ray's could be assumed to be parallel once they intercept the Earth. Was that a reasonable assumption to make at the time? I know, I know, this sounds like a really strange question. But, what was known about the distance to the Sun 2200 years ago? Could this not also be used as a way to measure the distance to the Sun, assuming a flat Earth? If the Earth is flat, then the data produces a distance to the sun of roughly 6520 km (whew...and you complain about the summer heat now! ). What was Eratosthenes' justification for such a large distance?

Regards,
AbbyNormal

Godbert

I think it was pretty established among greek scientists (if you want to call them that) that the earth is a sphere (ok, not exactly). Mostly because of the horizon I guess and also from seeing it's shadow during lunar eclipses. There was even a guy (Aristarch?) who had proposed the heliocentric arrangement around 280BC, which is why I don't see why Copernicus is celebrated so much for his plagiarism. But I digress. Those greeks surely did some impressive thinking that wasn't equalled for a long time. Of course they didn't have to put up with christianity, those lucky bastards

Shadowy Man

I'm pretty sure Aristotle suggested the Earth was round based on lunar eclipses in his On The Heavens circa 350 B.C.E.

AbbyNormal

I was just recently reading about the Ionian scientists of about 400 (ish) BC. This was the first I'd read about them and I was amazed at the advanced notions being put forth then, some of which would go into hiding and not resurface until the time of Keplar and Newton. Anaximander suggested that humans arose from other animals 2000 years before Darwin!
After reading about these acheivements (I'm reading Sagan's Cosmos) I felt cheated. The Ionians were making tremendous process and they valued nature and reason so highly.....and then along came mysticism and denied us of any meaningful advancement for 2000 years! I actually feel angry and resentful...can you imagine what we could have become had the Ionian tradition continued?
Okay, I'm done ranting.

Regards,
AbbyNormal

Jayjay

I think that the roundness of Earth was not exactly unheard-of in Eratosthenes' time. Greek sailors could observe it from the way the land "rises" from the horizon, for example... in fact, heliocentrism had been proposed before Eratosthenes made his estimate, which wouldn't work out very well for flat Earth.

capnkirk

While your description is true to the spirit of the experiment, you have inadvertantly switched the respective shadows for Alexandria and Syrene. Syrene sits almost on the Tropic of Cancer, with Alexandria 800km further north.
By making additional observations at the opposite solstices at each site, sufficient data could be obtained to sufficiently narrow the possibilities. The predicted difference between the longest and shortest lengths of noon shadow are different for a flat earth and nearby sun than for a curved earth and distant sun. That is: for a flat earth, if the observed difference at Alexandria was A, then the calculated difference at Syrene would be B. The difference at Syrene was NOT B. Ergo, the flat earth (nearby sun) presumption is false. For the curved earth (infinitely distant sun) case, the calculation predicts a difference of C. The observed difference is C. Assumption is true.