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It seems to me that the ancient Greeks knew that geographic location affected the apparent position of the sun in sky but given the lack of rapid travel or communications or reliable clocks, how was this ever discovered?

I am guessing that travelers might have noticed that their own internal clocks became out of synch with the local time but again, travel was so slow that I assume there really was no "jet lag" -- moving at most, aboard a ship which I think would have been the fastest was to move large distances, about 100 miles a day the change in local time from that of the traveler's origin would have been too slow for any jet lag-like phenomenon.

EDIT: I understand that once it had been concluded that the Earth was round and revolved, an unusually educated person would understand, perhaps, that this implied that the sun would be in a different position in the sky at the same time to people in two far-apart cities.

But I would suggest that very few people could visualize this idea. There was no way to communicate to the person in the distant city -- the idea of simultaneity might have been very hard to grasp, again, since time and solar position were so tightly coupled.

There was a very clever (I assert) inventor who devised a time-keeper for court in ancient Greece and he made provisions for the time-keeper to run differently at different times of the year -- he did not have, I think, the idea of an hour but rather some equal division of daylight. Abstract time may not have been a thing to the ancients and therefore simultaneity in two distant locations may not have been a thing.

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  • $\begingroup$ Can you confirm you really mean "longitude" ( i.e. the local time is different with the sun being a greater or lesser proportion of its east-west track across the sky) , and not "latitude" (i.e. the sun's highest point in the sky is higher or lower)? $\endgroup$
    – Spencer
    Commented Jul 8, 2020 at 19:06
  • $\begingroup$ i mean that people in cities significantly west/east of each other would see the sun in a different position at the same time which might have been a hard concept since time was tied to sun's position. $\endgroup$
    – releseabe
    Commented Jul 8, 2020 at 20:51
  • $\begingroup$ Certainly, ancient Greeks can't noticed change in internal clocks while traveling. Jet lag is called "jet" for a reason. Theoretically they could notice that moon eclipse was observed at different local time from different longitude. But I don't know that they actually made such observations. It's more likely that they knew that local time should be different just because they knew that Earth is round, but have not confirmed it experimentally (they have other confirmation that Earth is round). $\endgroup$
    – Alexei Kopylov
    Commented Jul 8, 2020 at 23:12
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    $\begingroup$ It is hard to miss that daylight duration drifts when you are sailing into or away from the Sun, even without precise clocks. More precise measurements of local time differences were always a problem, but the phenomenon was known to navigators. Hipparchus even proposed a method of calculating longitudes from local times based on observing lunar eclipses c. 250 BC, see History of longitude. Of course, Greeks accepted that the Earth was round for at least two centuries by then. $\endgroup$
    – Conifold
    Commented Jul 9, 2020 at 7:39
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    $\begingroup$ Bakker discusses this issue in Epicurean meteorology, p. 173. He suggests that Hipparchus's lunar eclipse method might have been known since c. 331BC (I mistyped above, Hipparchus was c. 150BC, not 250), and Pliny (c. 50AD) reports that local time difference was noticed when warning fires were lit west to east. But planetaria models were built since the time of Eudoxus (c. 350BC) and one would have seen in them that Sun's relative position depends on the meridian. So it might have been noted by astronomers first. $\endgroup$
    – Conifold
    Commented Jul 9, 2020 at 21:37

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This is an immediate logical consequence of sphericity of the Earth. Greek tradition credits sphericity to Pythagoras, but modern historians even doubt that he ever existed. So the question has no exact answer. Those Greek writers on astronomy and geography whose work survived take this fact as evident.

Chinese mathematicians who thought that the Earth is flat (in 2nd century AD) but the distance to the Sun is moderate (comparable with the size of the Earth) also concluded that the time will be different at different places. The only model which would give you the same time at all places is flat Earth and enormous distance to the Sun (so that the Earth size is negligible in comparison with this distance). As far as I know, nobody proposed this model.

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  • $\begingroup$ while it is, even a very bright person know that the Earth is spherical might not conclude that the sun is far enough away to be in a different position at distant points on the sphere or that that implied that , hey, while i'm still sleeping in the morning, they are already awake -- that would require powerful intuition, i think. $\endgroup$
    – releseabe
    Commented Jul 9, 2020 at 1:53
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    $\begingroup$ Why does it matter how far it is? If the Earth is spherical, Sun evidently rotates about it (we all see this), so it stands in the South direction at various times at various places. $\endgroup$
    – Alexandre Eremenko
    Commented Jul 9, 2020 at 1:56
  • $\begingroup$ Maybe I am not that sharp, but it sure would not occur to me if I had been an ancient Greek that at the same time the sun would be in a different place. do you accept the idea that abstract time would not have been a readily understood concept to people then or do you think simultaneity is just obvious? I am pretty sure it is a very modern concept that only existed in the minds of very intelligent people before the 19th century. $\endgroup$
    – releseabe
    Commented Jul 9, 2020 at 2:15
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    $\begingroup$ It is the same as if someone walks around your house, it's clear that when you look from different windows you will need to look in different directions to see that person. One don't have to have PhD to understand that. And you don't need to know whether Earth is revolved or Sun goes around the Earth. You only need to know that Earth is round. $\endgroup$
    – Alexei Kopylov
    Commented Jul 9, 2020 at 5:51
  • $\begingroup$ Or imagine how should it be arranged to have the same time at all places on the Earth: the Earth must be flat (say floating in a huge ocean, and Sun describing an arc of enormous radius (in comparison with the size of the Earth), setting to the ocean and rising every day. So that the directions from the various points on Earth to the Sun are approximately parallel. $\endgroup$
    – Alexandre Eremenko
    Commented Jul 9, 2020 at 11:44

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