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I assume this conclusion was hard to make empirically in the days of slow travel -- one could not as we can today fly from above the equator to below it and observe the difference in weather and even if one could, it would be hard to attribute this to a single cause as weather is very changeable.

I further assume some ancient understood the thing about the Earth being tilted on its access but even to me (not an astronomer or even very sharp mentally) it is far from obvious that this would affect the seasons -- I do think that only if one accepts Heliocentrism that this phenomenon can be accounted for.

Is it then possible that no one in western Europe in, say, 1500 believed that seasons were reversed and could explain it. I do not know what Indians or Chinese knew about this.

EDIT: I realized that a ship sailing south enough would still know what date it was and people might well note that it was colder or warmer than the same month where they came from. They might attribute this to local weather but if the differences were pronounced enough, at different longitudes, someone might suggest that indeed seasons were reversed without understanding why.

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    $\begingroup$ I would imagine that "international" trade (though 'national' is kind of a misnomer in the ancient world) would require planning for travel, including the right clothes to bring, food required, knowing what will be in season and available where, etc. If you assume you'll be traveling in summer in 3 months, and you get there three months later and it's winter, that would certainly stick in the mind! But this is all my own conjecture. $\endgroup$ Commented Nov 26, 2022 at 18:58
  • $\begingroup$ I think that if you came from a northern country, with serious winters like England, you would not fail to notice that February in, say, the Cape of Good Hope was sunny and I would assume that in that same cape they would also see snow in August, it would make quite an impression and indeed would have to be planned for. I doubt that the actual explanation was known until relatively recently. $\endgroup$
    – releseabe
    Commented Nov 26, 2022 at 20:35
  • $\begingroup$ I asked this somewhat-related question on HSE a few years ago: history.stackexchange.com/questions/46819/… $\endgroup$ Commented Nov 27, 2022 at 17:43
  • $\begingroup$ @AzorAhai-him-: I wondered I think also here about the average person in say 1600 having any sense that people living many degrees longitude that the local time was different for them -- very hard to accept probably even if explained. I sure remember being surprised by this as a little kid until maybe I spoke on the phone with relatives on the opposite coast who claimed (rightly, as it turns out) that it was already dark there. $\endgroup$
    – releseabe
    Commented Nov 27, 2022 at 17:55

2 Answers 2

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In the Purgatorio (c.1310) Dante places Mount Purgatory at the antipodes of Jerusalem and follows the rules of hard SF by thinking out all the consequences.

Longitude and time zones - this may be the first time in literature that someone has realised that 180° in longitude means 12 hours of time zone (he also has time zones for Rome and the Ganges).

Anticlockwise Sun / Sun to the north - Dante the character in the poem is thoroughly startled by this and Dante the poet has quite a bit of fun at his expense, as it all has to be explained to him.

Latitude and seasonal changes - Mount Purgatory being mostly above the Earth's atmosphere, it does not of course have weather. But Canto 4, in which Dante-the-character eventually manages to understand what is going on, handles the matter in terms of whether the Sun's path is "our side of the equator" or "the other side of the equator" - with "our side" and "the other side" changing places as we go from Jerusalem to the southern hemisphere.

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  • $\begingroup$ Remarkable -- I wonder what inspired Dante: something generally known in what would become Italy in those days or something Dante himself worked out? $\endgroup$
    – releseabe
    Commented Nov 25, 2022 at 15:09
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    $\begingroup$ @releseabe Even with some historical context, it’s hard to say. The aspect of Purgatory being antipodal to Jerusalem was, IIRC, somewhat novel, but most of the rest of La Comedia (not just Purgatorio) implies that most of Dante’s views on geography, meteorology, and other physical sciences were relatively ‘normal’ for the time period. $\endgroup$ Commented Nov 25, 2022 at 16:50
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    $\begingroup$ In his description of cosmos Dante certainly follows Ptolemy (or some exposition of his books). Almagest has been translated into Latin in 1175. $\endgroup$ Commented Nov 26, 2022 at 1:10
  • $\begingroup$ A popular textbook on astronomy, De sphaera mundi by J. de Sacrobosco was written in 1230. Most likely it was Dante's source. $\endgroup$ Commented Nov 26, 2022 at 1:17
  • $\begingroup$ Islamic astronomers were using astrolabes to determine precise daily prayer times synchronized with Mecca - ie, developing time zones - by the early 9th century at the latest; possibly several decades earlier. Islamic mariners were at the same time also exploring the Indian Ocean as far south as Zimbabwe and northern Madagascar, easily far enough south to recognize seasonal reversal. $\endgroup$ Commented Nov 27, 2022 at 2:14
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One does not need heliocentrism for this. The path of the Sun in the sky (as seen from the Earth) was well understood in Hellenistic times, and probably long before that. It was also understood how this path determines seasons. So probably these phenomena were known before Hipparchus. I say probably because very little astronomic work before Ptolemy survived to our day, so one cannot be sure who noticed this explicitly for the first time. Maybe even Thales who wrote books on solstices and equinoxes, but we have no way to know what was in those books.

Another thing is that this was not common knowledge, just a few astronomers (they were called philosophers at that time) understood these things. There is a famous story described by Herodotus (IV, 43) about an expedition which sailed around Africa, from the Red sea to Gibraltar by Indian and Atlantic oceans. Herodotus wrote: "According to their account (which I do not believe, let anyone believe who wants) they saw the Sun on their right hand side". It is exactly this "improbable" detail which is considered a proof that this expedition really happened:-)

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  • $\begingroup$ Maybe u want to edit this slightly? Do you mean "just a few astronomers..."? I would like to accept this answer but want to be sure about what is meant. $\endgroup$
    – releseabe
    Commented Nov 26, 2022 at 20:39

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