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When was the mechanism behind seasons on Earth discovered?

I'm talking about the angle between the Earth's rotational axis and its orbital axis. I read on Wikipedia here that

Earth's obliquity may have been reasonably accurately measured as early as 1100 BC in India and China.

But did they know this is related to the cycle of seasons?

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    $\begingroup$ "as early as 1100 BC in India and China" is wiki-nonsense. $\endgroup$ – fdb Jan 18 '17 at 0:19
  • $\begingroup$ related: hsm.stackexchange.com/questions/5565/… $\endgroup$ – fdb Jan 18 '17 at 0:24
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    $\begingroup$ @fdb - "as early as 1100 BC in India" might well be wiki-nonsense, but "in China" is not. One does not need to know the shape of the Earth to measure the obliquity of the ecliptic. Measure the solar zenith angle at local noon on the winter equinox and again on the summer equinox. The difference between these angles is twice the obliquity. $\endgroup$ – David Hammen Jan 20 '17 at 12:20
  • $\begingroup$ @user40085 -- You are asking two very different questions here. The title asks about the mechanism. The question at the end of the post asks whether ancients knew that the obliquity is related to the seasons. The geocentric concept of the obliquity of the ecliptic and the heliocentric concept of the Earth's axial tilt are two different views of the same thing. Alexandre Eremenko answered your second question. So which question are you asking? $\endgroup$ – David Hammen Jan 20 '17 at 12:28
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To explain seasons you do not need a precise measurement of the inclination of the ecliptic, it is sufficient to know the fact. I suppose that this fact was known to the first people who started agriculture, that is more than 12 thousand years ago. Indeed, agriculture strongly depends on the change of seasons. You have to know when to plant your plants for optimal results. 12 millennia ago, people did not have calendars (not speaking of internet), and did not even know the precise length of the year, and they did not count days (from which day will you begin your count anyway?). So how did they determine when seasons begin and end, and when it is appropriate to do seasonal agricultural works by observing the sky? Only by astronomical observations.

It is clear that you will get poor results if you rely on "today's weather" for sowing.

This is confirmed by the earliest texts on agriculture that we possess, like Hesiod. He does not write "On March 27 do this and this" but he writes "When Sun is in such and such position on the sky (with respect to such and such star, do this and this".

We do not know whether peasants did astronomical observations individually or were advised by their rulers or priests, perhaps the practice varied from one culture to another.

The beginnings of the seasons are determined by simple astronomical events: equinoxes and solstices. So any ancient agricultural society must have been aware of these events, and on the motion of the Sun on the ecliptic.

Position of the ecliptic in the sky determines that the days are longer and the Sun is higher in summer than in winter. So what can be more natural than to assume that this is exactly what CAUSES summer and winter? It is clear that we obtain all heat from the Sun (I am not counting firewood). Thus the warm season happens when the Sun stays higher and the days are longer. And this inequality happens exactly because the ecliptic is inclined under certain angle.

Of course we do not have written sources from 12 millennia ago to confirm all this, but it seems very plausible that people observing the sky were aware about connection between seasons and inclination of the ecliptic, since these observations began, that is since the beginning of agriculture, the latest. And the earliest known manual for agriculture (Hesiod) confirms this.

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  • $\begingroup$ This gets our ancestors conceptually from angle of the sun to seasons, but how do they get from angle of the sun to the tilt of the earth's axis? The axis tilt is root cause of seasons, the ecliptic inclination is just the observable by-product. $\endgroup$ – Nuclear Wang Jan 18 '17 at 18:17
  • $\begingroup$ It just depends on the point of view. In geocentric system it is the inclination of ecliptic, in Copernicus the tilt of the axis, it is the same angle anyway, what difference does the name make? $\endgroup$ – Alexandre Eremenko Jan 18 '17 at 19:01
  • $\begingroup$ It may date back to the Ancient Babylonians as well, as they had their idea of a 360 day year and the 12 lunar months. In their calendar they also have some kind of seasons. If you only consider the heliocentric view, at least Aristarchus of Samos (ca. 310 – 230 BCE) knew about it: universetoday.com/33113/heliocentric-model $\endgroup$ – Christoph S Jan 18 '17 at 20:25
  • $\begingroup$ @Christoph S: I was writing about much earlier times, before writing and formal calendar was invented. You cannot really do agriculture unless you practice some astronomy. $\endgroup$ – Alexandre Eremenko Jan 19 '17 at 7:39
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    $\begingroup$ @AlexandreEremenko I was thinking of places like Rwanda, with no pronounced seasons at least based on temperature (on second thought: possibly dry/wet seasons). I am looking for solid evidence that basic knowledge of astronomy is required for agriculture; that's not obvious to me. People could have used rules like "sow wheat when the so-and-so trees start to sprout leaves", i.e. using other biological clues in nature to time their agricultural activity. $\endgroup$ – njuffa Jan 19 '17 at 20:39

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