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It takes one year for the earth to go around the sun, but one year was already this long when they didn't even know that the earth revolves around the sun at all. So how did they figure it out?

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    $\begingroup$ The seasons, perhaps? $\endgroup$ – HDE 226868 Dec 31 '16 at 18:28
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    $\begingroup$ Duplicate of this: hsm.stackexchange.com/questions/2506/… $\endgroup$ – fdb Jan 1 '17 at 23:54
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    $\begingroup$ solstice? highest day of shadow of the year, equal days apart? quite a big marker of the year. they needed a system of days where festivals could be regular every year. it was of utmost importance. I live 15 miles from where the gregorian calander was signed by cat de medici or something on the river rhone. $\endgroup$ – com.prehensible Jan 4 '17 at 21:09
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Originally a year would be noticed as the time between two winters (or two harvests, two annual floods etc), although the exact number of days might not be known with any accuracy.

As people start to observe the sun more carefully, it would be noticed that the position of sunrise and sunset move regularly. Stonehenge and other ancient monuments seem aligned to midwinter sunset or midsummer sunrise. Since this would allow one to predict the seasons, it would have been very useful.

More careful observations led to the geocentric model, in which the sun takes just over 365 days to move around the Earth once relative to the background stars. The length of the year was known to be about 365.25 days since the times of the ancient Egyptians.

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  • $\begingroup$ Ah, of course. It occurred to me that the seasons and the position of the sun in the horizon were involved, but 365.25 days seemed like too precise a number to arrive at from just that. I forgot to consider the position of the other stars, haha. Thanks! $\endgroup$ – dbmrq Dec 31 '16 at 21:48
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    $\begingroup$ It's quite easy to make precise measurements, because you can use very large measuring instruments! Pick out a suitable marker on the horizon, maybe two or three miles away. An isolated tree will do fine, if there aren't any hill tops. Now line up the rising or setting sun with your marker, and put a stick in the ground to remember our postion. Hey, you just made a protractor for measuring angles - but unlike the little plastic one that you used in geometry class at school, this one is 3 miles in diameter. Repeat the experiment tomorrow, and your stick will be in quite a different place. $\endgroup$ – alephzero Jan 1 '17 at 9:33
  • $\begingroup$ @alephzero And tomorrow the tree will be a little bit taller... $\endgroup$ – Andrew Morton Jan 1 '17 at 21:33
  • $\begingroup$ The movement of the sun's position on the horizon is pretty noticeable. It wouldn't be unlikely that someone very early in our history decided to mark a position on the ground and go there daily to mark the sun's position. If they lived in a place with noticeable seasons, they'd associate the position of the sun pretty quickly. $\endgroup$ – Howard Miller Jan 6 '17 at 20:37
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There are several ways to measure a year. The time it takes for Earth to complete one orbit around the sun is called a sidereal year or 365.256363 days, and though it is a valid definition of a year, it is not the one our calendars measure, which is a tropical year. A tropical year is defined "as the period of time for the mean ecliptic longitude of the Sun to increase by 360 degrees," and is 365.24219 days in length. One way to think of the tropical year is the length of time it takes for the sun to complete its north-south journey through our sky.

There is a decent discussion of the various definitions of a year at Wikipedia. It's worth a read if you're interested.

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  • $\begingroup$ Interesting. That led me to this video, which is very nice: youtu.be/82p-DYgGFjI Happy new tropical year, then. :) $\endgroup$ – dbmrq Dec 31 '16 at 22:02

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