### How did they decide the length of one year even though it wasn't known that earth revolves around the sun?

• 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?

The seasons, perhaps?

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.

5 years ago

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.

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!

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.

@alephzero And tomorrow the tree will be a little bit taller...

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.