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redvillafranco t1_jab7ala wrote

Reply to comment by Successful_Box_1007 in Eli5: How did people know how long a year was in olden times? by Slokkkk

You can just look at where on the horizon the sun rises/sets every day. On the first day of summer, the sun rises the furthest north in the northern hemisphere. So you just count the days until it rises that far in the north again.

Also, you can measure the highest point of the sun in the sky each day. If you have the same length stick or rock on the ground. On the first day of summer, at high noon, the shadow will be its shortest length of the whole year. So you can just count the days until the shadow is that short again.

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Successful_Box_1007 t1_jab7kef wrote

And the ancients just assumed this meant that the earth had made a full rotation (or in their eyes, the sun had)? I mean why did they settle on that assumption? I am sure one could have made a dozen others.

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yogert909 t1_jab8ny7 wrote

They didn’t necessarily care about the rotation of the earth. They cared about how long until they should plant their crops again.

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redvillafranco t1_jab8rba wrote

They knew that a cycle of 365 days had passed. And this period of time was extremely consistent and repeatable. And they called that cycle a “year”. But they didn’t necessarily need to know what was causing the cycle.

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