How did they know how many days were a year in the past?


I know that the seasons were indicators but how did they know precisely to the day how much a year was.

Edit: Copying from a response I made:
“Thanks for the response!
But I still have a doubt cause most of the reponses are to measure it in certain way and wait until the sun goes back to its initial position, and I get how measuring its easy by doing it over a long period of time but the difference between 2 days seems kind of difficult to notice, like when the sun got back to its position and people were like “yeah it looks about the same as how it started” and then they observed the next day and it looked exactly the same, how did they decide a specific day.
I guess my question is more about how they achieved such precission rather than the method”

In: Earth Science

Set up a couple of rocks aligned with the Sun’s position on the horizon at sunrise. Keep a tally of days as you observe the rising Sun change its position north and south and north again until it’s back in the original position. Keep that up for several years; after four years, 1461 days should have passed. Keep it up longer for more precision.

They would count the days. They would have something that would help them tell the changes in the sun’s position over time. They counted the number of days it took to for the sun to return to the same position.

Here is an example, they would count the number of days It took for the sun to reach the maximum or the minimum and back again

They counted how many days until the sun got back to the same spot in the sky at the same time of day.

There’s lots of ways to do this. e.g. northernmost or southermost sunrise or sunset, maximum altitude at noon, etc.

Smart people have always been pretty smart. Have you seen the movie “Cast Away”?

Tom Hanks is stranded on an island with no real way to track the time/seasons. But he’s still able to track roughly what time of year it is, even on an island with no real seasons. He does this by tracking the position of the sun in a very smart way. His shelter cave has a small hole in it that allows a beam of sunlight in. At as close to the same time of day as he can manage, he marks on the wall where the sun beam falls. Throughout the year this spot will move on the wall as the sun is higher (summer) or lower (winter) in the sky. By marking the top and bottom positions year after year and counting the number of days between you get a pretty accurate count of the days in a year.

There are multiple different methods of performing this measure. You can have tall narrow object casting a shadow and mark the shadows tip at noon. You can use basic angle measurement device like an sextant to measure the angle of the sun at noon.

Different societies developed many different ways to perform this basic measure of the year throughout history.

You track how long it takes the sun to return to the same position in the sky.
There are a bunch of ways to do this. Some examples include:
1 – the sundial approach: Stick a stick into the ground, each day when the sun gets as high as it gets for that day (i.e. noon) mark the location where the shadow of the stick ends. When the shadow is at its longest/shortest for the year you’ve reached a solstice. Record the amount of time between equivalent solstices and you’ve measured a year.
2 – the Manhattanhenge approach – Find a nice clear area. Put a stick in the ground. Wait until it’s sunset and put a 2nd stick in the ground so that when you stand at the 1st stick the sun is directly in line with the 2nd stick. Wait. Note the days when when the sunset is inline with the 2nd stick while you stand at the first stick you will know you’ve passed exactly one year. If there is one such point per set of seasons then the time between them is a year. if there are two such points per set of seasons then the time between every other point is a year.

So actually **not** by measuring the sun. The Egyptians were the first to figure out that there were 365 days in a year. Before that calendars usually had 360 days because it more or less lined up with everything and the Babylonians used a base 60 number system that this fit well in. It’s also part of the reason we have 360 degrees in a circle. This was more based on observing the moon too. You’d have a new moon cycle roughly every 30 days, and 12 moon cycles would more or less line up with the sun being in sorta the same place and the seasons coming back around. Eventually this would fall out of sync since the moons cycle isn’t actually 30 days, and there’s not 360 days in a year. So sometimes they’d add an extra month here or there and it worked well enough.

But the Egyptians looked at the stars instead. They saw that the star Sirius appeared just above the horizon at dawn at the same time every 365 days. That lined up closely with the flooding of the Nile too, so it was significant beyond just being interesting. It gave them greater predictive power over important events. So they made a calendar with 12 lunar months, and then 5 extra days on top of that for 365 days total. They did know it came a day later every 4 years, but they just let that slide. The calendar fell behind a little bit every four years, but they could see by looking at Sirius when it had been a year and the Nile would flood again.

They put a lot of emphasis on observing the stars for largely religious reasons. So they could see pretty clearly when an important star appeared to rise in the same location. It’s more of a discrete event to them that they could see happened every 365 days than a measurement they had to try and get right with tools from 5000 years ago.