Eli5: How did ancient civilizations in 45 B.C. with their ancient technology know that the earth orbits the sun in 365 days and subsequently create a calender around it which included leap years?
They watched the sun. They knew about solstices (high point, low point of sun in sky). They tracked how many days between the solstices. They were interested in this because it correlated with growing seasons.
None of this happened overnight. There is always a large amount of trial and error involved in the development of ancient calendars. The idea of a leap year was a ‘fix’ to a calendar that wasn’t quite right. It seems like it happened instantly but if you look back, the trial-and-error time was often quite lengthy.
If you mark the ground or cave where specific shadows or sunbeam hits, those markings move and change through the year very very predictably to the inch. Each year specific points are marked and future validated. They also saw same star constellations and moon paths in same place through year, each year repeating
Oldest lunar calendar https://sservi.nasa.gov/articles/oldest-lunar-calendars/
Civilization had already been around for thousands of years by 45BC…. the Great Pyramid dates from ~2500 BC. Sumerian civilization had 360-day calendar… same origin as where we get the 360 degrees in a circle, and 3600 seconds in an hour.
For the calendar – The best indicator is that once you realize the days and the nights shift around in length you keep track and find the equinoxes and the solstices. Equinoxes are the days in fall and spring where night and day are the same length. The winter solstice has the longest night and the summer solstice has the longest day. So you start counting these and realize year after year they’re about the same number of days apart. After many years you realize that they are 365 or 366 days apart and with the right record keeping and math you pinpoint that once every four years is good.
Then a religion comes along which decided that certain days of the year should be holy days and they align with a fixed date on the calendar and also on a flexible date depending on the alignment of days of the week with phases of the moon. After several centuries you realize that these days are slowly migrating. So you look at the calendar again and look at all the records over the centuries and realize that the extra day out of four years is just a tad too much. So you remove the extra day every few centuries to get back on track.
Throughout the year the sun appears to move to the North and South because the planet is tilted. In the middle of Winter and the middle of summer it reaches the furthest point and changes direction, and these points are called the solstices.
The length of time between the solstices tells you the length of time of a year. And by the time people figured out the whole concept of leap years, they had been keeping track of the solstices for hundreds of years. So eventually they were able to figure out that the 365 days that they had used as an approximate length of the year was not quite accurate because every 4 years the Solstice moved by a day.