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?

Question

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?

In: 6494

Answer 1

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.