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

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.