How did the ancients know that New Year's Day happened to be the day when the earth went around the sun?

The ancients grasped the regularity by observing the positions and changes of the sun, the moon and the stars, and determined the four seasons and compiled calendars. Determining the length of the tropical year is the basis of the calendar.

Take our country as an example, our ancient calendar pays special attention to the solar terms of winter solstice, and accurately calculates the time of two consecutive winter solstice days, and the time interval between them is a tropical year. During the Spring and Autumn Period, China used soil gauge to observe the length of the sun shadow. At noon from the winter solstice, the sun is oblique and the surface shadow is the longest.