What did the ancients rely on to determine the solar terms?

The ancients used the bucket handle of the Big Dipper to determine the solar terms.

In The Circulation of Pipes, written by Guan Zi, a hermit of Chu State in the Warring States Period, there is such a clear record that the handle of a bucket points to the east and the world is spring.

Bucket handle guide, the world is summer; Bucket handle refers to the west, and the world is autumn; The barrel handle refers to the north, and it is winter all over the world.