What is the origin of summer solar terms?

The characteristics and origin of summer solar terms are as follows:

Features: "Bu" means that the extremely hot weather has come to an end and the summer heat has begun to fade. "Three summers" and "three days" both represent hot and humid weather, and the curves on the time axis and the temperature axis are basically the same: summer is coming, summer is coming; When the sky falls, summer is over.

Origin: In summer, the bucket handle of the Big Dipper points southwest (E position). When the sun reaches longitude 150, the node time is around August 23rd in Gregorian calendar. In summer, the direct point of the sun has moved from 23 26' north latitude to1128' north latitude. At this time, if you watch the Big Dipper at night, you will find that the curved bucket handle still points southwest. The arrival of the hot summer also means entering the second half of the dry calendar.