The ancients calculated that the starting point of the annual solar terms was

Since winter solstice, the days have become longer and longer. But the night is longer than the day. At the vernal equinox, the day is as long as the night. In the future, the days will be longer and longer, but the days will be longer than the nights. On the solstice in summer, the day is the longest. Since summer solstice, the days have become shorter and shorter. At the autumnal equinox, day and night are equal again. Until the winter of solstice. Cycle after cycle.

We have a saying here called "the solstice is long in summer and short in winter", which vividly illustrates this phenomenon.