system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the solstice the beginning of winter and not the middle?

If the solstice is the warmest/coldest time of the year, shouldn't it be in the middle of winter and summer, and likewise for the equinoxes and spring and fall? Instead winter started on the 21st.
gpt
The earth, ocean and atmosphere do not heat instantly in response to increased solar radiation.  That is to say, there is a lag between the maximum rate of energy input and the maximum temperature achieved.

Imagine a pot of water on the stove and slowly turning the flame up to maximum, and just as slowly turning it back down.  It continues to heat after the maximum flame, since just a lttle less than maximum is still plenty to increase the temperature.