If the winter solstice is the longest night of the year, why does it mark the beginning of winter, rather than the very middle of it?

Question

If the winter solstice is the longest night of the year, why does it mark the beginning of winter, rather than the very middle of it?

In: Earth Science

Answer 1

Societally it is because the seasons are things we invented, so we can decide it happens whenever we want*. Historically, it is because the solstices and equinoxes were easier to define astronomically than some arbitrary point halfway between them.

A more physically based answer is because the temperature/weather lags behind solar radiance. This is because of a concept called ‘thermal inertia’, which basically means that the environment still retains some warmth from summer and autumn. A simple model of weather would have the winter solstice be the day that the most heat was lost, but that means it gets colder the day after, and the day after that, etc. Actual weather is more complicated, but it still holds that the coldest day should occur a significant time after the solstice.

*cultures from more equatorial regions often defined the seasons differently. People in Sub Sahara Africa defined seasons by the direction of the crescent moon, which corresponded to the rainy and dry seasons. Ancient Egyptians defined their season/year by the star Sirius, which could first be seen predawn approximately 1 month before the Nile flooded.