And the solstices to give you a 4-way split.

But like you, I always assumed it to be this way too.
On the news last night they actually talked about this. There are different definitions of spring. Most use the astronomical definitions which are the equinoxes an solstice, but there is also a meteorological definition which varies from place to place depending on weather patterns. Here in Dallas meteorological spring starts on March 1st but it's late this year. In Chicago March 1st is still the dead of winter.



