When did Spring start?

One of our readers asks, "When did Spring start?"

Well, Ms. Yasir of Alexandria, Virginia, it seems that is a more complicated question than one might think.

Shouldn't spring always start the same exact second every year?

While it’s true that we’ve traditionally celebrated the beginning of spring on March 21, astronomers and calendar manufacturers alike now say that the spring season starts one day earlier, March 20, in all time zones in North America. Unheard of? Not if you look at the statistics. In fact, did you know that during the 20th Century, March 21 was actually the exception rather than the rule?

[ SPACE.com ]

And how do they figure all this stuff out? Math? Apparently it has its uses...