I was expecting the first day of Spring to be today, March 21, but it turns out it was yesterday. That got me wondering, what is the definition of the first day of Spring. This year, according to National Geographic, in the Northern Hemisphere spring officially begins at 7:44 a.m. ET on Friday, March 20, 2009—the vernal equinox, or spring equinox.

So many interesting tidbits!

The word equinox derives from the Latin words aequus (equal) and nox (night); in reality, the day is longer than the night at an equinox. Commonly, the day is defined as the period when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the centre of the Sun is below the horizon, its upper edge is visible. … The date at which the time between sunset and sunrise crosses 12 hours, is known as the equilux. Because sunset and sunrise times vary with an observer’s geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the equator.

But when is it?

Currently, the most common equinox and solstice dates are March 20, June 21, September 22 and December 21; the four-year average will slowly shift to earlier times in coming years. This shift is a full day in about 70 years (compensated mainly by the century “leap year” rules of the Gregorian calendar). This also means that in many years of the twentieth century, the dates of March 21, June 22, September 23 and December 22 were much more common, so older books teach (and older people may still remember) these dates.

And finally:

It is perhaps valuable for people in the Americas and Asia to know that the equinoxes listed as occurring on March 21, which occurred frequently in the 20th century and which will occur occasionally in the 21st century, are presented as such using UTC, which is at least four hours in advance of any clock in the Americas and as much as twelve hours behind Asian clocks. Thus, there will be no spring equinox later than March 20 in the Americas in the coming century.

Read lots more, complete with illustrations, at http://en.wikipedia.org/wiki/Vernal_Equinox