Tuesday, March 20, at 1:14 a.m., is the Vernal Equinox for 2012. This is the moment when the Sun crosses the Equator and those of us living in the Northern Hemisphere begin to see more daylight than darkness. Regardless of whether it has been sunny for weeks (as in many regions this year) or whether there are still snowdrifts piled high around, this day is the official start of spring.
So what does that mean? Essentially, our hours of daylight — the period of time each day between sunrise and sunset — have been growing slightly longer each day since the Winter Solstice in December, which is the shortest day of the year (at least in terms of light). Even after three months of lengthening days, though, we still see less light than darkness over the course of a day. The Vernal Equinox marks the turning point, when daylight begins to win out over darkness. For the next three months, our hours of daylight will continue to grow longer. In June (on June 20, 2012, at 7:09 p.m., to be exact) it will be the Summer Solstice, the longest day of the year in terms of hours of daylight.
After the Summer Solstice, the days will begin to grown shorter. It will take another three months, until the Autumnal Equinox (September 22, 2012, at 10:49 a.m.), for the periods of daylight and darkness to reach an equilibrium once again.
From the Autumnal Equinox, the days will continue to grow shorter, until we reach the Winter Solstice again on December 21, 2012, at 6:11 a.m., and begin the whole cycle anew!