Today is the first day of spring in the Northern Hemisphere. Though no guarantee of gorgeous weather that's not too hot and not too cold, the Earth's position relative to the sun says it's officially time for the birds to start chirping.
The first day of spring arrives on varying dates (from March 19-21) in different years for two reasons: Our year is not exactly an even number of days; and Earth's slightly noncircular orbit, plus the gravitational tug of the other planets, constantly changes our planet's orientation to the sun from year to year.
This year, spring starts Sunday, March 20, at 7:21 p.m. EDT (23:21 UTC). That's when the so-called vernal equinox occurs. Equinoxes (which mark the onset of spring and autumn) and solstices (which mark when summer and winter begin) are points in time and space that mark a transition in our planet's annual trip around the sun.
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