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 Tuesday, March 20, at 1:14 a.m EDT (05:14 Universal Time). 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.
At each equinox, the sun crosses Earth's equator, making night and day of approximately equal length on most of the planet. At the equator, the sun is directly overhead at noon on either equinox. However, day and night are not exactly equal on the date of the equinoxes. For instance, at higher latitudes in the Northern Hemisphere, the "equal day and night" occurs a few days before the spring equinox, while in the Southern Hemisphere that date comes after the March equinox, according to the National Weather Service. [Earth's Equinoxes & Solstices (Infographic)] The rest of this article can be found at: