The first day of spring arrived Tuesday with the 2012 vernal equinox, heralding the end of a winter that, for most of us, wasn't. The United States experienced the warmest winter of the 21st century so far and the fourth-warmest winter in 117 years of record keeping, according to the National Oceanic and Atmospheric Administration.

New Mexico was the only state with temperatures below its long-term 20th century average this winter. Most others were well above, with typically snow-capped locales like Grand Rapids, Mich., Grand Forks, N.D., and Green Bay, Wis., boasting their second-warmest winters ever.

As the vernal equinox arrived on Tuesday, the first day of spring proved to be equally mild with temperatures in the upper 60s and 70s across much of the U.S. East Coast.

The vernal or spring equinox marks the point in space and time when the sun moves across the celestial equator, an invisible circle projected into the sky above the Earth's equator. In the Northern Hemisphere, the days grow longer and the nights shorter. In the Southern Hemisphere, the same equinox marks the beginning of longer nights and shorter days and heralds the beginning of autumn.

In theory, the equinox should mark the time when day and night are equal, when both share 12 hours apiece. However, in practice that happens days earlier due to atmospheric effects. The date of the equinox varies each year from March 19 to 21 depending on location and corrections due to a discord between the Gregorian calendar (logging 365 days a year) and the actual duration of the Earth's orbit around the sun (which takes approximately 365.25 days to complete).

The occasion is celebrated in a myriad of ways across the globe. Press Start to have a look.