On Sunday, March 20, 2016, at 12:30 a.m. EDT, we welcome the official arrival of spring and the Vernal Equinox. This is the moment in the Northern Hemisphere when the Sun crosses the equator and the days and nights are equal in hours. Vernal translates to “new” and “fresh,” and equinox derived from the Latin aequus (equal) and nox (night). Regardless of what the weather is doing outside, this day is the official start of the spring season.
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.
On that day, the direct rays of the Sun are shining down on the equator producing the effect of equal day and night (give or take a few minutes). After the Vernal Equinox, the direct rays of the Sun migrate north of the Equator (with hours of daylight steadily growing longer) until they finally arrive at the Tropic of the Cancer (latitude 23.5 degrees north) on June 20th at 6:34 p. m. EDT. The migration of the Sun’s direct rays comes to a halt on that day; this is as far north as they will go. We call this the Summer Solstice (solstice is a suspension of the migration of the Sun’s direct rays). It is the longest day of the year in terms of hours of daylight.
After the Summer Solstice, the direct rays proceed to head south and the days begin to grow shorter. It will take another three months, until the Autumnal Equinox (September 22, 2016, at 10:21 a.m. EDT), for the periods of daylight and darkness to reach an equilibrium once again. The rays ultimately reach the Tropic of Capricorn (latitude 23.5 degrees south) on the day of the Winter Solstice (December 21, 2016, at 5:44 a.m. EST), and the whole cycle begins again!