Monday, March 20, at 6:29 a.m. EDT marks the date and time of the Vernal Equinox for 2017. 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 – hooray! Vernal translates to “new” and “fresh,” and equinox is derived from the Latin aequus (equal) and nox (night). Regardless of what’s going on outside your window (some of us still have snowdrifts piled high), 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 more darkness than light over the course of a day.
But on Monday, March 20th, the direct rays of the Sun will be shining down on the equator producing the effect of equal day and night (give or take a few minutes). After the Vernal Equinox, these rays 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 21, 2017, at 12:24 a.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, 2017, at 4:02 p.m. EDT), for the periods of daylight and darkness to reach an equilibrium once again.
From there, our days will continue to grow shorter until the rays ultimately reach the Tropic of Capricorn (latitude 23.5 degrees south) on the day of the Winter Solstice, December 21, 2017, at 11:28 a.m. EST, and we start the whole cycle all over again!