21 Mar Vernal Equinox, March 20, 2017
There are only two times of the year when the Earth’s axis is tilted neither toward nor away from the sun, resulting in a “nearly” equal amount of daylight and darkness at all latitudes. These events are referred to as Equinoxes and occurred today, March 20th at 4:29 am MDT (Vernal Equinox – the first day of Spring) and will again on September 22nd at 2:02 pm MDT (Autumnal Equinox – the first day of fall). The word equinox is derived from two Latin words – aequus (equal) and nox (night). At the equator, the sun is directly overhead at noon on these two equinoxes. The “nearly” equal hours of day and night is due to refraction of sunlight, or a bending of the light’s rays that causes the sun to appear above the horizon when the actual position of the sun is below the horizon. Additionally, the days become a little longer at the higher latitudes (those at a distance from the equator) because it takes the sun longer to rise and set.
Link to full article: https://www.nnvl.noaa.gov/MediaDetail2.php?MediaID=2011&MediaTypeID=1