The Vernal Equinox occurred early this morning (Monday, March 20, 2017) at 5:28 AM CDT. This was the start of astronomical spring in the Northern Hemisphere, as the sun's rays were directly overhead on the equator. Interesting tidbit, the word "equinox" is derived from two Latin words - aequus (meaning "equal") and nox (meaning "night").
On the two equinoxes (fall and spring), the length of day and night are nearly equal. The nearly equal hours of day and night are 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. The days will continue to slowly get longer over the next couple of months, as the direct rays of the sun move slowly northward.
NWS Milwaukee/Sullivan, WI