In March we come to the Vernal Equinox, when for about a month night and day are of about equal length, hence the word equinox.  There is one day when they are exactly equal and that is the Vernal Equinox, March 20. It is thereafter that one starts to gain dominance, for the Northern Hemisphere it is the day, while in the Southern it is night.  For the Southern their part of the globe actually moves away from the sun, hence the decreased light.