The March Equinox fell on 20 March this year. The Earth spins on its axis at an angle to the plain of the Solar System. This means that the poles of the Earth do not remain at 90° to the Sun, but rather the northern pole is tilted towards the Sun for six months of the year (the northern summer), and the southern pole for the other six months (the southern summer). This means that twice a year neither pole is inclined towards the Sun, on days known as the equinoxes.
How the combination of the Earth's equatorial plane (red circle) and the plane of the Earth's orbit (green circle) creates the Solstices and Equinoxes that we observe. Ohio State University.
The equinoxes fall each year in March and September, with the March Equinox being the Spring (or Vernal) Equinox in the Northern Hemisphere and the Autumn Equinox in the Southern Hemisphere, while the September Equinox is the Autumn Equinox in the Northern Hemisphere and the Spring Equinox in the Southern Hemisphere. On these two days the day and night are both exactly twelve hours long at every point on the planet, the only days on which this happens.
See also The Earth's Perihelion, The Lunar Perigee, The December Solstice, The September Equinox and The Lunar Apogee.
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