Equinox: (Latin: Equal night)
This example came about after a previous example to get the season for a given date. In the southern hemisphere, seasons are calculated simply by date. Summer down here begins on December first, Autumn on March first, Winter on June first, and Spring on September first.
This works well, and the seasons can be adjusted simply to transpose them into values that correspond to the nothern hemisphere. But these dates do not truely reflect the beginning of spring in the northern hemisphere, or autumn in the southern hemisphere.
Historically, in the northern hemisphere, the beginning of spring is marked by the spring, or Vernal, Equinox. The Vernal Equinox marks the point in time when the Sun crosses the celestial equator from south to north.
Calculation of the true Vernal Equinox is regarded by some as the true beginning of spring, while some churches relate to it in a totally different way.