<P> On an equinox, day and night are of approximately equal duration all over the planet . They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction . The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). </P> <P> The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator . As a result, the northern and southern hemispheres are equally illuminated . The word comes from Latin equi or "equal" and nox meaning "night". </P> <P> In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is exactly overhead at a point on the equatorial line . The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox . </P> <P> The equinoxes, along with solstices, are directly related to the seasons of the year . In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures and is considered the New Year in the Persian calendar or Iranian calendars as Nowruz (means new day), while the autumnal equinox (September) marks the beginning of autumn . </P>

When does the southern hemisphere have spring equinox