the first point of Aries


vernal equinox

vernal equinoxi. That point of intersection of the ecliptic and the celestial equator occupied by the sun as it changes from a south to a north declination, on or about March 21. Also called the March equinox or the first point of Aries.
ii. That instant the sun reaches the point of zero declination when crossing the celestial equator from south to north.