March equinox, when the sun crosses the equator while heading northward.

(astronomy) The moment when the sun crosses the celestial equator while heading southward, occurring on September 22–23. That would be autumnal equinox in the northern hemisphere and vernal equinox in the southern hemisphere.

(astronomy) The moment when the sun crosses the celestial equator while heading northward, occurring on March 19–21. That would be vernal equinox in the northern hemisphere and autumnal equinox in the southern hemisphere.