In the Northern Hemisphere, this is the fall or autumn equinox, and it marks the first day of fall. And, what do they call the fall equinox in the Southern Hemisphere? How the Fall Equinox WorksThe answer to each of these questions resides in Earth’s axial tilt. Parity will again be reached on March 21 or 22, the vernal equinox for the Northern Hemisphere, and the whole process will repeat itself. And welcome to fall, Northern Hemisphere!