Equinoxes
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
The two periods of the year (vernal equinox about March 21st, and autumnal equinox about September 22d) when the time from the rising of the sun to its setting is equal to the time from its setting to its rising.
See Dig. 43, 13, 1, 8.