The New Student's Reference Work/Equinox

E′quinox, the time when the sun is in the plane of the earth’s equator.  This occurs twice a year, once on March 20th, which is called the vernal equinox, and again on Sept. 22d, which is called the autumnal equinox.  The name is derived from the fact that the length of the night then exactly equals the length of the day, since the parallel rays of light from the sun, on these dates, fall upon the earth perpendicular to the axis of the earth.  The sun then appears to be on the equator, and hence the equinoxes are popularly spoken of as the dates on which “the sun crosses the equator.”