On two days in a year, the sun shines vertically over the Equator i.e., at an angle of 90°. This angle decreases as one moves away from the equator.
Autumn Equinox refers to, On September 23rd, the vertical rays of the Sun falls on the Equator, Therefore, the days and nights are equal in both the hemispheres. On 21st March, the vertical rays of the sun fall son the Equator, therefore, the duration of days and nights are equal in both the hemispheres.