Definition
The autumnal equinox (♎) is the intersection point opposite the vernal equinox. The Sun passes it around 22–23 September. At both equinoxes, day and night are nearly equal because the Sun illuminates both hemispopes equally.
Autumnal Equinox
The September equinox — Sun crosses the equator southward.
3D illustration
The two equinoxes lie 180° apart on the celestial equator.
Use the menu and time controls to explore. Based on Jean Meeus' Astronomical Algorithms.