"the sun being, to us, in its autumnal equinox"
Illumination of the Earth at an Equinox
Creative Commons Attribution Share AlikeIllumination of the Earth at an Equinox - Credit: Przemyslaw "Blueshade" Idzkiewicz

An equinox occurs twice a year, in autumn and spring, when the Sun is directly above the equator, thus illuminating half of the Earth and keeping the other half in shadow. At this time day and night are equally long.