Autumnal equinox explained: Fall begins with the arrival of the autumnal equinox

The transition from summer to autumn is usually marked by cool breezes and leaves changing color, though officially, fall does not begin until the autumnal equinox.

The Earth is split by the the equator into northern and southern hemispheres. According to National Geographic, the planet orbits the sun at an tilted angle of approximately 23.5-degrees, and it is this tilt that determines the seasons.