The four changes of the seasons, related to the position of sunlight on the planet, are captured in this view from geosynchronous Earth orbit by EUMETSAT's Meteosat-9 satellite. On March 20 and September 20, the terminator (the line between night and day) is a straight north-south line, and the Sun appears to sit directly above the equator.
Of course, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and the Sun. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.
Equinox means "equal night" in Latin, capturing the idea that daytime and nighttime are equal lengths everywhere on the planet.