Since the Earth is tilted 23.5 degrees, the sun’s rays hit the northern and southern hemispheres unequally. When the sun’s rays hit one hemisphere directly, the other hemisphere receives diffused rays. The hemisphere that receives the direct rays of the sun experiences summer; the hemisphere that receives the diffused rays experiences winter. Thus, when it is summer in North America, it is winter in most of South America, and vice versa.
The ancient Mayans were fascinated with measuring time, and even their pyramids were constructed to mark the time of equinoxes. The pyramid of Chichen Itza cast a shadow that reached the snakehead (foreground) at equinox.