Tonight at precisely 03:09 Universal Time (UTC) summer ends and fall begins on Earth. (Data source: U.S. Naval Observatory).
Fall or “autumn” starts with the “autumnal equinox”, an astronomical event during which the celestial equator and the ecliptic intersect. At this point in time, the tilt of the Earth’s axis is neither inclined away from nor toward the Sun, and the center of the Sun lines up with the Earth’s equator. When this happens, there is a point on the Equator where, if you stood there, the center of the Sun would be located exactly above you.
Two such moments (“equinoxes”) exist each year. In addition to the “solstices”, they mark the change of seasons.
Man’s knowledge of this goes back thousands of years, as we can tell from cult sites and edifices specifically constructed to mark these events.
Here is more about the Ring of Brodgar.