The equinoxes (from “equi,” meaning “equal,” and “nox,” or “night”) occur when the sun’s rays strike the equator at noon at an angle of 90 degrees.
In the Northern Hemisphere, the vernal or spring equinox occurs around March 20, and the autumnal or fall equinox occurs on September 22 or 23.
During the equinoxes, the sun is above the horizon for all locations on Earth for 12 hours. This year the fall equinox occurs on Sept. 22 at 9:29 p.m.
The tilt of the Earth’s axis is responsible for the seasonal variation in the amount of solar energy distributed at the top of the atmosphere and plays a key role in determining the seasonal variation in surface temperature.
The Earth’s axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because the Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of the Earth’s axis to the sun is always changing as the Earth orbits around the sun.
As this orientation changes throughout the year, so does the distribution of sunlight on the Earth’s surface at any given latitude, and this is the cause of the seasons.
On the equinoxes, the axis is not pointed at or away from the sun. This results in all areas experiencing a little more than 12 hours of daylight.
The September equinox is considered by many to be a sign of the beginning of fall in the Northern Hemisphere, and a marker of spring in the Southern Hemisphere.