I like to think of it as the Snake of Sunlight…
The word equinox was formed by two Latin words: "Equi" is the Latin prefix for "equal" and "nox" is the Latin word for "night." The equal refers to the fact that the amount of daylight and darkness on this day are almost equal. – 12 hours – all over the world. (For all practical purposes, the amount of daylight and darkness on the equinox can be considered equal. Scientists are quick to point out it is not exactly the same – but, let’s face it scientists are famous for splitting hairs and atoms and they want all of their science nerd friends to know that they know it’s not technically exact.)
The September equinox occurs the moment the Sun crosses Earth’s Equator – from north to south.
On any other day of the year, either the southern hemisphere or the Northern Hemisphere tilts a little towards the Sun. But on the 2 equinoxes, the tilt of Earth's axis is perpendicular to the Sun's rays, like the illustrations shows
Earth orbits the Sun at a slant, which is why equinoxes and solstices happen.
Shorter days ahead…The days (length of daylight) will now grow shorter until the Winter Solstice – the shortest day of the year, become equal at the Vernal (spring) equinox in March, then grow longer until the Summer Solstice – the longest day of the year in June.
The Autumnal equinox is known by many names - Fall Equinox, September Equinox, the beginning of fall. Keep in mind the Northern Hemisphere’s Autumnal Equinox is the Southern Hemisphere’s Vernal Equinox- but it is the September equinox for the whole planet.
The “Meteorological Seasons”. Meteorologists and climatologists have come up with their own calendar and named it after themselves. They don’t recognize the “astronomical seasons” like rest of the world -so they don't consider today the first day of fall. Heaven’s no – They have to start early. How else would they “predict” the weather and tell us what’s happening before it happens?
Weather folks start autumn on Sept. 1, the first day of winter Dec. 1, the first day of spring March 1 and the first day of summer June 1. Each of those seasons, known as "meteorological seasons," runs three full months and they are based on the annual temperature cycle instead of the earth's rotation around the sun.
Traditions and Folklore
Many cultures and religions celebrate holidays and festivals around the September equinox.
The Snake of Sunlight
The snake of sunlight at Chichen Itza, Mexico.The snake of sunlight on the stairs of the main pyramid at Chichen Itza, Mexico.
A famous ancient equinox celebration was the Mayan sacrificial ritual by the main pyramid at Chichen Itza, Mexico.
The pyramid, known as El Castillo, has 4 staircases running from the top to the bottom of the pyramid's faces, notorious for the bloody human sacrifices that used to take place here. The staircases are built at a carefully calculated angle which makes it look like an enormous snake of sunlight slithers down the stairs on the day of the equinox.