The Cusp of Autumn

Monday 23rd September marks the autumnal equinox and is, therefore, the astronomical beginning of autumn in the northern hemisphere, and Spring in the southern.  However, according to the meteorological definition of seasons, which is based on temperature cycles and the Gregorian calendar, the first day of autumn is September 1st.

The word “equinox” comes from Latin aequus, meaning “equal,” and nox “night.” On the equinox, day and night are roughly equal in terms of length.

During the equinox, the Sun crosses what is called  the “celestial equator”—an imaginary extension into space of Earth’s equator line. The equinox occurs precisely when the centre of the sun passes through this line. When the sun crosses the equator from north to south, this marks the autumnal equinox; when it crosses from south to north, this marks the vernal equinox.
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Throughout the world different cultures monitor or revere the celestial movements through the seasons; in the UK Stonehenge was probably geographically placed with the equinoxes and solstices in mind.  At Machu Picchu in Peru, there is an ancient stone monument called Intihuatana - which means “Hitching Post of the Sun and this serves as a solar clock to mark the dates of the equinoxes and solstices.  In Mexico, the Mayans built a giant pyramid called Chichen Itza (in Yucatan State). On the equinoxes, it looks as if a snake made of light slithers down the pyramid’s steps.
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Photos from websites:  English Heritage, Exploreandes and GetYourGuide