The term equinox comes from the Latin word ‘aequus’ and means ‘equal night’. There are 4 equinoxes each year, with the most well recognised being the summer and winter ones, though there are also a vernal equinox (spring) and autumnal equinox (fall). The autumnal equinox is also called September equinox and will occur around September 23rd this year.
In the USA it is also known as the first day of ‘fall’. There is an Irish proverb which proclaims ‘autumn days come quickly, like the running of a hound on the moor’, which is true, in that it seems we quickly move from summer to autumn in the blink of an eye! During an equinox the sun moves to a predicted part of the sky, which equally divides the northern and southern hemispheres. After the autumnal equinox the nights will get longer and the days shorter until the December equinox, around Christmas time.
Noticeable changes occur in the autumn, but they are not caused by changes in the weather, with the drop in temperature, but the amount of daylight and photosynthesis. Not all leaves change colour in the autumn, it depends of the types of plants they are on, but as the autumn days become shorter, the reduced light triggers chemical changes in plants and trees, causing the leaves to take less nutrients from the soil, until it causes the death of the pigment chlorophyll, which is what causes the colour green in the leaves! When the green is gone, 2 other pigments rise to the surface – carotene (yellow) and anthocyanin (red), which combine to give that beautiful autumnal colour we are all familiar with. Less light and colder weather affect the soil, so in a short time after the change of colour, the leaves will fall!
The celebration of the equinox goes back thousands of years. One famous ancient equinox celebration took place by the main pyramid, at Chichen Itza in Mexico. It was the site of Mayan sacrifices to the gods at the time of seasonal change. The pyramid was called El Castillo and had 4 staircases from the top to the bottom of the pyramid faces, where the human sacrifices took pace. Each of the staircases is built at such an angle that on the day of the equinox, the sun shines on the edges of the steps in such a way that it gives the illusion of a giant snake slithering down them!
Around the time of the autumnal equinox the changes can be seen in the sky, allowing the aura borealis display (northern lights) to be seen more clearly. From then, it is only a short time till we receive the winter equinox – and so life goes on!