There are two days every year when the northern and southern hemispheres of earth share the daylight equally. Today – September 22 – is one of those two days. This day marks the onset of Autumn which brings shorter and cooler days in the northern hemisphere, which houses over 90 per cent of the world population. People living in the other half will experience the opposite. For them, days will now be longer and warmer.
This year the September Equinox, also known as Autmnn Equinox or Fall Equinox, comes at 7 PM IST today. The other Equinox occurs between March 19 and 21. It marks the onset of spring in the northern hemisphere.
What are Equinoxes and how are they caused?
All of us know that the earth rotates along an imaginary axis that runs from pole to pole. But, this axis is not exactly vertical. It is tilted at about 23.5 degrees. Because of this, different parts of the earth are exposed to the sun at different angles. Some portions receive more sunlight compared to others at a given point of time. This is the reason we experience the various seasons.
During summer in the northern hemisphere, the southern hemisphere is tilted away from the sun and vice-versa. This causes opposite seasons in opposite hemispheres. Twice a year though, the earth’s axis lies perpendicular to the rays of the sun.
On these days, the length of the day and night are almost exactly the same. These two days are called Equinoxes. On the contrary, there are two days in a year when one hemisphere is closest to the sun, while the other is farthest. These are called Solstices. They occur at around June 21 and December 21. On these days, one hemisphere gets less than nine hours of daylight, while the other experiences it for more than 15 hours.
The solstices have the most extreme differences between night and day especially towards the poles. That is why Alaska has such long days in summer.
Another fun fact: If you stand at the equator today at the precise time of Equinox, which is 7 PM as per Indian time, your shadow will be at its absolute minimum. Across cultures in the world, Equinoxes hold significant value. In India, the Autumn Equinox marks the beginning of the festive season.