Astronomical Event / Autumnal Equinox Marks End of Monsoon, Start of Autumn

Bhubaneswar, Sep 22: A rare astronomical event will occur on Wednesday, when day and night will be nearly equal in duration across the globe.

Known as the Autumnal Equinox, this phenomenon marks the transition from monsoon to autumn and the gradual onset of winter in the Northern Hemisphere, including India.

On this day, both day and night will last about 12 hours each. From September 23 onward, nights will begin to grow longer while days shorten, ushering in cooler weather.

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Astronomers explain that the equinox occurs because Earth’s axis is tilted at 23.5 degrees as it orbits the Sun. Twice a year — during the Vernal Equinox in March and the Autumnal Equinox in September — the Sun’s rays fall directly on the equator, distributing light equally across both hemispheres.

After the equinox, the Sun gradually shifts southward, a movement known as Dakshinayan. This causes the Northern Hemisphere to receive sunlight at a slant, leading to shorter days and longer nights. The cycle continues until December 21-22, the year’s shortest day.

The equinox also signals the retreat of the monsoon in India. As rainfall decreases, mornings and evenings begin to feel cooler.

Interestingly, while equinoxes are popularly described as days when day and night are exactly equal, atmospheric refraction makes the day slightly longer — by about 6 to 15 minutes.

Earth’s atmosphere bends sunlight like a lens, allowing us to see the Sun a few minutes before it actually rises and after it sets. Thus, the precise 12‑hour balance occurs a few days later.

For skywatchers and cultural observers alike, September 23 is more than just a date on the calendar.