Autumn equinox 2026 today: Why September 23 marks a seasonal shift

The autumn equinox 2026 arrives on Wednesday, September 23, marking the beginning of astronomical autumn in the Northern Hemisphere. For India, the equinox occurs at 5:35 am IST, when the Sun crosses the celestial equator from north to south.

But what exactly happens on the equinox, and does it mean India will suddenly see cooler weather or exactly 12 hours of day and night?

What is the autumn equinox?

An equinox occurs twice a year, in March and September, when the Sun crosses Earth's equatorial plane. At the September equinox, the Northern Hemisphere begins its transition into astronomical autumn, while the Southern Hemisphere moves into spring.

The term equinox comes from Latin words associated with an “equal night”. The event is linked to a near-equal split between daylight and darkness, although the actual length of day and night is not exactly 12 hours everywhere on the date of the equinox.

The September equinox occurs at 00:05 UTC on September 23, 2026, which corresponds to 5:35 am IST in India.

What changes after September 23?

The most noticeable change is the gradual shortening of daylight hours across the Northern Hemisphere.

After the September equinox, the Sun's apparent path shifts further south and the Northern Hemisphere moves towards winter. This means days gradually become shorter and nights longer.

The change is already measurable in India. In the Indian subcontinent, daylight on September 23 is around 12 hours and six minutes, with the length continuing to decrease over the following days.

The rate of change also varies by location. Areas farther from the equator generally experience larger seasonal changes in daylight than places closer to it.

Is day and night exactly equal on the equinox?

Not quite.

Although the equinox is traditionally associated with equal day and night, sunrise and sunset calculations are affected by atmospheric refraction and the apparent size of the Sun. As a result, the actual date when daylight and darkness are closest to equal can differ from the equinox itself.

This is why the terms equinox and equilux are not interchangeable. The equinox is a specific astronomical event, while the equilux refers to the point when daytime and nighttime are approximately equal in duration.

Will the autumn equinox make India colder immediately?

Not necessarily.

The autumn equinox marks an astronomical change of season, rather than an instant change in weather. Temperatures do not suddenly drop simply because the equinox has occurred.

What does change is the amount and angle of sunlight received over time. As daylight gradually decreases in the Northern Hemisphere, seasonal conditions eventually shift towards winter.

For India, therefore, the key takeaway from the September 23 equinox is not an immediate weather change, but the start of the astronomical transition towards shorter days and longer nights.