New Delhi. The September Equinox, which occurs on September 23, is an important astronomical event in terms of the change in the length of day and night. Generally it is said that on the day of equinox, day and night are of about 12-12 hours, but in reality the duration of day and night is not exactly the same in every part of the world. The size of the Sun and the Earth's atmosphere cause differences in the times of sunrise and sunset.
According to NASA's astronomical calculations for 2026, the September equinox will occur at 5:35 am on September 23. At this time the Sun will cross the equator and move from north to south. After this, days will gradually become shorter and nights longer in the Northern Hemisphere. India, being in the Northern Hemisphere, will also be a part of this change.
What is Equinox?
Equinox occurs twice a year. The first in March and the second in September. During this time the Sun is above the Earth's equator and the sunlight received by both the hemispheres is almost equal. The Earth is tilted on its axis by about 23.5 degrees and due to this tilt during its revolution around the Sun, the duration of day and night keeps changing throughout the year.
At the time of September Equinox the Sun crosses the equator moving from north to south. In the Northern Hemisphere it is considered the beginning of astronomical autumn, while in the Southern Hemisphere it is the beginning of spring.
There will be an impact in India also
In India too, the duration of day and night will come very close to each other around 23 September. However, this does not mean that every part of the country will have exactly 12 hours of day and 12 hours of night. There will be difference in duration due to sunrise and sunset times at different places.
There are two main reasons behind this. First, the Sun appears not as a point in the sky but as a round disk. Second, Earth's atmosphere bends the sun's rays. Because of this, the sun appears a little earlier than its actual geometric position and the sunset also appears a little later.
For example, the day length in New Delhi on September 23 is expected to be around 12 hours 7 minutes. This means that even on the day of Equinox, the length of day and night is not exactly 12 hours each.
What will change after September 23?
After the equinox, the direct rays of the Sun will start moving southwards from the equator. This will affect the duration of day and night in the Northern Hemisphere. In India too, gradually the days will become shorter and the nights will become longer. This difference will widen as we approach the December solstice.
However, the change of weather in India is not determined only by the position of the Sun. Monsoon, temperature, oceanic influence and the local climate of different regions also influence weather patterns.
So the equinox on September 23 is certainly a special astronomical moment, but it would not be correct to see it as exactly 12 hours of day and 12 hours of night across India. Its actual effect varies depending on the location and after this the sequence of days becoming shorter and nights becoming longer continues in the Northern Hemisphere.