Autumn will begin in Azerbaijan on September 23 at 4:05:08 AM. According to the Department of Astrophysics of the Faculty of Physics at Baku State University (BSU), this marks the autumnal equinox.

At the autumnal equinox, day and night are approximately equal in length, while the Sun rises due east and sets due west. However, because of atmospheric refraction and the apparent size of the solar disk, daylight lasts slightly longer than 12 hours, meaning that day and night are not exactly equal in duration.

As the Sun moves along its apparent annual path, it crosses the celestial equator from the Northern Hemisphere into the Southern Hemisphere. From this point onward, astronomical autumn begins in the Northern Hemisphere, while astronomical spring begins in the Southern Hemisphere. Following the equinox, days gradually become shorter and nights longer in the Northern Hemisphere.

The change of seasons is not caused by variations in the distance between the Earth and the Sun. Rather, it results from the Earth's revolution around the Sun and the tilt of its rotational axis relative to the plane of its orbit, which is approximately 66.56°. This tilt causes the angle at which sunlight reaches the Earth's surface to vary throughout the year, resulting in the changing seasons.

In 2026, astronomical autumn will last 89 days, 21 hours, 45 minutes, and 1 second, ending with the winter solstice on December 22.

The winter solstice will mark the beginning of astronomical winter in the Northern Hemisphere and astronomical summer in the Southern Hemisphere.