The first day of autumn in 2025 will begin astronomically on September 22 at 20:19, while the calendar autumn starts on September 23.
Autumn 2025: Key Dates
Every year, the first day of calendar autumn falls on September 23, and it ends on December 21. Astronomical autumn, however, begins with the autumnal equinox.
Understanding the Autumnal Equinox
The autumnal equinox occurs when the Sun passes through the Libra point, with solar rays hitting the equator perpendicularly. This results in 12-hour days and nights at both poles.
The moment of the Sun’s passage through the Libra point is not fixed and occurs on different days between September 21 and 24 each year.
Why Does Astronomical Autumn Start on Different Days?
Damian Jabłeka, Deputy Director of the Śląskie Planetarium in Chorzów, explained that the start of astronomical autumn varies annually due to Earth’s orbit around the Sun not being a perfect multiple of its rotations on its axis.
This results in the need for a leap year every four years and causes small shifts in the start of autumn due to additional movements of Earth’s axis.
