As autumn arrives, we notice that the days shorten significantly. This phenomenon, although natural, is due to the way the Earth moves around the Sun and the tilt of its axis.
The Earth completes a full orbit around the Sun in a year, but its axis is tilted by about 23.5 degrees. This tilt is the main reason we experience seasons, and it also influences the length of the day throughout the year. When the northern hemisphere is tilted towards the Sun, as it is in summer, this region receives more direct sunlight, and the days are longer.
However, as the Earth continues its orbit around the Sun, the position of each hemisphere relative to the Sun changes. After the summer solstice, the northern hemisphere gradually tilts in the opposite direction to the Sun. This tilt means that sunlight strikes the Earth's surface more obliquely, and the Sun spends less time in the sky each day.
The effect is most noticeable around the autumn equinox, when day and night are of equal length. After this equinox, the northern hemisphere continues to tilt further away from the Sun until the winter solstice, the shortest day of the year, which occurs around December 21st.
Thus, in autumn, the progressive shortening of the day is explained by this combination of the Earth's rotation and its tilt. This annual cycle affects all regions of the planet, but the further you are from the equator, the greater the difference between the length of day and night across seasons.
It is a predictable astronomical phenomenon that shapes our perception of the seasons.