At 21:21, the autumnal equinox begins

Wednesday marks the autumnal equinox, the end of the astronomical summer, and the beginning of autumn. The Sun enters Libra at 21:21 CET, when the Sun’s center is above the Earth’s equator, illuminating both hemispheres equally. The day will be as long as the night. Days will continue to get shorter until December 21, when the winter solstice falls this year.

Seasons change regularly because the Earth’s rotational axis is inclined at an angle of 23.5 degrees to the orbit plane. Thus, as the Earth orbits the Sun, the northern and southern hemispheres of the planet alternately tilt towards or away from the Sun. 

When the northern hemisphere is tilted towards the Sun, it is summer in the Czech Republic and other countries in the northern hemisphere; when the southern hemisphere is tilted, it is winter in the Czech Republic.

Meteorologists have a different division of the seasons; autumn begins on September 1 and ends on the last day of November. Fixed three-month periods were introduced for uniform processing of climatological data.