The four seasons spring, summer, fall and winter correspond to four segments of Earth´s orbit around the Sun.
They are limited by the
solstices on 21st of June and 21st of December and the
equinoxes on 21st of March and 23rd of September.
They result from the change of the angle between the sunlight and the plane of the Equator (declination of the Sun).
Due to the tilt of the Earth axis to its orbital plane this angle
changes by 23,5° during each season.
An observer on Earth experiences seasons in form of the
Sun´s apparent path across the sky.
