September equinox

The annual instant when the Sun crosses the celestial equator southward, marking autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

Celestial equator: The projection of Earth’s equator onto the celestial sphere. The Sun crosses this projected circle as the September equinox occurs.
Tropical year: The time Earth takes to complete a cycle of seasons, about 365.2422 days. The equinox shifts against the civil calendar because a tropical year is not an integer number of days.
Autumn equinox: The equinox associated with the beginning of autumn in a given hemisphere. In the Northern Hemisphere, the September equinox is the autumn equinox.
March equinox: The annual instant when the Sun crosses the celestial equator northward. It is the opposite-direction crossing, associated with spring in the Northern Hemisphere.
Ecliptic: The apparent annual path of the Sun across the celestial sphere. The Sun’s southward intersection with the celestial equator defines this equinox.
Gregorian calendar: The solar calendar introduced in 1582 and now used internationally for civil purposes. Its leap-year rules keep the September equinox near the same calendar dates.
Spring equinox: The equinox associated with the beginning of spring in a given hemisphere. In the Southern Hemisphere, the September equinox is the spring equinox.
June solstice: The annual solstice when the Sun reaches its northernmost declination. Unlike the equinox, it marks the Sun’s greatest northward displacement.
Earth's axial tilt: The angle between Earth’s rotation axis and the perpendicular to its orbital plane. This tilt creates the changing solar declination that reaches zero at the equinox.
Precession of the equinoxes: The slow change in the orientation of Earth’s rotational axis, shifting the equinoxes against the stars. It gradually changes which constellations lie behind the Sun at this equinox.

Linked from 9 pages