The autumnal equinox 2017 marks the first day of fall in the calendar year.

We have already blown through the first two seasons of the year (plus the end of winter earlier on), and are entering the last full season of the year in the fall. The autumnal equinox 2017 will fall on September 22.

The equinox applies to those living in the Northern hemisphere of the planet, as the day comes in March for those in the Southern hemisphere. It marks a day in which the Sun shines directly on the Equator.

During the autumnal equinox 2017, the length of the daytime and night is nearly the same, but not exactly the same. From an astronomical standpoint, Sept. 22 is the moment in which the Sun crosses the celestial equator from north to south, and vice-versa in March.

The word equinox means “equal night” and it will also mark the time in which the temperature will begin to drop considerably in the night time, especially for those living in a place where winter is in the horizon.

“If the Sun were to shrink to a star-like point and we lived in a world without air, the spring and fall equinoxes would truly have ‘equal nights,'” says former Almanac astronomer George Greenstein. This statement essentially means that the Earth’s atmosphere is the reason why the night and day are not exactly the same length during the equinox.