According to the news today, Fall will officially arrives this coming Monday ( September 22) at 11.44 a.m. when the Sun cross the celestial equator in the sky heading from North to South. This will mark the beginning of Autumn in the Northern Hemisphere.
PARI Astronomer Bob Hayward said "There are two days of the year when the Sun is above the horizon for exactly half the day and is below the horizon an equal amount of time. At the moment, the Sun in its apparent path around the sky will stand directly over the equator of the Earth. That’s when the length of daylight is equal to that of the night, earning the name “equinox” from the Latin for “equal night.”
The hours of daylight will continue to shorten each after the autumnal equinox. This will continue until the winter solstice at 7:04 a.m. on Dec. 21. Following the solstice the days get longer until at the spring or vernal equinox - at 7:44 a.m. on March 20 - the day and night are once again equal in length.