The 2017 autumnal equinox is right now 4:02 p.m. EDT, EDT Sept. 22 according to the Old Farmer’s Almanac as the sun crosses the celestial equator. Fall has arrived in the Northern Hemisphere, and the days are now shorter than the nights and will continue to be so until the spring equinox.
The celestial equator is a circle concentric with the actual equator that extends infinitely to space. As the Earth has a 23 degree tilt the the northern half tilts towards the Sun during half its orbit and away the other half. The equinoxes occur when tilt switches.