The September equinox arrives/arrived at 14:21 UTC. or 10:21 EDT. Autumn in the north and spring in the south.
The image here came from timeanddate.com and is a good depiction of what is going on: Today at 14:21 UTC the equator of the Earth is pointed right at the center of the sun. As the Earth travels in its orbit the position of the Sun with respect to the equator changes. In September the tilt is such that the center of the Sun is moving south. By late December the Earth has moved sufficiently in orbit the apparent movement stops (the December solstice) and starts going back north.