In exactly one hour we slip over the edge from summer to fall, at 20:02 UTC (8 PM UTC, 4:02 PM EST, 1:02 PM PST, and go here for your local time).
So, you ask, what makes this the deciding moment?
It is the moment when the sun crosses the earth’s celestial equator, due to the tilt of the earth and it’s circling of the sun, and the sun begins to shine more on the southern hemisphere than on the northern hemisphere.
On the two equinoxes (spring=vernal, fall=autumnal) the earth’s axial tilt (look at the red dot on the *north pole*) is perpendicular (at 90 degrees) to the rays from the sun.