Depending on which source you reference, the autumnal equinox occurs this evening or early tomorrow morning an hour or two into the new day. Let’s think about what this means in astrophysical terms.
The earth’s orbit around the sun represents a circle, think of it as a circle drawn on a sheet of paper or plane. The center of that circle is the sun. You might imagine the earth’s axis, the line upon which it rotates, was perpendicular to the plane but it is not.
The axis is tilted approximately 23.4 degrees off perpendicular and that tilt is what gives us seasons. Winter is when our northern half of the globe is tilted away from the sun, Summer when it is tilted toward the sun.
The spring and autumn equinoxes are when the north and south poles are both the same distance from the sun. In other words, when the sun’s light is shining down vertically upon the equator and the periods of light and darkness are each 12 hours in both the northern and southern hemispheres.
Thus, tomorrow will be the first day of autumn in the northern hemisphere, and the first day of spring in the southern hemisphere. In our northern hemisphere the days will get shorter and shorter until, three months from now, the North Pole will be at its farthest from the sun. That will be the winter solstice, and winter will officially begin.
