With that definite nip in the air and leaves starting to turn colour, it can only mean summer's end is near. The first day of fall though officially arrives on or about September 21st and is called the autumnal equinox. The word equinox comes from Latin meaning "equal night" and refers to the 12 hour long day and night that occurs only on this particular day of the year.
Skywatchers looking at the mid-day position of the sun over the summer season will notice that it has been slowly sinking closer to the southern horizon, resulting in ever longer shadows. On the autumnal equinox the Sun reaches its halfway point in its migration towards its lowest point in the midday sky, which happens on the December solstice. It's only on the spring and autumnal equinox that the Sun rises due east and sets due west.
Astronomically speaking the September equinox marks one of the four major turning points in the cycle of seasons. The Earth spins on its axis, which is tilted at 23.5 degrees with respect to its orbital plane. On these days however the Earth's axis is neither tilted away nor towards the Sun, but has both northern and southern hemispheres experiencing equal amounts of sunshine.
The equinox is really a geometrical alignment of the Earth with the Sun, when the sun appears positioned right above our planet's equator. As autumn progresses the Sun appears to continue its travels south until the winter solstice, when it slowly begins its journey north.