In the Northern Hemisphere, you will sense the coming of the autumnal equinox because the days will be growing shorter. If you find a good place to watch the sunset twice a week during late summer, you will be able to spot the change when the sun stops moving south and sets due west on the day of the autumnal equinox. You will see it begin moving north after that. Look on a map for a street near your home that heads due west to east and has a clear view of the horizon to do this.
In Latin, "equinox" literally means "equal night." It occurs on the two days out of the year when the day is nearly the same length as the night. The autumnal equinox takes place between September 21 and 23 and is the first day of fall in the Northern Hemisphere. It is the first day of spring in the Southern Hemisphere. On the autumnal equinox, the sun is exactly midpoint in the celestial sky. After that, it starts its journey south, reaching the southernmost point on the winter solstice.
Just as two poles are on earth -- the North Pole and the South Pole -- two celestial poles are in the sky. You may have heard of the North Star, or Polaris, which hangs in the sky directly over due north. It's helped people navigate for centuries. The south celestial pole is invisible in the Northern Hemisphere.
The celestial equator is halfway between the celestial poles in the sky. It is actually a circle in the sky that crosses the horizon in exactly the middle of the poles at exactly due east and due west. The result of this is that the autumnal equinox is one of only two days a year when you can see the sun rising exactly due east and setting exactly due west.