Since this Summer hasn't been as hot as past years, maybe it is easy to believe that Fall starts Monday. The Autumnal Equinox marks the beginning of Autumn and that occurs at 10:44 AM.
Why is it such a specific time?
At that time the sun crosses the celestial equator, meaning the sun is directly in line with the equator at that moment.
Why Is It When Fall Starts?
But the reason this is a good time to say that the Fall season starts is because the sun will be at a much lower angle in the sky for us here in the Northern Hemisphere. This will mean less and less hours of sunlight in a day, which is why it will start to get cooler and of course why we like to say Fall starts with the Autumnal Equinox.
If this sounds like a physics lesson, I'm really not trying to make it confusing but simply explain why the Autumnal equionx matters. And if you're a visual person, here's a cool link that might help explain the sun angle and how it changes during the year, click here. (Give it time to load, click once on the white box if images don't appear)
To give you an idea of the loss of sunlight hours, we currently have about 12 hours of sunlight and by late December we only have about 9.5 hours of sunlight in a day. Bummer. Hey, it is what it is.