In my part of the world, and likely for most who stumble across this post, autumn is said to begin when the sun is directly over the Equator on its southward journey to the Tropic of Capricorn.
This occurs on September 21 or close to it; it does vary slightly. (
Of course, we refer to the sun as moving when, in fact, it is the earth.)
There are other notions of autumn, however. Aussies, for example, view autumn (actually their spring) as beginning on September 1st.
If we need to distinguish between the two versions, in Canada, what we call autumn (or more commonly, Fall) can be termed Astronomical Autumn. However, Meteoritical Autumn begins today, September 1st.
Again, here in the Great White North, it makes absolute sense to declare that September 01 begins autumn. It's when we go back to school and when temperatures begin to become more pleasant after the high heat of summer.
Of course, temperature change doesn't occur automatically on the 1st, but it isn't usually long afterward. I often began teaching on a very hot day early in September, but that heat soon dissipated into the much more agreeable autumn temperatures, not at all like July and August.
The meteorological definition makes sense to this poor clod. We always say that kids go back to school in Fall. Then we make it confusing by saying that Fall begins on the 21st or thereabouts.
The meteorological calendar divides into 3 month segments.
link
But that's confusing too, at least in this country, because November temperatures are definitely not autumn-like. Especially by month's end, it can be pretty doggone wintry. In many years, continuous snow cover begins before the end of November and continues into March. It has lasted into April a few times. Indeed, I personally think of winter, not as being 3 months but close to 5.
Once upon a time, in English-speaking countries, autumn was thought to begin on Lammas Day (August 01), and end around Hallowe'en. I am not entirely sure, but this may still be the way it is seen in the Irish/Gaelic calendar. Considering August to be autumn, may have worked in the more moderated climate of Eire and
Old Blighty, it wouldn't work for Canada where August is far too hot.
On top of all of this, I have my own definition based on sun observations in my own little den. I deem that autumn begins when I no longer have to pull the shade to keep the sun from glaring off my computer screen. We haven't quite reached that point, but it's getting close.