that silvery time again

What is it about the sunlight in September?

All of a sudden you notice it; the sunlight has changed to a silvery gleam and everything on which it shines takes on that crisp, cool shimmer: grass, leaves, even the air it seems. There is a sparkle in September that bridges the time between golden summer and snowy winter, even as trees are putting on a show of fiery reds, oranges, yellows and toasty browns.

Is it the southward movement of the sun at this time of year that creates this effect, and prompts deciduous trees to shed their leaves? Or is it because I just really love autumn and see things differently myself at this time of year?