re: Fall is upon us - how much further away from the Sun are we now?

We are actually getting closer to the sun right now, and we will be closest in early January. So, the variation in the orbit makes very little difference.

It is cooler in winter for the same reason it is cooler in the morning and evening than mid-day. A square foot of sunlight is spread over an area larger than a square foot (and so does less heating) as the angle it reaches the earth's surface changes.

All this is beside the fact that climate covers a period of time longer than a season.