earth-sun distance and heat involved

Hi,
I've a question which has been wandering in my head for a while. Maybe
this is not the right place to ask it, but maybe someone could point
me to a better place.

I'll try to ask it as easy as possible.

According to my calculations, the difference between the longest
distance from earth to the sun and the shorstest one are about 4.8
millions of kilometers.

Why the temperature doesn't drastically change because of this?

If this is because of radiation doesn't being too important this far
from the sun, Where can I find a equation which relates radiation with
distance?

Hope you can help me clear my mind.

Regards,
--
Beto