In article ,
Beto wrote:
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.

That is at least roughly correct.

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

Partly because Earth has a lot of thermal inertia: changes in the amount
of sunlight don't change its temperature quickly.

Partly because in the Northern Hemisphere, where most of the people live,
this trend is opposed to the seasons: Earth is closest to the Sun in
January -- Northern Hemisphere winter -- so the changes due to distance
are hidden by the normal seasonal changes.

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?

Light intensity varies (to a good approximation) inversely with the
square of distance.
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