You know that the sun is the source heat on the earth ,but why do you thing the heat varies morning?

Orbiting the sun has nothing to do with our season changes, IT CAUSED BY THE AXIS.
If orbiting the sun caused summer, then it would be summer all around earth. IF getting farther from the sun caused winter, it would be winter all over the earth, which is not the case. The change in seasons is caused by the tilting axis of the earth my friend. The angle at which the suns rays hit the surface is the cause of the flux in seasons and temp.
This is a scientific fact, google it...
It varies morning to night because the side that is reciving light at day time, it recieving the radiation, and the night side is not.. until it rotates around and vise versa.