Change of flux of SUNLIGHT?
Strength of the sunlight might vary depending on the distance of the Earth from the Sun. This turns out to be a very small effect. Althought orbits of planets are elliptical, the orbit of the Earth around the Sun is very close to circular.Flux involves the normal component coming through an area, so the flux should be greater in the summer when the sunlight is coming in at a more normal angle. It will be less in the winter when the normal component of the sunlight is smaller.