Earth's orbit is slightly elliptical, so the Earth-Sun distance varies over the year, causing the total solar radiation (insolation) reaching the top of the atmosphere to vary inversely with the square of that distance. Earth reaches perihelion (closest to Sun) in early January and aphelion (farthest) in early July, meaning December receives more total solar radiation globally (in both hemispheres) than June, independent of which hemisphere experiences summer or winter.