Kelper's Law

The popular belief that winter is caused as the Earth moves away from the Sun during the widest part of its orbit, and thus causes winter, is not true. In actual fact, winter occurs when the sun is at it's closest point.

The planet tilts 23°27' (23 degrees 27 minutes) and this causes different parts of the Earth's surface to be closer to the sun at various parts of its orbit. It is this variation that brings about the seasons. In winter, the northern hemisphere is tilted away from the sun and thus experiences colder temperatures. Since the southern hemisphere will be tilted toward the sun at this point, it's seasons are always the opposite of ours.

During the winter, the light rays coming in from the sun hit the Earth surface at a lower angle. Less energy is transferred to the surface as a result of the glancing nature of these rays. Basically, the same amount of light energy is spread out over a larger area. This effect is compounded by the larger distance this light must travel through the atmosphere, allowing it to absorb more of this already limited heat.