The seasonal temperature is ultimately due to the precession of the Earth around the axis. But what I'm curious about is... is it due more to the side experiencing winter being farther from the sun or is it more due to the fact that the days are shorter and the nights are longer?
[ The earth is actually closer to the sun in the Northern hemisphere's winter. The seasonal temperature variation is predominantly due to the angle the earth makes with the sun. In the northern hemisphere in winter the angle is such that the earth is tilted with north pole away from the sun and the sunlight hitting the earth is spread over a much larger area than if it was pointed towards the sun. Due to this tilt the sun is also lower in the sky and has the effect shown in the 2nd figure.