Actually, the major reason for the temperature difference is the ANGLE at which the sun's rays reach us. In the winter, when the sun is far to the south of us (as we see it), the rays have to penetrate quite a bit more atmosphere to get to us -- and the North Pole doesn't get any direct sunlight at all for part of that season. In the summer, when the sun is more directly overhead and it's always shining up at the North Pole, there's less atmosphere to absorb and diffuse the rays, and the polar region doesn't stay as cold.

Below the equator, the reverse is true -- which is why the folks down in Oz celebrate Christmas in their summer time, and people in the tropics where the sun is always more directly overhead don't have significant temperature changes year-round.

Our actual distance from the sun doesn't vary all that much from maximum to minimum, and even if that were a significant factor it would not explain why the seasons are reversed south of the equator.