The Earth doesn’t follow a truly circular path around the Sun; the real orbit is slightly elongated in to an oval that is slightly off-center. This means that Earth has an aphelion (furthest point) and a perihelion (closest point).
We just passed the perihelion (January 2/3 for this year) so we’re pretty much about as close as we get.
So why is it so bloody cold!?!?
This is one of the common misconceptions that often get passed around – sometimes even being taught in schools by people who ought to know better. The idea is that winter is caused by our elongated orbit and that the cold is caused by our being further away from the Sun.
To be sure, being closer to the Sun does mean that average temperatures across the world are around a couple of degrees higher than they otherwise would be. But right now I’m sitting at -5 (warming up from yesterday’s -17!), so what’s going on?
The answer to this apparent paradox is that winter doesn’t really have anything much to do with the Earth’s position relative to the Sun. It’s really to do with the tilt of the Earth’s axis. This tilt (approximately 23.5 degrees) means that at different parts of the year any given spot in the Northern Hemisphere will spend a greater or lesser time shadowed by the body of the planet itself.
This is what makes our days longer and shorter in different parts of the year too. As a result we have less of the warming effects we get from exposure to the sun and this is why we get winter and summer. It’s also this tilt that means that in the Southern Hemisphere right now it’s effectively their summer; the seasons below the Equator effectively being reversed from ours.
In addition to the fact that any particular spot spends more time under the Sun because of the tilt, there’s another part of the puzzle. The tilt also means that the Sun’s rays are more spread out due to the angle at which the light hits the Earth. As a result the light (and heat) per unit of land is lower in winter than in summer, and again this contributes to the lower temperatures in winter.
There are some good animations here, that show the effects clearly.