Its because of the earth's angle to the ecliptic plane (i think its about 23 degrees or thereabouts).

The earth is in a circular orbit and its not that the earth is further away in winter (or it would be winter all over the planet!). Its because the northern hemisphere is tilted away and the southern towards the sun and vice versa in our summer.

At the moment the sun is low in the sky as we are tilted away so the sunlight is hitting the surface at a much more shallow angle that if the sun were say directly overhead.

Hope this helps.