It is the fact that the earth is not "straight up and down" on its axis as it rotates around the sun that creates our seasons. As you know, the earth completes one circle around the sun each year. The "circle" is actually a bit off, and a little like an oval; despite this fact (that the earth is a little closer to the sun at some times than others), this is NOT what causes our seasons; that difference is small. But the earth, as it orbits the sun, is 23.5 degrees off of "straight up and down" as it revolves in its daily rotation--see the first link below for a diagram of this. When the sun's rays hit either the northern or southern hemisphere in more of a direct line, that hemisphere has summer. The northern or southern hemisphere has winter when it is tilted away from the sun; the rays then hit the earth at more of an angle, are less intense, and provide less warming. If the earth was straight up and down on its axis as it revolves around the sun there would not be much difference between summer and winter temperatures.