Despite a popular belief, the seasons have nothing to do with how far the Earth is from the Sun. If this were the case, it would be warmer here during January as opposed to July. Instead, the seasons are caused by tilt of the Earth on its axis, by as much as 23.5 degrees.

The Earth has an elliptical orbit around the Sun. The Earth is at its closest distance to the Sun in January (called Perihelion) and the furthest in July (the Aphelion). But this distance change is not great enough to cause a substantial difference in our climate. This is why the Earth's 23.5 degree tilt is all important in changing our seasons. Near June 21st, the summer solstice, the Earth is tilted such that the Sun is positioned directly over the Tropic of Cancer at 23.5 degrees north latitude. It is this time of the year the northern hemisphere "leans" toward and receives the most direct rays from the Sun. Since the Sun's rays enter the atmosphere at a higher angle, they don't have to travel as far before reaching the Earth's surface. This translates to less light being scattered by the atmosphere and more reaching the surface causing warmer temperatures. In addition, the high sun angle produces longer days.

The opposite is true in the southern hemisphere on June 21st, where a low sun angle produces more scattering of light, shorter days and colder temperatures. Therefore, near June 21st, the southern hemisphere is having its winter solstice because it "leans" away from the Sun.

On or near September 21st, the Earth reaches the autumnal equinox. As the Earth revolves around the Sun, it gets positioned such that the Sun is directly over the equator. Basically, the Sun's energy is in balance between the northern and southern hemispheres. The same holds true this time of the year on the spring equinox, as the sun is once again directly over the equator.

Lastly, on the winter solstice, near December 21st, the Sun is positioned directly over the Tropic of Capricorn at 23.5 degrees south latitude. It is this time of the year that the southern hemisphere receives the more direct sunlight, producing higher sun angles, longer days and thus warmer temperatures. The northern hemisphere "leans" away from the sun on December 21st producing low sun angles and the short days of early winter.