Many people believe that the seasons are caused by the Earth changing its distance to the Sun during its annual elliptical orbit. This is wrong. In fact, the Earth is closest to the Sun in January, when it is coldest in the northern hemisphere.
Instead, the seasons are caused by the Earth's axis of rotation not standing perpendicular to its plane of rotation around the Sun, but at an angle of 23° - see illustration below.
In this applet we created a tool to visualize the origin of the seasonal variation of the temperature. We select the reference frame of the Earth to do this and show where in the sky the Sun is as a consequence of the revolution of the Earth around it.
Click first on the month of your choice to put the Sun in the sky at the proper angle and relative distance to the Earth. (By changing the month, you can see that the angle changes widely from month to month, whereas the distance Earth-Sun remians almost constant - an expression of the fact that the excentricity of the Earth's orbital ellipse is quite small!)
Now you can click anywhere outside the surface of Earth and a line to the center of Earth is drawn, indicating your lattitude. In addition, the cosine of that line (the surface normal line) and the sun rays is displayed. This cosine is a direct measure for the intensity of the solar radiation that causes the variability in the surface temperatures on Earth.
© W. Bauer, 1999