Axial Tilt and the Reason for the Seasons

Following Newton's law of gravity and Kepler's laws of planetary motion, the earth orbits the sun in an elliptical orbit with a one year period. The moon in turn orbits the earth in about a month.

Because earth's orbit is elliptical and its distance from the sun consequently changes, it is tempting to think that the earth's changing distance from the sun causes the seasons. We have winter when earth is farthest from the sun and summer when it is closest. This idea is absolutely wrong.

The apparent logic of this idea makes it extremely difficult to dislodge it from students’ minds, but two facts directly contradict it:

1. Earth's closest approach to the sun is in January – the middle of the northern hemisphere’s winter.
2. The seasons are reversed in the northern and southern hemispheres. When it is winter in the northern hemisphere, it is summer in the southern hemisphere, and so on.

The seasons are caused by the tilt of the earth's axis. This tilt causes the sun to shine higher in the sky in the northern hemisphere and lower in the sky in the southern hemisphere during late June - the summer solstice. At this time of year the sun is also above the horizon for a longer period of time each day in the northern hemisphere and for a shorter period in the southern hemisphere. The more direct illumination for a longer time each day warms the northern hemisphere. The less direct illumination for a shorter time period means the southern hemisphere is cooler. Hence this solstice marks the beginning of summer in the northern hemisphere and the beginning of winter in the southern hemisphere.

In late December -- the winter solstice -- the situation is reversed. Hence, we have the beginning of northern hemisphere winter and southern hemisphere summer. Late September and late March mark the equinoxes when the sun is directly over the equator. They mark the beginning of autumn and spring which are also reversed in the northern and southern hemispheres. The dates of the equinoxes and solstices can vary by a few days each year. The 21st is the average, but they can occur from the 19th to the 23rd of the respective month.