Why Do We Have Seasons?

If you were to ask someone what a particular season means to them, they would probably mention the weather usually associated with it. In the winter, it's cold and snows outside. In summer, it's warm enough outside for people so they can go swimming. In the spring, it's warm enough to make the plants green again and in the fall, it's cool enough to make plants go dormant. Yet in some parts of the world, the average temperature remains warm throughout the entire year. A great many people who live in areas where brutally cold winters are normal make vacation plans during these winter months to the warmer parts of the world. How is it, that one part of the earth can be frigidly cold, yet another part of the earth feels as warm as summer? Are the seasons the earth experiences really attributed to the distance of the earth from the sun?

One of the biggest misconceptions concerning the Earth's orbit around the Sun and the changing of the seasons is that the Earth is closer to the Sun during the summer, causing summer's warmer temperatures. This is totally false. While the Northern Hemisphere has summer, the Southern Hemisphere has winter. While it's summer for the Northern Hemisphere, the Earth is actually slightly farther away from the sun than during the winter months. So the changing of the seasons is completely dependent on the tilt of the Earth, not the distance of the Earth from the Sun.

During the Northern Hemisphere's summer season, the North Pole is tilted towards the Sun. During the winter, it is tilted away. This tilt causes the Sun to appear higher in the sky during the summer than during the winter. The higher Sun causes more hours of daylight and more intense, direct sunlight, or hotter conditions on the surface of the Earth.

While the Earth rotates once a day about its axis between the north and the south poles, it revolves about the Sun in its orbit but once a year. As the Earth orbits the Sun, the orientation of its rotational axis is held fixed, so that if we imagine it extended into space, it is always pointing towards Polaris, the pole star. As the Earth orbits around the Sun, for a portion of the year, the Sun is in the same direction as the tilt happens to be. In other words, the Earth is in a part of its orbit where its tilt direction is towards the direction of the Sun. This means that the Northern Hemisphere will have warmer temperatures and hence summer. For the other portion of the year, the Earth has moved around the Sun, putting the Sun not in the direction in which the tilt happens to be. Or, the tilt direction is off into space behind the Earth (towards the outer planets). With the top of the Earth now tilted away from the Sun, the Southern Hemisphere can enjoy summer. There are, of course, times in between as the Earth is orbiting when it has the Sun neither towards nor away from the direction of the tilt. These times are when no hemisphere is experiencing any greater amount of sunlight than another is, and so this is spring or fall for the hemispheres.

A very good way to demonstrate the tilt of the Earth in relation to the sun is by measuring the amount of summer and winter sunlight. It is important to note that even without the tilt of the Earth, there would still be variations in temperature from one location to another, caused mainly by the curvature of the Earth. Locations closer to the equator would still, on the average, be warmer than locations closer to the poles. Light and heat (radiation) from the Sun would still strike Polar Regions at more of an angle than nearer the Equator. This angle tends to "spread out" the same amount of energy over a larger area, thereby decreasing its intensity and the amount of heat it brings to the Earth.

With light comes heat. The more light received in a hemisphere of the Earth, the higher the temperatures. A really good way to demonstrate this is by measuring how quickly and how much sunlight can warm two sheets of paper, one tilted, one not. For this project demonstration, you will need two sheets of black construction paper; two pieces of cardboard or plywood; bricks or blocks to prop up the board; masking tape; two thermometers.

Cut an inch-wide slit in the middle of each piece of construction paper. Tape one sheet of black construction paper to each of the cardboard or plywood boards. Place a thermometer into each slit such that the bulb is between the board and the paper, and the scale can be read without removing the thermometer. Tape the thermometers in place. Leave the assembled thermometers in the shade long enough so that they read the same outside temperature.

Tilt one board so that it faces the Sun and the Sun's rays fall nearly perpendicular to the board. The other should be flat on the ground or even tilted slightly backwards from the Sun if the Sun is especially high in the sky.

Periodically (every minute or so) record the temperature on each thermometer until the temperatures level off and stop climbing. Let the thermometers sit for a few minutes and record their final temperatures.

Which paper was heated more quickly? Which got warmer? You might want to try different angles. Beware of the effects of clouds and wind, as well as the shadows of trees and bushes. What do the results of this experiment tell you about the changes in temperature?