Bad Astronomy: The seasons are caused by the change in the distance of the Earth to the Sun.
Good astronomy: The seasons are mostly due to the axial tilt of the Earth. The change in distance of the Earth to the Sun is a very minor player.
How it works: This is one of the most pernicious types of ideas: one that sounds reasonable, and so it propagates easily. Unfortunately, it's wrong. Well, not completely wrong; certainly the Earth's distance from the Sun has something to do with the temperature, but it is a relatively minor effect.
First, a sanity check: The Earth's orbit is an ellipse. The Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (farthest point from the Sun) some six months later. If that were all that governed weather, we'd have summer in January, and Winter in July! This may be true for our Southern Hemisphere friends, but not up in the North. Something else must be going on.
We can check our qualitative conclusion above with some (simple!) math. The math involved in calculating a planet's gross temperature has been known for a long time. Basically, the temperature depends only weakly on distance changes; the temperature goes as the distance to the one-fourth power (the square root of the square root!). In other words, if you double the distance of a planet from the Sun, the temperature will drop by 2^(1/4) or 1.18. Doubling the Earth's distance from the Sun will only drop the mean temperature by about 44 degrees Celsius The Earth's average temperature is about 283 degrees Kelvin or 10 Celsius.
283 - (283/1.18) = 44 degree drop.
The Kelvin scale is absolute, which means it starts at 0, which is why I used it for the calculation.
At perihelion (nearest point) the Earth/Sun distance is about 146,000,000 km, and at aphelion (farthest point) it's about 152,000,000 km. The change in temperature is then (152,000,000 / 146,000,000 ) ^ 1/4 = 1.0085 or only 0.85 percent! This turns out to be only 2 degrees Celsius, which is quite a bit less than the temperature change we see between winter and summer! Obviously, something else must be going on.
The largest contributor to the change in seasons is the tilt, or inclination, of the Earth's spin axis with respect to its orbital plane (the ecliptic). The usual explanation is as follows: take a flashlight and a piece of paper. Shine the light straight onto the paper, so you see an illuminated circle. All the light from the flashlight is in that circle. Now slowly tilt the paper, so the circle elongates into an ellipse. All the light is still in that ellipse, but the ellipse is spread out over more paper. The density of light drops. In other words, the amount of light per square centimeter drops (the number of square centimeters increases, however, so the total amount of light stays the same-- you expect that, as the light from the flashlight has not changed).
The same is true on the Earth. When the Sun is overhead, the light is falling straight on you, and so more light (and more heat) hit each square centimeter of the ground. When the Sun is low, the light gets more spread out over the surface of the Earth, and less heat (per square centimeter!) can be absorbed. Since the Earth's axis is tilted, the Sun is higher when you are on the part of the Earth where the axis points towards the Sun, and lower on the part of the Earth where the axis points away from the Sun.
For the Northern Hemisphere, the axis points most toward the Sun in June (specifically, around June 21), and away from the Sun on December 21. This corresponds to the Winter and Summer Solstices, or the midpoints of summer and winter. For the Southern Hemisphere, this is reversed.
There is more, too. In the summer, the Sun is higher, and therefore the days are longer. This gives the Sun more time to heat the Earth, so it gets hotter. In the winter, the sun is lower, and the days are short, giving the Sun less time to heat the Earth. This is a secondary effect.
The distance of the Earth to the Sun is a smaller effect yet, but it does exist! So the Southern Hemisphere gets slightly hotter summers and slightly colder winters than the North. But only by a couple of degrees, and only on average.
Your mileage may vary!
Submitted by Grant Dixon
Hamilton Amateur Astronomers
Maintained by Rob Roy