We observe that temperatures vary in a set pattern over the year. This pattern is translated into seasons. We figure that the seasonal changes must be due to regular changes in insolation, given the variation of temperature being the most prominent feature of the changes.

So what causes the regular changes in insolation? We know that the Earth's orbit around the sun is elliptical, and sun is off-center; thus, the sun-earth distance varies during the year. By the inverse square law (the greater the distance from a point source of radiation, the lower the intensity), the Earth should be warmer at Perihelion (Earth closest to the sun) in January and cooler at Aphelion (Earth farthest from the sun) in July. Well, this may be true for the Earth as a whole, but we know that in the Northern Hemisphere, it's winter in January, when the Earth is closest to the sun, and summer in July. In addition, the difference in the distances at Perihelion and Aphelion is about 3-4% (147 vs. 152 million kilometers), which affects the overall Earth temperature minimally, if at all. So it's not the Earth-sun distance change that causes the seasons...

...it's the angle at which sunlight hits the Earth's surface. The cosine law relates the angle of incidence of radiation on a surface to the intensity of the radiation illuminating the surface. Basically, the shallower the angle (or, the greater the zenith angle or the smaller the declination angle), the small the intensity since the radiation in the beam gets spread over a larger area (illustration "b"). In the righthand diagram, there are two identical beams of sunlight hitting two different latitude regions of the Earth's surface. The red sticks are perpendicular to the Earth's surface. The beam hitting the equatorial region (the lower beam) is striking the Earth's surface perpendicularly (so the zenith angle is 0°) and should produce the highest intensity at the surface. The other beam in the higher latitude is at an angle to the red stick, so the radiation is spread over a larger area and thus, the intensity of the sunlight at the surface is lower. This just results in an intensity variation vs. latitude, but how can this change over the entire Earth's surface over the course of a year? See next page -->