Closer to the Fire?

If you live in the Northern Hemisphere, as we do in the United States, Earth is actually closest to the sun during winter. The Earth's orbit isn't a perfect circle. The planet reaches perihelion -- the closest point to the sun -- in early January, and aphelion -- the farthest point -- in July. We receive about 3 percent more solar radiation in January.

So why is it so cold? You've probably noticed that the sun is lower in the sky at noon in winter. Therefore, the sun's rays must pass through our atmosphere at a greater angle before striking the ground. The heat and energy are somewhat diluted along the way. (See Tying Down the Wind, pages 131-132.) In places where the sun's rays are more direct, such as at the equator, sunlight is much more intense.

It all comes back to the tilt of Earth's axis. Here's one way to visualize it: Try pointing a flashlight at a wall. If you hold the flashlight close to the wall, you'll get a small, intense circle of light. Now tilt the flashlight at an angle. The light covers a greater area, but it's weaker. The greater the angle, the weaker the light.