by Tenebris

Created on: June 15, 2008

As a child growing up in the northern hemisphere, one aspect of summer I always found confusing was that the days were hottest when the earth was furthest away from the sun. Surely the earth ought to be hotter when it was closer to the sun?

Seasons and consequently longer and shorter periods of daylight result from the earth's axial tilt. What this means is that the poles of the earth do not point straight up and straight down relative to the sun, but are actually on an angle. The greater the angle of a planet's tilt, the more contrast there will be between its seasons. The angle of the earth's tilt is approximately 23 degrees, or slightly less than a quarter of a right angle ('L'). If you divide the earth along the equator into a northern hemisphere and a southern hemisphere, one hemisphere will always be inclined toward the sun, and the other will always be inclined away from the sun. The temperature differential caused by this tilt is enough that the hemisphere inclined toward the sun will experience summer, while the hemisphere tilted away from the sun will experience winter.

(The axis of the earth also 'wobbles', in the same manner as a gyrating top. This should not be confused with axial tilt. Precession is the reason the North Star has not always been Polaris, and will be Vega at the opposite end of its cycle. One complete precession cycle takes 25,800 years. The actual angle of the earth's tilt is also not constant, but fluctuates within a range of between 22.5 degrees and 24.5 degrees. This cycle takes approximately 41,000 years to complete.)

From the surface of the earth, what this means is that at noon, the sun is usually not directly overhead. In the northern hemisphere, the sun will usually remain to the south, while in the southern hemisphere, the sun will usually remain to the north. The only places on the entire earth where it is possible for the sun to be directly overhead is between the Tropic of Cancer and the Tropic of Capricorn, which just happen to be approximately 23 degrees north and south of the equator respectively.

The closer the sun comes to being overhead at noon, the longer the period of daylight will be. The further the sun is from being overhead at noon, the shorter the period of daylight will be. In the area of the world called the tropics, between the Tropic of Cancer and the Tropic of Capricorn, the difference between the longest periods of daylight and the shortest will be fairly small. The further outside this region, the greater

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