by Tenebris

Created on: June 15, 2008   Last Updated: June 18, 2008

They tell about the apprentice who was watching a construction worker heat a nut to free it from its bolt. When the apprentice asked why, the worker patiently explained that as the nut was heated, it would grow larger and ease its grip on the bolt. "So things get larger when they get hot?" asked the apprentice; to which the worker answered dead-pan, "Yes. That is why days are longer in summer and shorter in winter." After a longish pause, the apprentice's face suddenly cleared: "You know, I always wondered about that!"

It is true that days do tend to get hotter as daylight grows longer. But why does daylight grow longer in the first place?

Seasons - and consequently longer and shorter periods of daylight - result from the earth's axial tilt. 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'). As the earth's orbit carries us around the sun, this tilt causes the orientation of the northern and southern hemispheres to change with respect to the sun. When one hemisphere is tilted toward the sun, the other will be inclined away from it. 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, the part that we can observe directly is that in the parts of the earth which experience warm/cold seasons (ie. outside the tropics), the sun is never directly overhead. In fact, 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 lie approximately 23 degrees north and south of the equator respectively, at the same degree of latitude and longitude as the axial tilt.

Over the course of a year, the closer the sun comes to being directly overhead at noon, the longer will be the hours of daylight at that location.

The day of the year with the longest period of daylight is called the summer solstice. In fact, the word 'solstice' directly derives from the Latin for sun ('sol') and standing still ('stitium'), this being the time of year when the sun seems to stand still during the long, lazy day while it comes closest to being directly overhead. In the northern hemisphere, this day falls sometime between June 20 and 24, while in the southern hemisphere, the summer solstice falls between December 20 and 24.

The further poleward we go, the more extreme the seasonal difference in daylight becomes. In fact, north of the Arctic Circle and south of the Antarctic Circle, located at just over 66 degrees north and south latitudes respectively, on the summer solstice, the sun never sets at all!

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