by Tenebris

Created on: June 15, 2008   Last Updated: June 17, 2008

On a Midsummer Day over two millennia ago, Eratosthenes of Cyrene observed that in Syene, Egypt, far up the Nile from where he lived on the Egyptian coast, the noon sun could be seen reflected in a deep well: which meant that there it was almost directly overhead. However, in Alexandria where he then lived, on the coast of the Mediterranean, the midsummer noon sun still cast a shadow. He measured the shadow, and knowing the distance between the two cities and using the basic geometrical rules of triangles, he was able to determine that the circumference of the earth must be 252,000 Egyptian stadia, or approximately 39,690 km: within 1% of our modern measured value.

The apparent movement of the noontime sun and consequently the longer and shorter periods of daylight which cause our seasons result from the earth's axial tilt relative to the plane of its orbit. 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 the planet 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 further a hemisphere is inclined toward the sun, the longer will be its length of daylight. Thus the temperature differential caused by this tilt is enough that when one hemisphere is inclined toward the sun it 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 consists of the interaction of several independent factors, and takes approximately 41,000 years to complete.)

The summer solstice occurs when a hemisphere is inclined such that it receives the maximum possible exposure to the sun. From the surface of the earth, the sun will be as close to overhead at noon as it ever gets in that part of the world, and thus both the intensity of the sunrays and the length of daylight will be as high as it ever gets. In fact, above the Arctic Circle and below the Antarctic Circle, at the summer solstice the sun never sets. Because the sun appears to hang in the sky so very long during the summer solstice, the word 'solstice' itself is derived from the Latin for sun ('sol') and standing still ('stitium').

In the northern hemisphere, the summer solstice occurs sometime between June 20 and 24, while in the southern hemisphere, the summer solstice occurs sometime between December 20 and 24. Traditionally this day marks the beginning of summer.

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