Circle of Illumination

WORLD SUNLIGHT MAP

In the hallway outside the Geography Department office at Bridgewater State University is a fascinating device known as Geochron. Manufactured decades ago, it is a combination map and clock that is configured so that it always represents the areas of the earth in sunlight and darkness. At all times, exactly one half of the earth is illuminated by the sun, though cloud cover can affect how much reaches the surface and latitude can influence its intensity. The 50 percent in sunlight is known as the circle of illumination, because the boundary between sunlight and darkness forms a circle.

On the equinoxes, the circle of illumination intersects both the north and south poles, but during the rest of the year, the circle appears tilted first one way and then another, as the earth orbits the sun at a constant 23.5-degree angle. During this orbit, the apparent position of the sun changes steadily, and it appears directly overhead at only one place at a time on the planet. That location is known as the sub-solar point, and it oscillates from the Tropic of Cancer on the summer solstice to the Tropic of Capricorn on the winter solstice (as defined in the northern hemisphere -- people in the south use the opposite terminology).

On the original, mechanical Geochron, a small dot represents the single place on earth -- always in the tropics -- where the sun appears to be directly overhead. Of course, there is only one such point at any time, known as the sub-solar point because it appears to be under the sun.It moves from the Tropic of Cancer on our summer solstice to the Tropic of Cancer on our winter solstice, crossing the equator on the equinoxes.

Two digital versions of the GeoChron are readily available online. The World Sunlight Map shown above also represents current cloud cover and is available in several different projections. The NIST version from the National Institute of Standards is accompanied by the current, official time as used for U.S. Government purposes. (Be sure to adjust the time zone or display options if necessary.)

With parental permission, it is also now possible to install a miniature version of the GeoChron as a desktop gadget on your computer, allowing you to track the sun at all times.

We invite EarthView participants to view the mechanical GeoChron in our geography department, in order to see the circle of illumination in more detail and to marvel at the clever, compact design. On the mechanical version, it is also possible to see how the sun's noon position advances and retreats through the year not only by latitude but also by longitude, tracing the figure-eight shape of the analemma.