(© copyright 1996-1999, Timothy G. Feeman and Elaine F. Bosowski.)
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Geodesy; Geoid; Spheroid; Eratosthenes; Latitude; Longitude;
Time and time zones;
Solar time;
Mean
time; Spherical coordinate system.
Images:
The following diagram shows another view of how Eratosthenes determined the latitude angle of Alexandria (relative to Syene). The sketch shows the angle between the gnomon and the sun's rays at solar noon, when the shadow of the gnomon is the shortest. The angle t° in the sketch is the difference in latitudes between the location where the measurement is being made (Alexandria, in Eratosthenes' case) and the location where the sun is directly overhead at noon on that day (Syene on the summer solstice, in Eratosthenes' case).
How do you know the latitude at which the sun shines directly overhead at noon on any given day? You use an analemma, a tool designed for just that purpose. Here is a copy of an analemma. Just find the day you want on the figure 8; the latitude where the sun is overhead on that day is shown to the left. (Or, pick your favorite latitude and use the figure 8 to telll you the days when the sun will be overhead there.)
A look at the geoid. More about the geoid.
The analemma. An analemma applet.
A biographical sketch of Eratosthenes; more on Eratosthenes' work; info on classical Greek geography/geometry
