Astronomical Polar Measurements of the Earth

Measuring the earth is a very old problem. The first measurement of the earth, based upon logical reasoning, was made in Egypt by Eratosthenes about 250 B. C.

(Figure 1). At the time of the summer solstice at Syene he observed that the noon-day sun cast no shadow in the bottom of a well. Syene, which is the modern Aswan, lies very near the Tropic of Cancer, so that the midday sun on the longest day of the year is almost overhead.

At Alexandria some years later, the same day in June, Eratosthenes found that the noonday sun was 70 12' or 1/50 of a circle south of the zenith, so the difference of their latitudes is 1/50 of the earth's circumference. Since Alexandria is