To know the exact size and shape of the earth has long been a goal of scientists. Man's attempts to locate himself and to determine the configuration of his planet date from at least the time of Eratosthenes, about 200 BC. He observed that on a particular day in mid summer the sun shone directly down a well in Syene, Egypt (thus the zenith angle=0°). Supposing the earth to be a sphere, he measured the zenith angle on the same day to be 7.2° at Alexandria. His calculation of the distance between the points, based on the travel time of a camel caravan between them and on the assumption that both were on the same meridian, gave him enough information to compute the earth's circumference: 7.2°/360° = l/50. Therefore, the distance between Syene and Alexandria was 1/50th of the earth's circumference. His calculations were about 16 per cent too large. Considering, however, the primitiveness of his measurements and assumptions, his value was remarkably close. Two later determinations were made using this “astrogeodetic” principle, but with little increase in accuracy.