Men have always been curious to know the shape and size of the planet upon which they live. Most people have heard of the experiment by which Eratosthenes determined the circumference of the Earth, and the principle of his method underlies all subsequent determinations. What he did amounted to finding the difference of astronomical latitude between two places and measuring their distance apart in terms of a standard length (Fig. I). Astronomical latitude may be denned here as the complement of the angle between the vertical at the point in question and the direction of the Earth's axis.