Geodesy, in geophysics, specialty that seeks to determine the precise size and shape of the earth. The ancient Greeks attempted to measure the earth, and Eratosthenes (3rd century B.C.) obtained a very accurate figure for the circumference. Geodesy began, however, in the 18th century, when surveyors found that they needed accurate reference points. Geodetic work consists of refining previous estimates of the shape of the earth. The earth is approximately an oblate spheroid—that is, it bulges somewhat at the equator because of its rotation and is correspondingly somewhat flattened at the poles (by about 13 mi/ 21 km). But topographical irregularities cause the curvature of the earth to vary from point to point. Such bulgings and flattenings are noted by geodesists carrying out surveys over great arcs of the globe. Since 1957 satellites have provided information about the earth's shape.