Refining the Earth's Pear Shape

QUITE soon after the launching of the first artificial satellites, analysis of their orbits led to the reshaping of the world. The orbital plane of Sputnik 2 rotated less rapidly than expected, indicating¹ that the polar flattening was about 0.35% less (the polar diameter about 150 m greater) than had been thought. The variations in the perigee height of Vanguard 1, as perigee moved successfully north and south of the equator, showed² that the polar section of the geoid-the profile of the sealevel surface revealed by a cut through the poles-was not symmetrical about the equator but had a slight pear-shape tendency, with the stem at the North Pole. Floating at sealevel at the North Pole, one would be about 40 m further from the equator than an equally undauntable explorer who bored down to sealevel at the South Pole.