Number 75 (Story #2), April 10, 1992 by Phillip F. Schewe and Ben Stein EARTH'S INNER CORE OSCILLATES SLIGHTLY , according to new sensitive gravity measurements by Douglas E. Smylie at York University in Ontario and by scientists at other labs. Smylie's gravimeter used superconducting magnets to levitate a small niobium ball, whose motion, sensitive to the tug of a slightly changing gravitational field, provides a measure of possible pendulum-like oscillations of the Earth's solid core within the fluid medium of the outer core. Using his own data and data from gravimeters in Brussels, Frankfurt, and Strasbourg, Smylie has deduced three translational modes for the inner core with periods of 3.58, 3.77, and 4.01 hours. Smylie has also used this information to calculate a density of 12.96 g/cm3 and a radius of 1221 km for the Earth's core. The gravimeters, it should be noted, can measure changes in gravity down to the level of a nanoGal (10-9 Gal, where Gal is the unit of acceleration, 1 cm/sec2, named after Galileo). In these units the gravity at the Earth's surface is about 1000 Gal. Thus the gravimeter can measure the difference in gravity brought about by lifting the device up by a distance equal the width of a human hair. (Science, 27 Mar. 1992)