Comets: A Chronological History of Observation, Science, by Donald K. Yeomans

By Donald K. Yeomans

Incomplete dossier: pages 141-358, 474-496 are passed over

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However, these predictions can only be made over a finite period into the future. If there is any uncertainty about the current positions of the planets—which there inevitably is—this uncertainty grows over time as the simulations progress. As the positions of the planets become more uncertain the further one looks into the future, the perturbations which they exert on one another also become more uncertain. This triggers a snowball effect—because we don’t know exactly where the planets will be, we also don’t know exactly what forces they will exert on one another—and at some point in the future this causes the simulations to rapidly lose all certainty of where the planets will lie along their orbits.

After two false starts—a group at MIT erroneously believed it had bounced radio waves off Venus in 1958, and Jodrell Bank made a similar claim in 1959—the first unambiguous detection of such reflections was by the Jet Propulsion Laboratory (JPL) on March 10, 1961. The Space Age 21 The work of making such measurements, feeding them into computer simulations of how the planets move under gravity, and using these simulations to determine the paths of the planets, has remained a staple part of the work of JPL since.

The Earth’s surface is slightly higher around the equator, and lower at the poles, by about 20 km. In other words, the Earth is shaped like an M&M—a shape technically called an oblate spheroid—with a bulge around the equator. This spheroidal shape is a result of the Earth’s rotation: material on the equator is revolving at high speed around the Earth’s center, and feels an outward centrifugal force—rather like a car perpetually turning around a corner. The material at the two poles, meanwhile, moves no distance at all as the Earth rotates.

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