The Astronomical Unit

In 1767, at the age of 18, Pierre Simon Laplace was appointed as professor of mathematics in Paris, where he devoted the majority of his attention to a comprehensive study of the manner in which the planets perturb one another gravitationally, with a view to determining whether the observed arrangement of the Solar System was stable. He concluded that while the eccentricities of the individual orbits would vary over time, the system would adjust to compensate, and was therefore stable. In addition, using Newton's law of gravitation, he was able to use the magnitude of these perturbations to calculate that the Earth was about 150 million kilometres from the Sun - a value that was designated the Astronomical Unit (AU) because, when combined with Kepler's empirical laws of orbital motion, it gave the basis for a scale by which to measure the Solar System. Upon applying this scale, it became evident that Saturn orbits at the astonishingly large distance of 1.4 billion kilometres. The subtended angles indicated that the planet's diameter was of the order of 100,000 kilometres and the ring system was about 270,000 kilometres in diameter. Laplace published his collected researches in Celestial Mechanics in five instalments between 1799 and 1825.

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