The Satellites of Jupiter, from Galileo to Bradley

  • John North


The satellites of Jupiter are hardly to the forefront of the typical astronomer’s consciousness, despite the remarkable findings of the Voyager mission. Even historians of astronomy tend to pass over the satellites in silence, perhaps thinking of them as nothing more than a trivial extension of the solar system—and paralleled by roughly comparable systems of moons around Mars, Saturn, Uranus, and Neptune. In the seventeenth century, however, they carried a cosmological message of great importance, for they were first seen at a time when the old and the new world systems were contending for the favor, not merely of astronomers, but of a significant fraction of the educated world. They were seen by Galileo in 1610. In 1676 Ole Rømer made use of them to show that light takes time to travel. My account runs for roughly half a century beyond this date, stopping at Bradley because, as I hope to explain, he marks the end of the first, largely empirical, phase of investigation of the satellites. I do not mean by this that no further empirical work was done—on the contrary, the most dedicated work of this sort was still in the future. Bradley’s proof of the aberration of light nevertheless clinched the argument for the finite velocity of light, at least in the eyes of reasonable men. He was one of the first to allow for the velocity of light in tables of the four satellites then recognized, and he it was who first saw that the inequalities in their motions are interconnected—and thus possibly a consequence of gravitational interactions. In a sense, therefore, he opened the way to the theoretical studies of this problem by Euler, Bailly, Lagrange, Laplace, and others.


Satellite Motion Finite Velocity Outer Satellite Satellite Problem Apsidal Motion 
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Copyright information

© Plenum Press, New York 1983

Authors and Affiliations

  • John North
    • 1
  1. 1.University of OxfordOxfordEngland

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