This is the full text of the inaugural lecture for the Jacksonian Chair at the University of Cambridge, given by A. H. Cook in 1973, with the subject of the Astronomer as Natural Philosopher.

Planets have excited the minds of man since prehistory. In our own time planetary science has become a rapidly developing area of astronomical research, as the instruments carried by spacecraft have vastly increased our knowledge of planetary surfaces and interiors. the rocky planets of the inner solar system bear countless craters, scars of their encounters with innumerable meteorites, although the active surface of the earth has contrived to erase these features from our own planet. The outer giants, particularly Jupiter, have vigorous atmospheres, while Io, a satellite of Jupiter, has...