26 THE FOUNDATIONS OF SCIENCE 



And now, let us return to Galileo's great discovery 

 that, left to itself, a body moves on in a straight 

 line with uniform velocity. The need for vortices 

 to maintain the motion of the planets was thus done 

 away, and it became clear that what cried for ex- 

 planation was not the continued motion, but the 

 continual deviation from the straight course involved 

 in circular or elliptical orbits. Some force must be 

 acting, drawing the planets towards the centre. 



Isaac Newton (1642-1727), a young graduate of 

 Trinity College, Cambridge, had returned for a time 

 to his home in Lincolnshire, driven out of Cambridge 

 by the Great Plague of 1665. In this retirement he 

 set himself to apply Galileo's dynamical principles 

 to known astronomical events. At that time the 

 Ptolemaic system of astronomy, which made the 

 earth the centre of all things, had recently been 

 given up by all competent philosophers in favour of 

 Copernicus' (1473-1543) revival of the theory that the 

 sun is the centre of our system. Tycho Brahe (1546- 

 160i) had collected an immense number of observa- 

 tions on the motions of the planets, and, by a life- 

 long study of his data, John Kepler (1571-1630) had 

 formulated laws which described the motions: laws 

 such as the statement that the planets describe 

 ellipses with the sun in one focus. 



Taking the simplest case, the revolution of the 

 moon in a nearly circular orbit round the earth, it 

 occurred to Newton that, as the moon moves in a 



