6. GALILEO AND THE TELESCOPE--NOTIONS OF GRAVITY BY HORROCKS, ETC.


It is now necessary to leave the subject of dynamical astronomy for a
short time in order to give some account of work in a different
direction originated by a contemporary of Kepler's, his senior in fact
by seven years. Galileo Galilei was born at Pisa in 1564. The most
scientific part of his work dealt with terrestrial dynamics; but one
of those fortunate chances which happen only to really great men put
him in the way of originating a new branch of astronomy.

The laws of motion had not been correctly defined. The only man of
Galileo's time who seems to have worked successfully in the same
direction as himself was that Admirable Crichton of the Italians,
Leonardo da Vinci. Galileo cleared the ground. It had always been
noticed that things tend to come to rest; a ball rolled on the ground,
a boat moved on the water, a shot fired in the air. Galileo realised
that in all of these cases a resisting force acts to stop the motion,
and he was the first to arrive at the not very obvious law that the
motion of a body will never stop, nor vary its speed, nor change its
direction, except by the action of some force.

It is not very obvious that a light body and a heavy one fall at the
same speed (except for the resistance of the air). Galileo proved this
on paper, but to convince the world he had to experiment from the
leaning tower of Pisa.

At an early age he discovered the principle of isochronism of the
pendulum, which, in the hands of Huyghens in the middle of the
seventeenth century, led to the invention of the pendulum clock,