Newton makes the simple calculation, and the result is … 9.8 metres per second per second! The same acceleration as in Galileo’s experiments for falling bodies on Earth. Coincidence? It can’t be, reasons Newton. If the effect is the same – a downwards acceleration of 9.8 metres per second per second – the cause must be the same. And so: the force which causes the little moon to turn around its orbit must be the same as that which causes objects to fall to the ground on Earth. We call the force causing objects to fall gravity.
