Is the same force that pulled the apple to the ground was tugging on the moon.
But the moon is far away and its reasoned that the force weaker there. It much strong enough to bend the moons motion into nearly circular path around the earth. That apple just fell but its in the earth.
oh wait a single force explained a falling apple on earth and the movements of heavenly bodies as well. Well then this is the force of Gravity.
In 1687 he published a book about gravity. The book was known as the Principia.