The moon - Like any other celestial object are bounded by gravity. Gravitational attractive force is pulling moon closer to earth but this force is countered and balanced by the outward rotational (centrifugal) force which results an orbit.
Because of the three immutable laws, Newton is able to synthesise a new equation that describes the motions of planet. This is what we affectionately call “Newton’s Universal Law of Gravitation”. This law is powerful because it is so short – barely five characters in the equation. Enough to predict how much force it needed to propel a rocket to space, how much force pulls us on the surface of the earth, how much force the moon need to stay in orbit. In fact, Since the discovery, the equation is still being used now by all space agencies to launch anything into orbit! Here’s the nasty bit:
Here’s how it works. The F which means the attractive force between two bodies in space is equal to the product of mass 1 (object A in space) and mass 2 (object B in space) divided by the square of distance between them. Do not mind the G. It’s a constant which has a fixed value. What this means is that for any two objects in space, now there’s a formula to apply and we get the value for the forces between them. The motions of everything in this universe became calculable.
Newton’s law stated that gravitation is always attractive. That is bodies in space will always attract each other no matter what they are made of. This has a logical flaw which is pointed out by Richard Bentley, another brilliant thinker at his time, sending a letter to Newton. That innocent looking letter packed with such a load that blows Newton back thinking about this: “If all things in this universe are attractive in nature, shouldn’t the stars all come onto each other crashing? The universe is unstable.”
In reply towards Bentley’s letter at 1691, Newton argued that such case will only happen if there are finite number of stars and finite volume of space between them. So, he proposed that in the universe, there should be infinite stars separating from each other in equal distances and space is infinite in volume. Also, God has to intervene once in a while to nudge the stars so that they will not move towards one another destroying the stability of the universe.
Newton’s law is elegant, in the sense that the law is simple yet it could predict the motions of stuff in our experiences. Take a stone and throw for example. Using Newton’s law (knowing the mass of the stone) we could predict how high the stone will fly and with a computer, we can draw a path of its precise trajectory. Ironically, it was precision that render Newton’s law at describing some astronomical observations, wrong.