Believe it or not for many centuries many people asked questions about why things fell to the ground and what keeps things like the moon in our night sky. Then a man by the name of Sir Isaac Newton came along with a novel idea in 1666. The idea was to imagine that the Earth's gravity influenced the Moon and all objects here on Earth. This was a very radical idea back in its day.

Sir Issac Newton was talked into writing a paper on his new physics ideas and how it related to astronomy. After about a year of working on this paper Newton published the Philosophiae naturalis principia mathematica or Principia as it is always known in 1687.

The Principia is looked at and stated to be one of the most important scientific papers ever written. Sir Isaac Newton's paper looked at the motion of objects in resisting and non-resisting situations under the influence of a centripetal forces. ( A centripetal force is a force that pulls toward the center of a mass.)

Newton's papers results were applied to orbiting bodies (aka planets), projectiles, or free-fall objects near the Earth. He demonstrated that the planets were attracted toward the Sun and then he generalized that all heavenly bodies (planets) and matter in general was mutually attract towards one another.

Gravity: The force of attraction between objects due to their mass.

The Law of Universal Gravitation:
All objects in the universe attract each other through gravitational forces. Thee size of the force depends on the masses of the objects and the distance between these objects.

Gravitational force is SMALL between objects that have a small mass	Gravitational force is LARGE between objects that have a large mass

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