Gravity is the force of attraction that occurs between all objects because of their mass.  Because of gravitation, objects near or on Earth are attracted to it. Gravitation also keeps planets, and stars in orbit around their center.  Although the effects of gravity is easy to see, an explanation was quite hard to find .  The ancient Greek philosopher Aristotle taught that heavy objects drop  faster then light ones. But in the 1600`s Galileo  came up with the correct view of gravity.  According to Galileo, all objects fall at the same speed unless air resistance or some other force slows it down.  When famous British scientist Sir Isaac Newton was 23 years old he was sitting by a tree a falling hit him in the head.  Newton started to question how far the force of gravity reaches.  He realized that the force that pulled the apple to the floor is responsible for keeping the moon in orbit around the Earth.  From laws discovered by Johannes Keppler, Newton calculated what the force is that attracts the moon towards the Earth.  This force proved to be the same that gave the apple its acceleration.  Although all objects fall at the same time, the bigger the planet is, the faster things fall and the heavier.  Although the smaller the planet is, the lighter everything is,  and the slower everything falls.