The Invention of Gravitation Phenomenon

In the early of 1600's, a german mathematician and astronomer, Johanes kepler (1571 - 1630) was succeeded to analyze the astronomical data of the planets motion revolving the sun made by his teacher , Tycho Brahe. Based on result of analysis, Kepler proposed three laws about the planetary motion, those are as follows. 

The path of each planet revolving the sun is elipptical with the sun as the center lies on one of the focus point (Kepler's I law); an imaginary line connecting the center of the sun to that of the planets will sweep out the equal areas in equal time interval (Keppler II law); and the ratio of the square of two planets periods is equal to tehe ratio of their average distance from the sun cubes (Kepler III law). 

Altough those laws good enough to explain the planetary motion in the solar system, but they were still unable to explain how the planets can stay on their elliptical orbit when revolving the sun.

Around 1686, Sir Isaac Newton (1642 - 1727) the british physicist and mathematician, thought of the planets revolve the sun while staying on their elliptical orbits. beside that, he also thought of how the moon could stay on its circular orbit while revolving the earth. 

Based on his first law, Newton concluded that there must be some force acting on the moon and planets so that they could move with such path, because if there was no such force then they would move on a straight - line path. 

When Newton was thinking about the problem of attractive force on an object that seemed to have no relation with a force acting on the moon and planets, he observed that if an abject at a certain altitude above the earth surface released, then that object will always fall freely toward the earth surface. Newton conclude that there is an attractive force coming from from the center of the earth which acts on an object so that it will always fall to the center of the earth. this attractive force is called gravitational force. Based on tis observation, Newton tried to find why the moon can move inits circular path, by illustrating it using a cannon ball. 

If the cannon ball is shot horizontally from a very high mounth with no air resistance and there is no such thing as gravitation, then the cannon ball would travel a straight - line path. Since the gravitation exists, then it would move downward with straight - line path and eventually falls to the earth. 

Now if the cannon ball is shot horizontally again, yet with a greater speed, then it would travel a farther path from the earlier until it eventually falls on the earth. if there is a speed on the cannon ball so that it can revolve the earth following the curvature of the earth. then if the speed is increased then it whould revolve the earth without crushing each other and becomes a satelite orbit in circular motion. if  its speed is very great at the launch, then it whould orbit the earth on an elliptical path. 

Based on those illustrations, it was proved that gravitational force keeps the canon ball on its circular path, because if there is no gravitational force acting upon it, it would move according to the straight - line path.

The falling of the cannon ball to the earth because of gravitational force is an analogy of the moon motion orbiting  the earth. if the moon orbit can be analogized with the falling of the cannon ball, then it can also be analogized to the objects falling on the earth surface.

After finding that the earth gravitation has influence on the moon, Newton tried to compare the magnitude of the gravitational force attracting the moon and objects on the earth surface. From his observation, Newton concluded that it is the gravitational force that causing the moon to stays on its circular orbit when revolving the earth and planets revolve the sun while maintaining their elliptical paths.