DIAGRAM TO ILLUSTRATE NEWTON S LAW OF GRAVITATION 



(E represents the earth and A the moon. Were the earth's pull on the moon to 

 cease, the moon's inertia would cause it to take the tangential course, AB. On the 

 other hand, were the moon's motion to be stopped for an instant, the moon would 

 fall directly towards the earth, along the line AD. The moon's actual orbit, result- 

 ing from these component forces, is AC. Let AC represent the actual flight of the 

 moon in one minute. Then BC, which is obviously equal to AD, represents the dis- 

 tance which the moon virtually falls towards the earth in one minute. Actual 

 computation, based on measurements of the moon's orbit, showed this distance to be 

 about fifteen feet. Another computation showed that this is the distance that the 

 moon would fall towards the earth under the influence of gravity, on the supposi- 

 tion that the force of gravity decreases inversely with the square of the distance; 

 the basis of comparison being furnished by falling bodies at the surface of the earth. 

 Theory and observations thus coinciding, Newton was justified in declaring that the 

 force that pulls the moon towards the earth and keeps it in its orbit . is the familiar 

 force of gravity, and that this varies inversely as the square of the distance.) 



