## Strength and change of movement

• Astronomy and mechanics
Newton made his greatest discoveries in the astronomy. Galileo had stated that a body, once driven, moves unaccelerated; a change in the state of motion only results when a new force acts. The gravitational pull of the sun seemed to guide the planets in their orbit; an opposing centrifugal force seemed to prevent them from falling against the sun. From the third law of Kepler, which relates the orbital times of the planets to their solar distances, Newton concluded that the attraction of the sun decreases with the square of the distance.
"Inferring further from this, I compared the force required to keep the moon in its orbit with the force of gravity on the earth's surface and found that they are almost exactly the same."
So Newton suspected that the attraction that causes the bodies to fall reaches as far as the moon and constantly distracts it from the linear movement. Since the moon is at a distance of 60 earth radii, the gravity there should be 3600 times weaker than on the earth's surface. Its second "fall distance" to earth could be calculated from the size of the orbit and the orbit of the moon. The value obtained times 3600 should correspond to the path that a body falls through in the first second on the earth's surface. The calculation was too small a number; it seemed to Descartes' whirl to stop the falling of the moon.

In 1682 Newton learned that the earth was larger than had previously been assumed. A degree measurement of Jean Picard in France gave the new value. The moon, which is 60 earth radii away, was thus further away, the orbit longer, the second fall distance greater. Now the comparison with the falling space of bodies on earth was correct; the eddies separated and the movements of the planets and moons could be recorded mathematically. Newton completed his main work within a very short time 'Philosophiae naturalis principia mathematica' (Mathematical foundations of natural science). Through further intensive thought work he suddenly realized - after the Legend of looking at the falling apples - that to calculate it one must think of the gravitational pull of the earth's mass as being united in its center. From there, it acts along the straight line connecting to the center of another sphere, e.g. the moon, and vice versa. The forces must be in proportion to the masses.

Newton's handwriting was available at the meeting of the Royal Society on April 28, 1686. Astronomer Edmond Halley took care of the organization and cost of printing. He had been in contact with Newton for a long time and knew that tasks had been solved here that he and others had tried in vain. The work consists of three parts. The first two deal with, from the Newtonian axioms starting out, mathematically the movement of the body, while the third explains the structure of the world in a more generally understandable way. That's how it is there Principle of general gravity explained, according to which two bodies attract each other in proportion to their masses and in the inverse proportion of their distance. Not just sun and planet or planet and moon, but also sun and moon or planet and planet. The inequalities of the lunar orbit and the orbital disturbances of the planets result from changing mutual positions.

Newton distinguished them Concepts of mass and weight. At the surface of the earth, the centrifugal force of the rotating planet counteracts the attraction. The same stone must be heavier at the North Pole than at the equator. The weight is reduced by the centrifugal force; the mass remains the same. The earth should be flattened at the poles.