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i am bummed out if you dont answer
sry was looking up something have a look at this page http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html
basically kepler thought the center of the solar system was the sun...but it's not that simple so kepler modified his 3rd law to say that the sun is at a focal point of an ellipse in which all planets orbit He modified this law based on Newton's law of universal gravitation and it's explained in more detail on that page
gravity keeps the closest and farthest planets in orbit instead of them floating of into space
Here's the WHOLE story"It is said that one day Newton were siting under an apple tree when suddenly an apple fell on his head. Then Newton asked himself "This apple fell on my head, but why the Moon don't fall on my head?". It was the key question leading Newton to invention of the Universal Law of Gravitation. This is one version of the famous story about Newton and an apple that inspired him to the great invention. The question is whether the story is true. Many believe that it is only a good anecdote. However, according to the notes and memoirs of his contemporaries, it seems that Newton was really inspired by apples falling on the ground. His acquaintance William Skukeley in his memoirs describes an episode of tea drinking in the garden. Stukeley attributed the thinking on this occasion "why should that apple always descend perpendicularly to the ground,...why should it not go sideways, or upwards? but constantly to the Earth's center?" to Newton. (Source: Wikipedia, the article "Isaac Newton") Newton's assistant and husband of his niece also described the event in his writing, including Newton's idea that the force causing fall of an apple is the same force that cause Moon's motion around the Earth. Fig. 1. The story of Newton and an apple is inspiration for many. How to explain Kepler's laws of planetary motion? After a German astronomer Johannes Kepler (1571-1630) showed that planets move along an ellipse, a new problem appeared. Why planets move that way? Namely, in that time the circular motion was considered to be a natural movement. Even Kepler himself tried to answer this question. Kepler assumed that the Sun is a large magnet, and a planet under the influence of the Sun is moving around by magnetic forces. However, the theory had a huge deficiency. It was known that the Sun is a hot body, and that bodies lose its magnetic properties when they get hot. Then a French mathematician Rene Descartes (1596-1650) suggest another theory. Descartes proposed a model according to which the entire space is filled with subtle and invisible particles of fluid material. However, Newton later shows that this theory is not in line with Kepler's laws. Finally, Newton showed that gravitational interaction is the cause of planetary motion. This law, together with the three laws of motion he published in his monumental work "Philosphiae Naturalis Principia Mathematica". He actually solved this problem some twenty years earlier, but it was not published until 1687. Fig. 2. "Principia", one of the most famous scientific books ever. Newton waited for decades to publish his discoveries. A brief note on Isaac Newton Isaac Newton (1642-1727) enrolled in the study of mathematics at Trinity College, Cambridge. Already as a student he made spectacular achievements in mathematics, revealing the binomial theorem and calculus. The 1665th in Cambridge, there appeared smallpox and the university was closed. Newton is temporarily returned home to the country. During the one-year isolation Newton formulated his most important ideas in mechanics. He formed a clear idea of the first and second law of motion and gravity law. Summary - Newton's law of gravitation in the class Newton's law of universal gravitation The law of universal gravitation is a physical law derived from empirical observations. Newton was aware, from his first law of motion that some net force had to be acting on the Moon. Otherwise, the Moon would move straightforward instead along an elliptical orbit around the Earth. Newton's law of universal gravitation states that every two particles in the universe attract each other. That force is proportional to the product of their masses and inversely proportional to the square of the distance between them. More precisely, if masses of particles are m1 and m2 and r is the distance between them then equation in Figure 3 holds. In this relation G is the gravitational constant, whose value is G = 6.673 x 10 -11 Nm2/kg2 Found At:http://flysky.hubpages.com/hub/Newtons-Law-of-gravitation