Newton, Gravity, Relativity, 13 September 2013!After Kepler solved the motion of the planets in Astronomia Nova(1609), the 1.new question that occupied western scholars was why the planets moved as they did. !Everyone thought the sun had some power that made everything move A.around it.!One incorrect proposal that amounted to nothing was that the sun's a.brightness somehow kept the planets orbiting around it. !A couple decades later, Isaac Newton would hit upon the real reason.!b.(1642-1727) Isaac Newton !2.Background: !A.Born 1642 in a small manner house in central England. !a.Though he was premature, he survived to display great intelligence as a b.child. !His intelligence earned him a scholarship to cambridge, where he aced c.all of his classes and was immediately hired back by the university as an assistant professor (called a "tutor").!When Newton was 24, he was forced to return to his family home in B.Lincolnshire, where he figured out that the motion of the planets was due to gravity. !Cambridge University was temporarily suspended due to an outbreak of a.Black Plague (which people knew hit crowded areas like cities the hardest). !All of the people of Cambridge were told to disperse into the 1.country until the plague died down. !Also in the same year (1666): Great Fire of London and the English 2.Civil War (between Cromwell and the House of Stuart).!Newton had a lot of free time, so used it to think about the major b.scholarly issues of the day, such as why the planets move the way they do.!One day, while contemplating why the moon orbited the earth, an 1.apple from the tree he was under fell (to the ground, not on him).!This gave Newton the idea that the force that pulled the apple to A.the ground was the same force that kept the moon from moving away from the earth.!He called the force "gravity," and then figured out how gravity a.worked. !Having solved a major issue for scholars of those days, Newton 2.amazingly did not publish it or even tell anyone.!No historian or scientist one has competently explained why.!A.Some suggestions: !B.It was perhaps to spite by Royal Society group that had just a.formed in London, who Newton didn't like.!During this time Newton also did some work on optics, mechanics. 3.and theology.!After a year and a half, Cambridge resumed and Newton had a glorious c.mathematical career.!Meanwhile, scholars were still trying to figure out the planets. !1. Almost 20 years later, the secretary of the royal society finally convinced C.Newton to publish his ideas in Philosophiae Naturalis Pincipia Mathematica (Principia) in 1687.!Edmond Halley, the secretary of the royal society, paid to publish the a.book personally since the royal society was broke.!After the scholars of Europe managed to pick through Newton's turgid b.Latin prose, they realized that it had the answer to the big problem of the era.!Even though there have been changes since then, Newton's work is 1.the basis for almost all research on the motions of everything in the universe. !You need to know this!!A.Everyone agrees this is the most important book in the history of c.science: !There are other gems in this book about how nature works, but the 1.idea of gravity is especially important to astronomy. !"Gravity is boss": A great deal of astronomy is gravity's story, ie the A.consequences of gravity.!If you don't understand gravity, this course won't make sense!B.The fact that you could use his Principia to understand how planets 2.moved and all of his other laws of motion, really impressed all of scholarly Europe.!It inspired other scientists to find formulas that mathematically A.represent the regularities of nature in their fields. !This is why math is the language of science today. !a.Isaac Newton's Principia: !D.The force of gravity between two objects is:!a. directly proportional to the product of their masses, and !1. inversely proportional to the square of the distance between them!2. F = G m1m2 / r^2 !3.in convention, m1 is the larger mass and m2 is the smaller mass!A.Orbits:!b.Illustration of Orbits: !1.Imagine an extremely tall mountain on earth with an extremley A.powerful cannon on top. If a cannon ball is shot sideways from the top of the mountain with enough force, the cannon ball will enter a circular orbit around earth. !With enough force, the cannon ball will fall around the earth a.without hitting the earth. The cannon ball has enough momentum to not hit the earth, but not enough to go straight out into space.!With more force, the cannon ball can enter an elliptical orbit b.around the earth.!With more force, the cannon ball can escape the trajectory c.altogether (and head off into space). !Rule: the amount of momentum an object has determines its orbit:!2.Without achieving orbital velocity, the object will fall back down A.and intersect the earth again. !With enough energy, the object will go into orbit. !B.With even more energy, the object can escape the trajectory of C.earth, and even the sun. !Five human made space probes have that much momentum. !a.If you start an object orbiting orbit another way (not a cannon D.from the earth), objects can settle into orbits. and not everything has been learned for how that happens !Because gravity works as an inverse square law (1/r^2), the pull of gravity c.decreases spectacularly with distance.!It is a highly localized effect: it works a lot on near by things, but you 1.cannot detect it from many light years away!Newton explained planetary motion as accurately as anyone needed for at E.least 200 years (until the1800s).!Nothing in that time was observed that contradicted Newton's ideas in a.any manner !NOTE: The fact that there was a perfect explanation for everything seen b.in the sky, made the term "laws of planetary motion" an oxymoron: !Recall that "planet" comes from the greek word for "wanderers," so 1.named because their motion could not be predicted by any laws.!Now, they are no longer technically wandering, because scientists A.could predict their motion. !Keeping the name planet is an example of Astronomy holding onto 2.the first name of something, even as more is learned about that something which proves the name inaccurate. !This happens because objects and phenomena are often given a A.preliminary label based on astronomer's first impressions of them, which can be wrong.!This is the root of many problems with terms in astronomy, and B.also other fields in science. !For example: Douglas Firs are not
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