Lecture 4 -- Newton and the Physics of Astronomy Henry Throop / University of Colorado 7-Jun-2000 ASTR 1110 !! Please keep in mind that these notes are a supplement -- there's a lot of information covered in class or in the textbook that is not in these notes !! Announcements ------------- Planetarium tomorrow. Class lunch afterwards. Reading -- class and book don't correlate 100%, but should know material. We will talk about almost all of it. HW #1 due today, now. Points off for late -- but better to understand & late than turn in bad HW. Today ----- Motions of the Sky -- what did the Greeks see, what could they think? Retrograde motion History of Astronomy -- quick tour of all the major players Ellipses Outside demo -- the heliocentric system explains it all Paradigm shifts and the history of Discovery --- Ellipses History of Astronomy -------------------- Aristotle -- 350 BC Started science, wanted symmetry & perfection. Want everything to be a sphere, to be smooth. But he didn't observe! He could have seen moon is not perfect! He could have seen that gracity didn't make heavy things fall faster! Aristarchus -- 300 BC Distance to Sun, stars (very far), sun=center from retrograde, 300 BC. Also got sizes from eclipses. Aristarchus' Parallax Demo: hands up in front, fingers, motions thru trees What is _logical_: motion of earth thru stars is like motion of person thru the trees: the patterns of trees _should_ shift. Unless the trees are far away, or motions are so small. Which is in fact the case. But, trees/stars being _so_ far away that their relative motions -- parallax -- was not conceivable. Use parallax all the time in car: speed of nearby grass vs. distant mountains, to estimate speed. Eratosthenes -- earth is round, 200 BC. Alexandria & Syene. Off by 1/50th of a circle, concluded 6700 km, not 6400 km. Also figured out prime #'s Ptolemy -- 150 AD Put together a decent book (almagest = greatest compilation, stolen by arabs from alexandria library) which defined most of astronomy with a working model for 1300 years. Explained all the basic motions, but used a complex epicycle method. Also, sun was not at center, but earth was. Concept of a celestial sphere! Hipparchus -- 130 BC Copernicus -- 1500 AD Proposed complex method as Ptolemy's, but with sun at center. Not terribly accurate, not terribly simple, but was a start at a competing theory. Thought everything was circles! Tycho Brahe -- 1600, Danish observer Golden nose (duel over mathematics) Died at king's Died not knowing whether his incredible observations (no telescopes yet) could be used for some good or not! Essentially a lab technician -- think of these people now as eccentrics. But it was only with his detailed observations that... Only measured _position_ of stars & planets & moon; not shapes or sizes or colors. Kepler -- 1600 Ellipses! A slight difference from an epicycle, a different shape. But, he could explain everything perfectly! Didn't want to -- wanted to stick to Harmony of the Spheres, and circles. But believed Brahe's observations, and simply couldn't fit them. Came up with three laws: 1. Each planet moves in an ellipse, with sun at one focus 2. Line between sun and planet sweeps out equal area in equal time 3. A^3 = p^2, where A = `distance' (AU), & P = period (years) Galileo - 1610 Used telescope to show that some things orbited around something other than Earth Saw the individual planets as their own worlds! Total non-earth-centricist! Built telescopes (didn't invent them) Discovered Jupiter had moons Found Saturn's ring was a ring Figured out that the `milky way' was just more stars All in about two months' time Gaileo ------ The title page of Sidereus Nuncius served, to a certain extent, as an advertisement and was directed to Grand Duke Cosimo. The text of the title page demonstrates Galileo's skills as a self-publicist: SIDEREAL MESSENGER unfolding great and very wonderful sights and displaying to the gaze of everyone, but especially philosophers and astronomers, the things that were observed by GALILEO GALILEI, Florentine patrician and public mathematician of the University of Padua, with the help of a spyglass lately devised by him, about the face of the Moon, countless fixed stars, the Milky Way, nebulous stars, but especially about four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness; which, unknown by anyone until this day, the first author detected recently and decided to name MEDICEAN STARS. "Most Serene Prince. Galileo Galilei most humbly prostrates himself before Your Highness, watching carefully, and with all spirit of willingness, not only to satisfy what concerns the reading of mathematics in the study of Padua, but to write of having decided to present to Your Highness a telescope ("Occhiale") that will be a great help in maritime and land enterprises. I assure you I shall keep this new invention a great secret and show it only to Your Highness. The telescope was made for the most accurate study of distances. This telescope has the advantage of discovering the ships of the enemy two hours before they can be seen with the natural vision and to distinguish the number and quality of the ships and to judge their strength and be ready to chase them, to fight them, or to flee from them; or, in the open country to see all details and to distinguish every movement and preparation." -- Galileo to Prince of Venice Mar 1610 Newton -- 1700 Wasn't satisfied with describing _how_ it worked, but wanted to know _why_ it worked that way. Developed calculus Developed differential equations Developed most of optics Developed most of physics Developed laws of motion ('Newton's laws') Figured out gravity Applied that all to the planets Decided that Moon was not a `jewel', orbits didn't have to be round, Sun had `spots' on it, etc. and thus a `messy', ungeometric system was ok. Simple, but ugly. Squashed Aristotle (as did Galileo) Current day -- 2000 Structure of solar system & fundamental laws of physics have not changed. Few minor changes because of relativity Mercury's position -- relativity says that it's different if we have high speeds, strong gravity. Mercury has both these -- but it's an error that took several 100 years to detect & fix. 43 arcseconds per century -- after 10,000 years, mercury's position would be wrong by one degree. Concl ----- Took a long time for the ladder to get started -- but once they started climbing with Copernicus, the rooftop was in sight with ease, and the earth-centered (geocentric) world view was inevitably lost forever. It was a rapid series of discoveries. Not over yet -- we don't know _why_ gravity works, for instance -- but we have the large-scale structure down well. Perhaps we will be laughed at 1000 years in future. Q: At every point, they thought they 'knew 99% of it'! Ptolemy's model was good but complex. Copernicus' was better but didn't fit the orbits quite right. Newton/Galileo didn't match mercury right. We can't explain why gravity works, or what happens beyond / before start of universe. Hubble Einstein Hawking Demos: model solar system: retrograde motion Goal of demos: Explain most of the same concepts that Greeks saw: o Stars rising & setting o Moon rising & setting o Not yet: ecplises, moon phases o Venus: Morning / Evening star o Other planets: up all night o Slow shift of constellations over period of year Do some more group work: color cards VG's ---- Pictures of epicycles Pictures of Ptolemaic model, earth at center Diagram of retrograde motion Siderius Nuncius Letter regarding telescope Ellipses