Great Minds in Astronomy - Week 5 Notes - ASTR 1345
Great Minds in Astronomy - Week 5 Notes - ASTR 1345 ASTR 1345-004
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This 5 page Class Notes was uploaded by Sera (Notetaker) on Saturday October 1, 2016. The Class Notes belongs to ASTR 1345-004 at University of Texas at Arlington taught by James J Davis in Fall 2016. Since its upload, it has received 7 views.
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Date Created: 10/01/16
Note taker: Sera ASTR 1345 Great Minds in Astronomy Ptolemy (90AD-168AD) o Greek astronomer and geographer o Mathematician who sought to rectify the Aristotelian universe model; wanted to make the model fit observed reality o “Mathematical Syntaxis” Also known as “The Almagest” and “The Greatest” Compiled the world’s astronomical knowledge o Astronomy for the next 1400 years would be based on the work of Ptolemy o Supported and strengthened the geocentric model o The Ptolemaic Universe Realized Aristotle’s Universe did not follow observations The Equant – Earth is slightly offset from the center of the universe Planets Planets is Greek for “wanderer” 7 known moving objects: Sun, moon, and the 5 planets up to Saturn (Earth was not included in this as it was not considered a planet) Retrograde Motion – when a planet moves Eastward with respect to the stars, stops, and reverses redirection, repeating this several times throughout its orbit o All planets do this at different rates Retrograde motion --> The Ptolemaic Cycle explained retrograde motion in planets with epicycles o An epicycle is an orbit on top of an orbit o Ptolemy kept adding epicycles onto Mars’ orbit in order to explain its motion in the sky every few years – in the end Mars had 37 epicycles Other difficulties with models of the universe o The evening/morning stars (Mercury and Venus) o Mercury and Venus were never visible late after sunset or long before sunrise Note taker: Sera ASTR 1345 The Astronomy Revolution The Aristotelian Universe o First embraced out of respect for Aristotle o Later embraced out of religious dogma With the birth of universities, new models of the universe began forming “Science” o Falsifiability – must be able to propose a contrary case, to prove a theory wrong with experiment Nicolaus Copernicus (1973-1543) o The first “Astronomer” o Polish mathematician o Proponent of a heliocentric theory It eliminated complicated epicycles Explained retrograde motion, that it occurs from the Earth orbiting the sun while other planets orbit the sun as well, and since Earth orbits faster than superior planets, we are constantly catching up to them in their orbit, creating retrograde motion o Explained inferior/superior planets (why Mercury and Venus were always near the sun) Tycho Brahe (1546-1601) o Dutch Astronomer o Greatest naked-eye astronomer o Measured star and planets positions with great position using parallax – the apparent motion of distant objects due to an observer’s movement o Theorized that Earth was the center of the universe, the Sun orbited the Earth while everything else orbited the Sun Johannes Kepler (1571-1630) o Found an interest in Astronomy while in theology school o Worked under Tycho Brahe o Argued for the Copernican model of the Universe o Figured out that the orbits of planets were elliptical and not perfect circle Brahe’s Legacy Measured relative distances to planets Jupiter was 5.2x further from the Sun than the Earth Mars was 1.5x further away Proved there was no celestial spheres Showed “guest stars” were very far away (not objects in the atmosphere) meaning the heavens could change Earth’s atmosphere bends light Note taker: Sera ASTR 1345 Taming Mars: Johannes Kepler Mars had a notoriously hard to predict orbit due to retrograde motion Kepler stole Tycho’s data logs upon his death Kepler realized that the observations of Mars’ orbit did not fit within a circle, but did fit within an ellipse An ellipse is a flattened circle with two centers o The “flatness” of an ellipse is called the eccentricity o The centers are called foci A New Era End of Greek Astronomy, start of Astronomy of science Idea of celestial spheres had been abolished Belief in geocentrism was waning Realization that the sun somehow controlled the planets Sun remained stationary, did not orbit any body Planets orbited faster when they were nearer to the sun Kepler’s 3 Laws 1 Law o The orbit of a body about the Sun (or any star) is an ellipse with the Sun’s center of mass at one focus o Circular orbits are extremely rare o Larger planets have eccentricities very close to zero (near circular orbits) 2 Law o A line joining a planet and the Sun sweeps out equal areas in equal time internets 3 Law o The square of any planet’s period is proportional to the cube of its orbital radius o Radius = Period 2 o In other words, the amount of time it takes to orbit the sun is effected by how close you are to the sun Galileo Galilei (1564-1642) o Pioneer in the field of observational astronomy o Pioneer in experimental physics o The DeMedician Stars While observing Jupiter through a telescope, he noticed the presence of 4 “persistent stars” that were always present and changing position around Jupiter (they were later realized to be moons, renamed the Galilean moons) Noticed they followed Kepler’s laws and had very short orbital periods o Mountains of the Moon Note taker: Sera ASTR 1345 Disproved Aristotle’s claim that the Moon is a perfect celestial object His observations implied that the Moon is simply a rock, just like Earth o Phases of Venus Recorded full, gibbous, and crescent Venus Noticed that Venus sometimes appears larger, sometimes smaller Proof that Venus was an interior planet which orbited, the sun, not Earth o Sunspots Galileo observed sunspots on the sun These are “cooler” regions of the sun created by magnetic fields pushing gas away Proved that the sun was not perfect and did change Sunspots act on an 11-year cycle Isaac Newton (1643-1727) o Mathematical interests Created calculus which allowed him to solve problems with multiple changing variables o Physics interests Motion – Why does motion happen? Gravity – How does the sun affect all of the planets? Optics o The Question of Gravity Astronomers argued whether gravity could hold the planets in elliptical orbits Newton provided compelling and thorough proofs of gravity’s interactions in his text “Principia” Newton’s Laws of Motion First coherent scientific description of motion in recorded history Acceleration is a force, motion is caused by forces These laws were universal and deterministic o Could make useful predictions anywhere Critical velocity o Newton theorized that a projectile with enough energy (velocity) would fall toward the Earth at a rate at least matching the curvature of the Earth (aka orbit the Earth) Gravity o Gravity is universal o Gravity attracts objects towards the center of mass o Objects in orbit around one another orbit a common barycenter – the center of mass of the system of two objects o Gravity is mutual: gravitationally bound objects exert the same force on one another Note taker: Sera ASTR 1345 o The strength of gravity depends on the mass of the objects and the distance between the obj2cts o F = G M M 1 r 2 F = force of gravity G = Gravitational constant M 1 mass of object 1 M 2 mass of object 2 R = separation distance Mass & Weight o Mass – the number of particles contained in an object o Weight – the force of gravity felt by an object st Newton’s 1 Law o An object in motion stays in motion and an object at rest in rest stays at rest unless acted on by an outside force o Law of Inertia Newton’s 2 Law o The acceleration of an object is proportional to the force acting on it o F = ma F = force M = mass A = acceleration rd Newton’s 3 Law o For every action there is an equal and opposite reaction from the object being acted on Zero G o This is being weightless, but not free of gravity (aka freefall)