A Brief History of Astronomy: From Ancient Greek Times to Modern Physics - Prof. Gregory B, Study notes of Astronomy

Download A Brief History of Astronomy: From Ancient Greek Times to Modern Physics - Prof. Gregory B and more Study notes Astronomy in PDF only on Docsity! 1 From Aristotle to Newton The history of our knowledge about the Solar System (and the universe to some extent) from ancient Greek times through to the beginnings of modern physics. Astronomy Picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html TU24 seen by the GBT Eratosthenes Determines the Size of the Earth in about 200 B.C. Syene Alexandria Sun's rays 7.2oS N Earth He knows the distance between the two cities is 5000 "stadia”, where 1 stadia = 6.25 km From geometry then, 7.2o 360o Earth's circumference 5000 stadia = => circumference is 250,000 stadia, or 40,000 km. So radius is: 40,000 km 2π = 6366 km (very close to modern value, 6378 km!) Clicker Question: Who was the first person to use a telescope to make astronomical discoveries? A: Aristotle B: Brahe C: Kepler D: Gallileo E: Newton Clicker Review: What time of day does the first quarter moon set? A: 6am B: noon C: 6pm D: midnight E: Never sets 2 "Geocentric Model" of the Solar System Aristotle vs. Aristarchus (3rd century B.C.) Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth. Ancient Greek astronomers knew of Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Aristotle: But there's no wind or parallax. Difficulty with Aristotle's "Geocentric" model: "Retrograde motion of the planets". Aristarchus: Used geometry of eclipses to show Sun bigger than Earth (and Moon smaller), so guessed that Earth orbits the Sun. Also guessed Earth spins on its axis once a day => apparent motion of stars. Aristarchus: Yes, sir Planets generally move in one direction relative to the stars, but sometimes they appear to loop back. This is "retrograde motion". But if you support geocentric model, you must attribute retrograde motion to actual motions of planets, leading to loops called “epicycles”. Ptolemy's geocentric model (A.D. 140) "Heliocentric" Model ● Rediscovered by Copernicus in 16th century. ● Put Sun at the center of everything. ● Much simpler. Almost got rid of epicycles. ● But orbits circular in his model. In reality, they’re elliptical, so it didn’t fit the data well. ● Not generally accepted at the time. Copernicus 1473-1543 Illustration from Copernicus' work showing heliocentric model. Copernican model was a triumph of the Scientific Method Scientific Method: a) Make high quality observations of some natural phenomenon b) Come up with a theory that explains the observations c) Use the theory to predict future behavior d) Make further observations to test the theory e) Refine the theory, or if it no longer works, make a new one - Occam’s Razor: Simpler Theories are better -You can prove a theory WRONG but not RIGHT Observation TheoryPrediction 5 Newton's Second Law of Motion When a force, F, acts on an object with a mass, m, it produces an acceleration, a, equal to the force divided by the mass. a = Fm or F = ma acceleration is a change in velocity or a change in direction of velocity. Newton's Second Law of Motion Demo - Measuring Force and Acceleration Newton's Third Law of Motion To every action there is an equal and opposite reaction. Or, when one object exerts a force on a second object, the second exerts an equal and opposite force on first. Newton's Third Law of Motion DEMO: CART Newton's Law of Gravity For two objects of mass m1 and m2, separated by a distance R, the force of their gravitational attraction is given by: F = G m1 m2 R2 F is the gravitational force. G is the "gravitational constant". An example of an "inverse-square law". Your "weight" is just the gravitational force between the Earth and you. Newton's Correction to Kepler's First Law The orbit of a planet around the Sun has the common center of mass (instead of the Sun) at one focus. 6 Clicker Question: A flaw in Copernicus’s model for the solar system was: A: It didn’t explain retrograde motion. B: He used circular orbits. C: The Earth was still at the center. D: He used the same mass for all the planets. E: All of the above Clicker Question: Why didn’t my hand get crushed by the hammer? A: My bones are actually stronger than steel. B: The plate has a lot of inertia C: The plate is very strong D: The force of gravity kept the plate from moving Clicker Question: Suppose Matt weighs 120 lbs on his bathroom scale on Earth, how much will his scale read if he standing on a platform 6400 km high (1 Earth radius above sea-level)? A: 12 lbs B: 30 lbs C: 60 lbs D: 120 lbs E: 240 lbs Escape Velocity Velocity needed to completely escape the gravity of a planet. The stronger the gravity, the higher the escape velocity. Examples: Earth 11.2 km/s Jupiter 60 km/s Deimos (moon of Mars) 7 m/s = 15 miles/hour Consider Helium Gas at room temperature (300 K) E = kT = 4.1 x 10-14 erg E = 0.5 m v2 = 4.1 x 10-14 erg so v = 1 km/sec on average, but sometimes more Timelines of the Big Names Copernicus Galileo Brahe Kepler Newton 1473-1543 1546-1601 1564-1642 1571-1630 1642-1727