1. Evolution of the Solar System Nebular hypothesis, p 10 a. Cloud of atoms, mostly hydrogen and helium b. Gravitational collapse contracted it into rotating disc c. Heat of conversion of gravitational to thermal energy fired Sun into star in center of nebular cloud d. Cooling allowed condensation of rocky particles in inner solar system, that collided to become planets accretion i. Their accretion created gravitational heat of contraction warm but fairly small ii. Unable to attract much lighter elements e. Outer nebular cloud stayed cool i. Ices of methane, carbon dioxide, ammonia and water ii. Cooler at greater distances from Sun, 1. these solid particles collided to become outer planets. 2. Large quantities existing there allowed these planets to become enormous, and attract and hold lighter elements 2. Big Bang happened before all of this a. created the universe, the early galaxies, and the first-generation stars b. Remember, Sun is a second or third generation star the nebular cloud of dust and gases was created by a supernova of a preexisting star 3. Earth Structure a. Compositionally: Crust, Mantle, Core i. Most of Earth is mantle rocky shell 2900 km thick ii. Crust has less iron, more silica thin veneer iii. Core is mostly metal iron and nickel, ~3500 km radius b. Physical properties of these zones change with depth, due to increase in pressure at greater distances from surface i. Lithosphere is the crust and uppermost mantle that behaves in a brittle manner (100-250 km) ii. Asthenosphere is a soft (but not melted) zone of the upper mantle that the lithosphere moves around on (to a depth of about 600 km) iii. Increased pressure keeps the lower mantle rigid to its boundary with the core. iv. Outer core is liquid (molten) metal (~2260 km thick) v. Inner core is under so much pressure that it is solid (1220 km radius) 4. History of Astronomy ch 21 a. Ancient civilization ideas that Earth was at the center b. Age of Reason beginning to recognize Sun is center c. Age of Technology exploring wide reaches of the universe History of Astronomy Early Years 2006, Sept 28 Page 1 of 5
5. Ancient Greeks thought planets rotated around Earth in perfect circles a. Anaxagoras, 5 th century BC, reasoned that Moon had phases because it is a sphere, and is lighted by Sun b. Aristotle, tutor of Alexander BC 382-322, i. knew Earth was round from shadow of Earth during lunar eclipse ii. helped cement the Geocentric view of the solar system because 1. Earth has no apparent movement 2. Things do not fly off the surface 3. Looks like the stars are moving 4. There is no stellar parallax 5. Earth must be stationary c. Aristarchus BC 312-230 i. Determined relative sizes of Sun and Moon by shadows during eclipse events ii. Greater size of Sun lead him to believe Sun was center Heliocentric iii. Believed Earth s axis was tilted to orbit around Sun, and this creates seasons iv. Hypothesis discounted due to observations of Aristotle, etc. d. Erastosthenes BC 276-194 Librarian in Alexandria i. Determined size of Earth 1. Found notes indicating Sun is directly overhead in Syene on summer solstice 2. Sun was about 7 o south of overhead in Alexandria 3. this is about 1/50 of a circle, so Earth s circumference must be 50x that distance ii. 39,400 km ~ 40,075 km pretty good for 2200 years ago e. Hipparchus i. Star catalog: Location, Brightness 1-6 ii. Length of year within minutes of actual iii. Prediction of lunar eclipse events f. Cladius Ptolemy 85-100 to 165-180 AD i. Mathematician, geographer, astronomer ii. His Great Work (Syntaxis, Amalgest) preserved by Persian scholars through middle ages iii. Geographer 1. Maps were very accurate 2. first known attempt to project sphere onto a plane 3. inspired Columbus iv. Mathematician work with circles and chord theorems v. Codified Geocentric View model that planets orbit around Earth History of Astronomy Early Years 2006, Sept 28 Page 2 of 5
1. Perfect circle orbits discounted because of retrograde motion with respect to Earth a. Looks like planet going backward in orbit b. Actually different speeds of planets orbiting Sun 2. devised idea of epicycles to explain this a. small loops in orbits could explain backward motion b. best predictor of location of planets until Kepler (1609) c. accepted by Roman Catholic Church, and Church punished those who reasoned against it History of Astronomy Early Years 2006, Sept 28 Page 3 of 5
6. Age of Reason a. Nicolaus Copernicus 1473-1543 i. Commenteriolus manuscript circulated from 1512 1. unpublished 2. Heliocentric hypothesis ii. On the Revolutions of the Planets published year of his death 1. incorporated some epicycles to account for lack of perfect agreement with perfectly circular orbits around Sun (did not consider ellipse shape of orbits) 2. publisher s preface declared it hypothesis to predict location of planets 3. not much notice taken by Church at that time 4. Galileo began Copernican Crusade in early 1600 s that lead to it being banned from 1632-1835 by Catholic Church b. Tycho Brahe 1546-1601 Danish astronomer i. Observations very exact thorough 1. 1572 Supernova in Cassiopeia 2. Comet in 1577 convinced him Aristotle s hypothesis was wrong celestial sphere not solid and unchanging ii. great quadrant and corrections for refraction of atmosphere allowed him to plot very accurately 1. calculated Earth s axial tilt to 1/100 of degree 2. tropical year length to one second accuracy iii. rejected Ptolemaic system, and Copernican system 1. Tychonian model a. Sun orbits Earth b. All other planets orbit Sun c. Earth is fixed there is no stellar parallax iv. Took on assistant Johannes Kepler in 1600 c. Johannes Kepler 1571-1630 i. Used Tycho s observations to formulate laws of planetary motion 1. Planets move in elliptical orbits around Sun 2. Planets sweep equal area in equal time period 3. distances of planets to Sun are proportional to period of orbit around Sun (p 2 =d 3 ) ii. fanatic about finding causes for natural observations hindered him at times iii. Vigorous supporter of Galileo s observations of the moons of Jupiter History of Astronomy Early Years 2006, Sept 28 Page 4 of 5
d. Galileo Galilei 1564-1642 i. Heard about lenses being used to magnify objects 1. created his own telescopes to 30 power not the inventor! 2. looked at planets and Sun ii. Planetary observations 1. discovered planets are discs, not points 2. found Jupiter has moons a. implication that Earth not the only center of orbit b. disputes the argument that if Earth orbits Sun, then Moon would be left behind 3. Venus has phases, and this supports heliocentric hypothesis also. He noticed that when Venus is full, it appears smallest, because it is farther away than when it is crescent 4. Moon surface is cratered and mountainous 5. Sun has sunspots, that move around on Sun s surface iii. Dialogue of the Great World Systems published in 1630, banned by Church, he was house-arrested for the rest of his life exonerated in 1992 e. Isaac Newton 1642-1727 i. Formulated and tested the Law of Universal Gravitation every body in the universe attracts every other body with a force proportional to their masses, and inversely proportional to the distance between them ii. Accounted for why Kepler s laws worked iii. Also explains perturbations of orbits due to other bodies f. Foucault s pendulum proved that Earth rotates on axis i. Pendulum continues to swing in same plane unless acted upon by outside influence ii. Pendulum set into motion changes apparent position over full day period iii. It doesn t really swing in a changing plane, Earth is rotating under it. History of Astronomy Early Years 2006, Sept 28 Page 5 of 5