Ancient Astronomy: Uses of the Sky and Early Models of the Universe - Prof. Randall Espino, Study notes of Astronomy

Download Ancient Astronomy: Uses of the Sky and Early Models of the Universe - Prof. Randall Espino and more Study notes Astronomy in PDF only on Docsity! The Ancient Roots of Science/Astronomy Chapter 3 Astronomy in Ancient Societies • Uses of the sky • Track of time and seasons • for practical purposes, including agriculture • for religious and ceremonial purposes • In aiding navigation Ancient Science • Scientific approach: to infer information about Nature from observations of the sky • Example: measuring the radius of Earth. 1 2 3 Summer Solstice Winter Solstice Spring/Fall Equinox Eratosthenes measures Earth • On the longest day of the year, at exactly noon, shadows (and the position of the Sun) look different depending on ones latitude • Eratosthenes knew that in Syene, on that day, there was NO shadows. (Syene is exactly at 23 1/2° of latitude) • Eratosthenes knew that at that moment in Alexandria objects DID project a shadow! Eratosthenes measures Earth 7° 4 5 6 Ptolemaic Model • Ptolemy (A.D. 100–170) • Sufficiently accurate to remain in use for 1,500 years A model of epicycles • Planets move in a small circle (epicycle), which in turn moves along a larger circle called a deferent. • Planets really do go backward in this model as they retrograde. • It also explains variations in distance The Copernican Revolution • After the dark ages Copernicus (1473-1543) proposed a Sun-centered model (published in 1543). • He used the model to determine the layout of the solar system (planetary distances in AU). • But...was no more accurate than Ptolemy because it used circular orbits. 13 14 15 Tycho Brahe’s naked eye observations • Tycho (1546–1601) compiled very accurate (1 arcminute) naked eye measurements of planetary positions. • He saw NO evidence of motion of the Earth: NO stellar parallax. • Recognized that other planets go around Sun. Johannes Kepler • Keppler (1571–1630) first tried to match Tycho’s observations with circular orbits. • But an 8-arcminute discrepancy led him eventually to ellipses. recognized that other planets go around Sun. A Circle 16 17 18 A Circle center radius • Special point: CENTER • The distance to the center is the same for any point in a circle. • This distance is the radius of the circle An Ellipse focusfocus Mayor Axis Minor Axis An Ellipse focusfocus semi-mayor Axis semi-minor Axis 19 20 21 Kepler’s 3 laws 3. More distant planets orbit the Sun at slower average speeds: p2 = a3 • p = orbital period in years • a = average distance from Sun in AU (a is also the semi-mayor axis of orbit) Kepler’s 3 laws 3. More distant planets orbit the Sun at slower average speeds: p2 = a3 • Example: Earth • p = 1year; a = 1AU • (1)2 = (1)3 Kepler’s 3 laws 3. More distant planets orbit the Sun at slower average speeds: p2 = a3 • Example: Jupiter • p = 11.85 years; find a • (11.85)2 = a3 • 140.42 = a3 • a = ! 140.42 = 5.2 AU 28 29 30 Galileo Galilei • Galileo (1564–1642) built one of the first telescopes. Modern astronomy begins! • Overcame major (aristotelian) objections to the Copernican view. Law of Inertia 1. “Earth could not be moving because objects in air would be left behind.” • Aristotle: natural state is rest. • Galileo: Objects will stay in motion unless a force acts to slow them down (Later became Newton’s first law of motion — Inertia). • Galileo’s experiments showed that objects would stay with a moving Earth. On Perfection 2. “Noncircular orbits are not perfect as heavens should be.” • Tycho had already seen NON circular orbits: comet. • Using his telescope, Galileo saw: • Sunspots on Sun (“imperfections”) • Mountains and valleys on the Moon (NOT a perfect sphere) 31 32 33 Stellar Parallax 3. “If Earth were really orbiting Sun, we’d detect change in the position of the stars” • Galileo showed stars must be much farther away than Tycho thought— using his telescope saw that the Milky Way is made of countless individual stars. • If stars were much farther away, then lack of detectable parallax was no longer so troubling. Galileo: Non-geocentric Universe • Annotations by Galileo of his observations of 4 moons orbiting Jupiter • Not a geocentric Universe! Galileo: Non-geocentric Universe • In Ptolemeic model one could only see Venus in crescent phase, never in full phase! (Because Venus is always between the Sun and Earth) 34 35 36