Ancient Astronomy Many ancient cultures were interested in the night sky • Calenders • Prediction of seasons • Navigation 1 Timeline Astronomy timeline • ∼ 3000 B.C. Stonehenge • 2136 B.C. First record of solar eclipse by Chinese astronomers • 613 B.C. First record of Halley’s comet by Zuo Zhuan (China) • ∼ 270 B.C. Aristarchus proposes Earth goes around Sun (not a popular idea at the time) • ∼ 240 B.C. Eratosthenes estimates Earth’s circumference • ∼ 130 B.C. Hipparchus develops first accurate star map (one of the first to use R.A. and Dec) 2 Geocentric model The Geocentric Model • Greek philosopher Aristotle (384-322 B.C.) • Uniform circular motion • Earth at center of Universe Retrograde Motion • General motion of planets east- ward • Short periods of westward motion of planets • Then continuation eastward How did the early Greek philosophers make retrograde motion consistent with uniform circular motion? 3 Ptolemy Ptolemy’s Geocentric Model • Planet moves around a small circle called an epicycle • Center of epicycle moves along a larger cir- cle called a deferent • Center of deferent is at center of Earth (sort of) Ptolemy’s Geocentric Model • Ptolemy invented the device called the eccentric • The eccentric is the center of the deferent • Sometimes the eccentric was slightly off center from the center of the Earth Ptolemy’s Geocentric Model • Uniform circular motion could not account for speed of the planets thus Ptolemy used a device called the equant • The equant was placed the same distance from the eccentric as the Earth, but on the opposite side • From the viewpoint of the equant the center of the epicycle appears to move with uniform angular motion Problems with Ptolemaic model • Inconsistent use a various devices (eccentric, deferent, equant) • Centers of the epicycles for Mercury and Venus are fixed on a line joining the Sun and Earth • Theory did not fit data as accuracy of observations increased • Assumption of uniform circular motion not based on observations but on philosophy Predictive value of Ptolemaic model Prediction of phases of Venus 2 No full phase ! 4 Copernicus Nicolaus Copernicus • 16th century Polish cleric • Dissatisfied with inconsistencies of Ptole- maic model • Displeased at use of equant • Reexamined heliocentric model Simple Copernican Model • Earth and planets orbit Sun • Sun and stars stationary • Only the Moon orbits Earth Simple Copernican Model Copernican model explains: • Retrograde motion • Varying brightness of planets 3 Copernican model • However, Copernicus kept the idea of uniform circular motion and epicy- cles • Copernicus’s astronomical tables were no better than ones calculated using Ptolemy’s methods Copernican model Why the Copernican model was not initially accepted • No observable stellar parallax (at the time) • Predictive accuracy was not better than using Ptolemy’s methods • Earth does not feel as if it is moving Copernican model Prediction of phases of Venus 5 Galileo Galileo Galilei • Italian mathematician and philosopher (1564-1642) • Willing to test ideas • Built a telescope in 1609 and aimed it at the sky 4 Galileo’s observations • Observed that the Moon has mountains, valleys, and craters Moon watercolor from Sidereus Nuncius manuscript(1610) Galileo’s observations • Moons of Jupiter Galileo’s observations • Phases of Venus consistent with Coperni- can view Istoria dimostrazioni intorno alle macchie so- lari (1613) Galileo’s observations • Sunspots • Inferred that the Sun was rotating Istoria dimostrazioni intorno alle macchie so- lari (1613) 5.