ASTR 101 Lecture 4 Outline of Last Lecture I Days and Seasons A Summer Solstice B Winter Solstice C Spring Equinox D Fall Equinox II Phases of the Moon III Eclipses Outline of Current Lecture I The Scientific Thinking of Astronomy II Astronomical Observations Benefiting Ancient Societies III IV A Ancient civilizations achievements in astronomy Greek Roots Copernicus Tycho and Kepler A Kepler s three laws of planetary motion B How did Galileo solidify the Copernican revolution Current Lecture Chapter 3 The Science of Astronomy Scientific thinking is based on everyday ideas of observation and trial and error experiments How did astronomical observations benefit ancient societies o For practical purposes including agriculture o For religious and ceremonial purposes o In aiding navigation o In keeping track of time Ancient people of central Africa 6500 B C could predict seasons from the orientation of the crescent moon What did ancient civilizations achieve in astronomy o Daily timekeeping ex Egyptian obelisk o Tracking the seasons and calendar o Monitoring lunar cycles o Monitoring planets and stars o Predicting eclipses o And more In what ways do all humans employ scientific thinking o Scientific thinking involves the same type of trial and error thinking that we use in our everyday lives but in a carefully organized way Our mathematical and scientific heritage originated with the civilizations of the Middle East Greeks o Socrates 469 BC 399 BC o Plato Greek philosopher mathematician studied under Socrates o Aristotle What is an ellipse An ellipse looks like an elongated circle Our goals for learning 1 Why does modern science trace its roots back to the Greeks a Greeks were the first people known to make models of nature b They tried to explain patterns in nature without resorting to myths or the supernatural 2 How did the Greeks explain planetary motion a Underpinnings of the Greek geocentric model i Earth at the center of the universe ii Heavens must be perfect objects move on perfect spheres or in perfect circles iii But this made it difficult to explain the retrograde motion of the planets iv The most sophisticated geocentric model the Ptolemaic model sufficiently accurate to remain in use for 1500 years So how does this model explain retrograde motion Planets really do go backward in this model It had each planet move on a small imaginary circle whose center moves around earth on a larger circle Our goals for learning 1 How did Copernicus Tycho and Kepler challenge the Earth centered idea a Copernicus proposed the sun centered model b He used the model to determine the layout of the solar system planetary distances in AU c But the model was no more accurate than the Ptolemaic model in predicting planetary motions because it still used perfect circles d Tycho Brahe compiled the most accurate 1 arc minute naked eye measurements ever made of planetary motions He still could not detect stellar parallax and thus still thought Earth must be at the center of the solar system but recognized that other planets rotated around the Sun He hired Kepler who used Tycho s observations to discover the truth about planetary motion e Kepler first tried to match Tycho s observations with circular motion But an 8 acr minute discrepancy led him eventually to ellipses 2 What are Kepler s three laws of planetary motion a Kepler s first law The orbit of each planet around the Sun is an ellipse with the Sun at one focus b Kepler s second law As a planet moves around its orbit it sweeps out equal areas in equal times this means the planets move faster closer to the Sun and slower when its farther from the Sun c Kepler s third law More distant planets orbit the Sun at slower average speeds obeying the relationship p2 a3 3 How did Galileo solidify the Copernican revolution a He overcame major objections to the Copernican view Three key objections rooted in the Aristotelian view were the following i Earth could not be moving because objects in air would be left behind ii Noncircular orbits are not prefect as heavens should be iii If Earth were really orbiting the Sun we d detect stellar parallax
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