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History of Astronomy Our Universe • Earth is one of nine planets that orbit the sun • The sun is one star in 100 billion stars that make up our galaxy- The Milky Way • Our galaxy is one of billions of galaxies in the universe • Views of our galaxy and Universe have changed tremendously throughout time Early Astronomy • Astronomy- science that studies the universe • Deals with properties of objects in space and the laws through which the universe operates Early Astronomy- Ancient Greeks • Aristotle (384-322 B.C.) – Concluded that Earth was round from observations of the curved shadow on the moon when Earth passes between sun and moon • Eratosthenes (276-194 B.C.) – Calculated circumference of the Earth • Used angles of noonday sun in two locations on the planet Early Astronomy- Ancient Greeks • Hipparchus (2nd century B.C.) – Determined the location of 850 stars and categorized them according to brightness – He measured the length of a year and developed a method for predicting lunar eclipses Models of the Universe • Greeks believed in the Geocentric model of the universe • Earth is the center and planets, sun, and moon orbit around Earth • Aristarcus (312-230 B.C.) – First to propose a heliocentric model- Earth and other planets orbit the sun • Still- the Geocentric model dominated for nearly 2,000 years Ptolemaic System • Claudius Ptolemy published the Ptolemaic system in 141 A.D. • This system accounted for the movements of the planets, but was still a Geocentric model which was determined to be inaccurate • Ptolemy discovered retrograde motion Retrograde Motion • Planets appear to stop, reverse direction, and resume normal motion Modern Astronomy • Nicolaus Copernicus (1473-1543) – Concluded that Earth was a planet and proposed a model of the solar system with the sun at the center (Heliocentric) – Tycho Brahe (1546-1601) • Used instruments to measure the locations of “heavenly bodies” • His observations were far more precise than any made before • Johannes Kepler started his astronomy career as an assistant to Brahe Modern Astronomy • Johannes Kepler (1571-1630) • 3 laws of planetary motion – Discovered that planetary orbit around the sun is not in a circle but an ellipse (oval shape) – Also discovered that speed of planets changes as revolve around the sun • Faster as they get closer to the sun (perihelion) slower as it gets further from the sun (aphelion) Kepler’s 3 Laws of Planetary Motion • Law 1 – The path of each planet around the sun is an ellipse, with the sun at one focus, and other focus symmetrically opposite Kepler’s 3 Laws of Planetary Motion • Law 2 – Each planet revolves around the sun so that it sweeps over equal areas in equal time intervalstherefore planets travel faster as they near the sun Kepler’s 3 Laws of Planetary Motion • Solar distances of the planets can be calculated when their periods of revolution are known • Distances are expressed in astronomical units (AU) Galileo Galilei (1564-1642) • Created the first telescope in 1609- previous to this invention all astronomical discoveries were made without aid of a telescope – Telescope magnified objects 3X Galileo’s Discoveries • Four moons orbiting Jupiter – Disproved that Earth was center of universe • Planets are circular disks and Earthlike – Disproved that planets were points of light • Venus has phases like the moon – Shows that Venus orbits its light source (the sun) • Moons surface is not smooth • Sun has sunspots, or dark regions – Helped to calculate the rotational period of the sun Sir Isaac Newton (1642-1727) 1. Was the first to formulate and test the law of universal gravitation – This helps to explain why planets move in ellipses and not in a straight line • Gravity is directly proportional to mass of an object and inversely proportional to distance • Greater mass = Greater gravitational attraction • Greater distance = less gravitational attraction – Gravity Pulls objects towards the sun = elliptical orbits