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## Birth of Cosmological Models

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**The History of Astronomy ~ Chapters 4 & 5**Birth of Cosmological Models • Babylonians • 1600 B.C.: first star catalogs compiled; recording of planetary motion • 800 B.C.: planetary locations with respect to stars of zodiac • 240 B.C.: observation of eclipses**Birth of Cosmological Models**• Greeks ~600 B.C. • geometrical, physical models of cosmos – based on the ideal of the sphere as the perfect form • Pythagoras used geometry to develop a model of the cosmos • a series of concentric spheres centered around the earth – GEOCENTRIC**Birth of Cosmological Models**• Greeks • 427–347 B.C. Plato – heavenly bodies move at a uniform rate • 384-322 B.C. Aristotle – used 56 spheres and took into account physical ideas of motion: natural motion & forced motion**Birth of Cosmological Models**• Greeks • ~300 B.C.: Greek astronomer Aristarchus proposed a HELIOCENTRIC (sun-centered) model of cosmos • much of his writings were lost; model attacked because it contradicted Aristotle’s physics**Birth of Cosmological Models**• Greeks • ~A.D. 125 Ptolemy – first astronomy textbook • earth is spherical and at center of cosmos & doesn’t move • predicted planetary motion in a GEOCENTRIC cosmos with accuracy • accepted for 1,400 years**Copernicun Revolution**• Copernicus proposed a heliocentric universe • Marked the birth of modern science • Long association with the church • Reluctant to publish ideas that contradicted church teaching**Copernicun Revolution**• 1507 – started his writings • Didn’t authorized publishing until 1543 – he was dying • Hypothesis was correct – sun at center of cosmos • Model was inaccurate • His belief in uniform circular motion did not accurately explain the motion of the planets**Galileo Galilei**• Concentrated more on terrestrial physics than telescope observations • Used telescope to give support to Copernican model • Refuted two Aristotelian ideas: • Observations of moon geography dismissed the idea of a perfectly spherical body • Observations of Milky Way dismissed the idea of a set number of stars • Discovered four new “planets” • really the moons of Jupiter, therefore, things revolve around something other than the earth**Galileo**• His evidence didn’t actually support the Copernican model, but it provided evidence against the Aristotelian-Ptolemaic model • Investigated and proposed “new” properties of physics: speed, velocity, acceleration, inertia, free-fall due to gravity**Isaac Newton**• Provided laws of motion and gravity that apply on earth and in the celestial realm (goes against Aristotle) • Discovered: • laws of gravity that explain planetary motion. • that the force of gravity varies depending on the inverse square of the distance between two bodies. • The strength of the gravitation force depends directly on the product of the masses of the two objects and inversely on the square of the distance between their centers – this explains the elliptical orbits – supports Kepler’s third law**Newton**• justified the Kepler-revised Copernican model • accurately predicted the motions of the planets • Impact of Newton’s Laws: mathematical tool for: • placing satellites in orbit around earth • setting trajectories of spacecraft in solar system • defining the escape speed on an object from a planet (or the solar system itself)