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 The History of Astronomy ~ Chapters 4 & 5

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 – 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. 125Ptolemy – 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)