Bellwork 1.Who is credited with the revolutionary model of a HELIOCENTRIC solar system? A. Aristotle B. Ptolemy C. Galileo D. Copernicus 2.The planets loop backwards in their orbits. A.TRUE B.FALSE 3.During which months is Earth closest to the Sun? 1

The story so far…. Ancient Greeks had a GEOCENTRIC model of the solar system with Earth in center and Sun and planets in perfect circles. Ptolemy add many epicycles to explain the looping of planets during retrograde motion. Galileo used a telescope to see phases of Venus, Jupiter’s moons, rotating sunspots Copernicus made a HELIOCENTRIC model of the solar system with the Sun in the center so the planets were NOT looping backwards, Earth’s faster orbit was ‘passing’ the slower outer planets. 2

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Kepler’s Model of the Solar System 4

Tycho Brahe (1575) After seeing the total eclipse on August 21, 1560, and became a master of careful astronomical OBSERVATIONS. Lost part of his nose in a duel and replaced it with a metal one. Made DETAILED naked eye observations of the motions of the planets. In the course of 30 years, he had amassed the most accurate astronomical DATA of the day 5

Mathmatician - Kelper's mathematical skills were extraordinary. He could not get Tycho's very careful observations to fit Copernicus’ model. He constructed 3 Laws about planets orbits: SHAPE, SPEED, TIME Kepler ‘inherited’ the observations after the sudden death of Tycho Johannes Kepler’s Johannes Kepler’s (1600) 6

Kepler’s 3 Laws of Planetary Motion 1 st Law: Ellipses! The SHAPE of the orbital paths of each planet is an ellipse (NOT a perfect circle) (with the Sun not exactly in the center, but at one focus) 7

hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/ x/787 78/Kepler_Nav.swf::Keplers%20Second%20Law%20Interactive 8

2 nd Law: SPEED of the planet An imaginary line connecting the Sun to any planet sweeps out equal areas in equal time. Area 2 Area 1 90 days Planets have variations in their orbital speed 9

Kepler’s 3 rd Law: a planet’s orbital TIME is proportional to its distance. P 2 (years) = A 3 (AU) 1 Astronomical Unit = The Earth-Sun Distance (93 million miles) In other words……the farther away the planet, the longer its ‘year’ 10

Kepler’s Third Law Object a (AU) P (year) a3a3a3a3 P2P2P2P2 Mercury Venus Earth Mars Jupiter Saturn Uranus ,080.7,060. Neptune , , ,200. Pluto , , ,500. = = = = = = = = = 11

12 Heliocentric Solar System Using data construct a model to show the correct SIZE and SCALE