1. MODELS OF THE SOLAR SYSTEM

2. ARISTOTLE Greek philosopher (384-322 BC) He promoted an earth centered called geocentric, model of solar system He said the sun, stars and the planets revolved around sun Didn’t explain retrograde motion - some planets appear to reverse direction and move backward

3. CLAUDIUS PONTIUS Greek astronomer (approx. 722 BC) He proposed that the earth had two motions Planets revolved around earth Planets rotated in circles called epicycles ; this motion caused earth to appear to move backward

4. COPERNICUS’S MODEL Proposed a heliocentric, sun centered, model of solar system Earth and other planets revolve around sun All planets orbit in same direction, but each at own speed and distance from sun Confirmed in 1600s by Galileo Galilei, Italian scientist, with telescope

5. KEPLER’S LAWS Tycho Brahe, Danish astronomer, made detailed observations of positions of stars and planets Johannes Kepler was a German astronomer who was hired by Brahe, developed three laws that explain planetary motion

6. LAWS OF ELLIPSES Kepler’s first law States that each planet orbits the sun in a path called an ellipse An ellipse is an elongated oval whose shape is determined by two points within the figure; each point if called a focus Sun is at one focus of a planet Orbits are ellipses, planets are not always same distance form sun Perihelion - point in orbit closest to sun Aphelion - point in orbit farthest from sun Average distance between earth and sun is known as one astronomical unit (AU)

7. LAW OF EQUAL AREAS Kepler’s second law Describes speed at which planets travel at different points in their orbits Orbit of earth almost nearly a perfect circle, sun is off-center Earth moves fastest when closest to earth

8. LAW OF PERIODS Kepler’s third law Describes the relationship between average distance of a planet from sun and orbit period of planet Orbit period is time it takes for planet to make one revolution around the sun Kepler’s third law : K x r 3 = p 2 K is mathematical constant r is distance of planet from sun p is period Radius of earth’s orbit (distance from sun) is 1AU and its period is one year

9. NEWTON’S APPLICATION OF KEPLER’S LAWS Isaac Newton explained why planets move the way Kepler described A moving body will only change its motion if an outside force causes it to do so Tendency of body to move in a strait line at constant speed until outside forces act upon it is called inertia (tendency of a body to stay at rest until outside force acts on it) Planets do not travel in strait line, must have a force acting upon them Outside force acting on planets is gravity, attractive force that exist between objects in universe Gravitational pull of sun keeps planets in orbit