Early Ideas

Early Astronomers Ancient astronomers could recognize the difference between stars and planets These astronomers believe that the Sun, planets, and stars orbited a stationary Earth This is called the geocentric model (Earth centered)

Geocentric Model

Early Astronomers (cont.) Normally planets, as observed from Earth, move to the east Sometimes a planet will look like it moves the opposite direction (west) This is retrograde motion In 1543, Nicolas Copernicus suggested the sun was the center of the solar system This is the heliocentric model (Sun centered)

Heliocentric Model

Kepler’s First Law The ideas of Copernicus were not very accepted unit further evidence was discovered Tycho Brahe made very accurate observations of planetary positions to help him out Johannes Kepler discovered that the planets travel in ellipses, rather then a circle Each planets elliptical orbit is different in size and shape The distance between the Sun and Earth is 1 AU (astronomical unit)

Eccentricity Because planets have elliptical orbits they have points that they are closer and farther away from the sun Perihelion is when the planet is closest Aphelion is when the planets are furthest away

Kepler’s Second Law Kepler found an imaginary line between the Sun and planets sweeps out equal amounts of area in equal amounts of time

Kepler’s Third Law Kepler also figured out a relationship between the size of it’s orbit and the time it takes the planet to orbit the Sun (P2 = a3) This was his 3rd law While Kepler was figuring the solar system out mathematically Galileo Galilei developed the telescope His discoveries proved the heliocentric model

Gravity and Orbits Isaac Newton started figuring out gravity by looking at the Moon’s motions He figured out that any two bodies attract with a force that depends on their masses and distance apart This is known as the law of universal gravitation