22.1 Early Astronomy

Solar System Models geocentric model: model of the solar system that states that the Earth is the center of the solar system believed until 1600’s

heliocentric model: model of the solar system that states that the sun is the center of the solar system and the planets revolve around it

the stars don’t seem to move in our sky; planets do because they are much closer than stars planets move in an eastward movement sometimes planets appear to move in a backward movement, this is called retrograde motion a loop occurs because each planet travels around the sun at different speeds

Watch retrograde motion! BBC Video

The path a planet follows around the sun is called its orbit

Kepler’s Laws of Planetary Motion 1st law : planets orbit the sun in a path called an ellipse

perihelion aphelion

astronomical unit: the average distance between the earth and the sun 150 million km 93 million miles

2nd law: planets move fastest when they are closest to the sun in their orbits

3rd law: the farther away a planet orbits, the longer the planet takes to revolve around the sun