 Definition - Earth is at the center of the solar system being orbited by the Sun and other planets  (sketch a picture of a geocentric solar system)  Scientists  Aristotle - believed that if Earth were moving, then the stars would move  Ptolemy - planets move at constant speeds in perfect circles around Earth Heliocentric  Definition – Earth and other planets orbit the Sun  (sketch a picture of a heliocentric solar system)  Scientists  Aristarchus – tried to measure the relative distances between Earth, the moon, and the Sun  Copernicus – developed the 1 st detailed heliocentric model  Galileo – saw the phases of Venus and 4 of Jupiter’s moons

 Larger objects have a stronger gravitational force.  The Sun is the largest object in the solar system, so it has the greatest gravitational pull. DISTANCE  The closer together two objects are, the stronger the gravitational force.  The moon is closer to Earth, so we can see the effect of its gravitational pull.

 The Law of Orbits  Planets orbit the Sun in an ellipse with the Sun at one focus.

 The Law of Areas  Planets cover equal areas in equal time.  A planet will move faster in its orbit when it’s closer to the Sun and slower when it’s farther away.

 The Law of Periods  Planets closer to the Sun will complete an orbit faster than planets farther away.

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