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 Motions of the Planets
 

Ancient theory -How Earth relates to the sun and the planets
   · Ancient Greek astronomer named Ptolemy believed that the planets and the sun traveled in orbits around the earth
   · This is called the Geocentric theory of the universe
        ¨ Geo – means earth
        ¨Centric – means center (Earth is in the center of the universe)
   · Ptolemy also believed that the universe was unchanging and divine

Modern theory -How Earth relates to the sun and the planets
   ·About the year 1500, and astronomer named Copernicus developed our current theory, the earth along with other planets are in orbit around our star, the sun
   · This is called the Heliocentric theory of the solar system
        ¨ Helio – mean sun
        ¨ Centric means center (The Sun is in the center of the solar system)
   · Every planet took a different amount of time to travel around (orbit) the sun
   ·He also believed that the planets orbited the sun in perfect circular orbits

Elliptical Orbits
   · German mathematician, Kepler supported the Heliocentric theory, the planets orbit the sun, however they do not travel in circular orbits
   · The time it takes for a planet to orbit or revolve around the sun is called the period of revolution.
   ·He proved that planets travel in an ellipse, (egg-shaped orbit)
   · Each planet travels counter-clockwise in an elliptical orbit
   · The closest planets travel the shortest distance, shortest period of revolution, the farthest planets travel the longest distance, longest period of revolution.

Inertia and Gravity
   · In the 1600’s, Isaac Newton explained why planets stay in their orbits
   · Law of Inertia – A moving object will not change its speed or direction unless acted on by another force.
   ·Gravity pulls the planets towards the sun and each planet has its own gravitational pull.
   ·Inertia and gravity keep the planets in their orbits

Period of Revolution
   · Time it takes to make one complete orbit around the sun
   ·The period of revolution for a moon is the time it takes to make one complete orbit around its planet
   ·A planet’s period of revolution is called a year on that planet
   ·Mercury – closest planet has a period of revolution of 88 days and Pluto, normally the most distant planet has a period of revolution of 248 earth years.

Period of Rotation
   ·All planets spin on their axes
   ·The period of rotation is the time it takes for a planet to make one complete spin on its axis
   ·A planet’s period of rotation is called a day on that planet