There are 8 recognized planets. They can be divided into 2 groups.

Inner Planets 4 closest to the sun. Mercury, Venus, Earth and Mars Also called terrestrial planets Traits: small, but dense Rocky surface

Outer Planets 4 farthest from the sun. Jupiter, Saturn, Uranus, and Neptune Also called the Jovian planets Traits: Large, but not very dense Gaseous surface Atmosphere of hydrogen and helium

To Be a Planet Has to be rounded by gravity Has to orbit a star Has to have everything pushed out of its orbit (nothing is in its way) Which one does Pluto not meet?

Earths Movement

Barycenter The point between two objects where they balance each other The center of mass where two or more celestial bodies orbit each other. When a moon orbits a planet, or a planet orbits a star, both bodies are actually orbiting around a point that lies outside the center of the larger body. (1,710 km) http://ecx.images-amazon.com/images/I/71PWRTtmO8L.gif

The Sun is not stationary in our solar system. It actually moves as the planets tug on it, causing it to orbit the solar system’s barycenter. The sun never strays too far from the solar system barycenter. http://www.youtube.com/watch?v=1iSR3Yw6FXo

Precession Change in the direction of the axis of the Earth, but without any change in tilt This changes the stars near (or not near) the Pole, but does not affect the seasons The Earth’s axis currently points at Polaris as the “Northern Star”, but it wont always be.. Each gyration around the cone takes 26,000 years

Nutation Wobbling around the precessional axis Change in the angle: ½ degree one way or the other Occurs over an 18 year period and is due to the Moon Very slightly increases or decreases seasonal effects