Unit 8 - Earth Science Astronomy words. Tools A B Celestial Object Anything outside of the earth's atmosphere Celestial Sphere An imaginary circle surrounding the earth. Rotation Earth spinning around on it's axis Revolution Time to travel around the sun. Zenith A point right above the observer Terrestial Rock planets Jovian Gas Planets Auroras Solar flares brought to our planet by our magnetic field Satellites Any smaller object revolving around a larger object Retrograde Motion The apparent movement of planets moving backwards. Comet Big ball of dirty snow Meteor Light made by a meteroid as it's burned by our atmosphere Asteriod large masses of rock found on a belt between Mars and Jupiter Orbit The path in which all celestial objects move. Elliptical Orbit An Ellipsed shape orbit around the sun Heliocentric The model in which the sun is in the center of the solar system Geocentric Model in which the Earth is the center of our solar system Perihelion Point in which the Earth is closest to the sun. Aphelion Point in which the Earth is farthest away from the sun