21.1 Structure of the Solar System 
      The sun is the center of our solar system
      The sun and 9 planets make up our solar system 
      Planets “wander” or orbit around the sun at different distances 
      Sometimes when we view stars, we actually see planets that appear to be stars
      Venus is the closest planet to Earth and is easily seen most nights
      Stars are much farther away than planets so appear as tiny specks of light 
      Objects in the Solar System 
      Sun: largest star in the solar system-makes up 99%, 10x larger than Jupiter, made of gases (hydrogen) 
      The sun produces energy through nuclear fusion and applies gravity to all planets 
      Planets: reflect sunlight, spherical shaped object that orbits sun, classified/grouped as inner, outer, dwarf
      Asteroids: small rocky objects orbiting sun between Mars & Jupiter (belt), range in size
      Comets: Gas, dust, and ice orbiting sun and producing a glowing tail (melting), trillions in space 
      Planets
      Inner: terrestrial/rocky: Mercury, Venus, Earth, Mars; smaller than outer planets 
      Outer: gas giants: Jupiter, Saturn, Uranus, Neptune; larger than inner planets 
      Dwarf: tiny mass, made of rock and ice: Pluto, Ceres, Eris, Makemake
      Measuring Space 
      AU-astronomical unit is used to measure distance in space 
      It is the average distance from Earth to the sun
      About 150 million kilometers
      Each planet’s AU increases as it gets farther from the sun 
      See Chart p. 764
      Motion of Planets 
      Sun’s gravitational pull on each planet prevents planets from drifting apart and keeps each planet on a curved 
      path around the sun 
      Revolution/revolve-is the time it takes a planet to travel once around the sun (year) 
      Rotation/spinning on axis-is the time it takes planet to spin once on the axis (day) 
      Copernicus proposed the idea that Earth and other planets orbit the sun (heliocentric model) instead of Earth 
      being in the center (geocentric model) in 1500’s 
      Kepler discovered in 1600’s that planet’s orbit is an ellipse and not a perfect circle and speed increases near 
      the sun