planet earth

• the shape of the earth is described as an obselete spheroid (a sphere slightly flattened at the poles)
• the polar diameter is shorter than the equitorial diameter by about 40km
• structure of the earth

  • crust
  • mantle
  • outer core
  • inner core
• Lattitude lines

  • are parallel to one another and never meet
  • run in an east-west direction
  • cross the prime meridian at right angles
  • measure distance north/south of the equator
  • lie in planes that cross the Earth's axis at right angles
  • get shorter toward poles with the equator as the longest great circle
  • measured in degrees between 0 and 90N or 90S
• longitude lines

  • known as meridians
  • run in a north-south direction
  • measure distance east/west of the prime meridian
  • are farthest apart at the equator and meet at the poles
  • cross the equator at right angles
  • are equal in length
  • lie in planes that pass through the earth's axis
  • are halves of great circles
  • measured in degrees between 0 and 180E or 180W
• mean diameter of earth (approx) = 13000km
• • 1° = 60’ (each degree has 60 minutes)
  • 1’ = 60’’  (each minute has 60 seconds)
• arctic circle = 66.5°N and 23.5° from the north pole
• tropic of cancer = 23.5° N
• tropic of capricorn = 23.5° S
• equator lies at 0°
• antarcitic circle = 66.5° S and 23.5° from the south pole
• temperate areas are between 23.5° N/S and 66.5° N/S
• sun is directly overhead at noon at the equator on the autumn and spring equinox (March/September 21/22)
• sun is directly overhead at noon at the tropic of cancer on the summer solstice (June 21/22)
• sun is directly overhead at noon at the tropic of caporicorn on the winter solstice (December 21/22)
• skyglow


  a form of light pollution that appears a rusty orange haze cast by lights near human settlements
• dark adaptation

  how our eyes become adapted to night vision
• 2 types of photosensitive cells in the retina

  1. colour sensitive cones
  2. non-colour sensitive rods
• astronomers use red filters on flashlights and screens to be able to consult books and computers while maintaining dark adaptation of their eyes. The light/screen appears a dark red which is comfortable to view at night
• local glare

  bright lights at our location that ruin dark adaptation (eg- streetlights, motorway lights, sports floodlights)
• Twinkling of stars

  1. as the air is constantly flowing it's density varies over time as masses of air rise and fall continuously which causes randpm fluctuationc in the direction of the light coming from stars dues to refraction
  2. these fluctuations make stars seem like they are 'twinkling' and bring about 'seeing conditions'