Orbiting Bodies

• The planets and moons of the Solar System are visible from Earth when they reflect light from the Sun

  • The outer regions of the Solar System are around 5 × 10^12 m from the Sun, which means even light takes some time to travel these distances
• The light we receive on Earth from the Sun takes 8 minutes to reach us

  • The nearest star to us after the Sun is so far away that light from it takes 4 years to reach us
  • The Milky Way galaxy contains billions of stars, huge distances away, with the light taking even longer to be seen from Earth
• The speed of light is a constant 3 × 10^8 m/s

  • Therefore, using the equation: speed = distance/time
• The time taken to travel a certain distance can be calculated by rearranging to:

  • time = distance/speed
• The speed of light is very fast.

  • This is why in our everyday life things like switching on a light seem to be instant
  • This is only because the light travels very fast and the distances are very small
  • In large, astronomical distances which can be millions or even billions of kilometres, the limit of the speed of light starts to have an effect
• Orbits of planets, minor planets and comets are elliptical

  • An ellipse is just a 'squashed' circle
• Planets, minor planets and comets have elliptical orbits

  • However, the Sun is not at the centre of an elliptical orbit
  • This is only the case when the orbit is approximately circular
• Planets and comets travel in elliptical orbits, but the Sun is not at the centre of these orbits

  A) foci

  B) ellipse

  C) sun

  D) planet

  
• You will not be asked to do any calculations with elliptical orbits. If you are asked to calculate the time period, orbital speed or radius of an orbit, it can be assumed that it is circular.
• Over many years, data about all the planets, moons and the Sun have been collected
• This is not just for general interest, but to indicate:

  • Factors that affect conditions on the surface of the planets
  • Environmental problems that a visit (using manned spaceships or robots) would encounter
• Although you don't need to memorise any of the planet data, you must be able to confidently analyse and interpret it

  • Look out for trends such as one variable increasing whilst the the other decreases (or also increases