Orbiting Bodies (CIE IGCSE Physics)

• 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 × 10¹² 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 × 10⁸ m/s
  • Therefore, using the equation:

• • The time taken to travel a certain distance can be calculated by rearranging to:

Step 1: State the equation for the time taken for light to travel a certain distance

Step 2: Substitute in the values

• • The distance travelled is the radius of the orbit
    • Distance, d = 5.8 × 10⁹ m.
  • Speed = the speed of light, v = 3.0 × 10⁸ m/s

Step 3: Round up the answer and include units

EXTENDED

• 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

EXTENDED

• 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 

Table of Data for Planets in our Solar System

Planet	Orbital distance / million km	Orbital duration / days or years	Density / kg/m3	Surface Temperature/ °C	Uniform Surface Gravitational Field Strength/ N/kg
Mercury	57.9	88 days	5427	350	3.7
Venus	108.2	225 days	5243	460	8.9
Earth	149.6	365 days	5514	20	9.8
Mars	227.9	687 days	3933	–23	3.7
Jupiter	778.6	11.9 years	1326	–120	23.1
Saturn	1433.5	29.5 years	687	–180	9.0
Uranus	2872.5	75 years	1271	–210	8.7
Neptune	4495.1	165 years	1638	–220	11.0