Orbital Motion (Edexcel IGCSE Physics)

• There are many orbiting objects in our solar system and they each orbit a different type of planetary body

Orbiting Objects or Bodies in Our Solar System Table

Gravitational attraction causes the Moon to orbit around the Earth

• Gravity causes different astronomical orbits to orbit certain others
• In general:
  • Planets orbit the Sun
  • Moons orbit planets
  • Comets orbit the Sun
  • Artificial satellites to orbit the Earth or any body in the Solar System

Orbits of Planets

• There are several similarities in the way different planets orbit the Sun:
  • Their orbits are all slightly elliptical (stretched circles) with the Sun at one focus (approximately the centre of the orbit)
  • They all orbit in the same plane
  • They all travel the same direction around the Sun

• There are also a few differences:
  • They orbit at different distances from the Sun (different orbital radius)
  • They orbit at different speeds
  • They all take different amounts of time to orbit the Sun

• The further away a planet is from the Sun, the slower it travels and therefore the longer it takes to orbit

The orbits of planets around the Sun

Orbits of Moons

• Moons orbit planets in a circular path
• Some planets have more than one moon
• The closer the moon is to the planet:
  • The shorter the time it will take to orbit
  • The greater the speed of the orbit

Comets

• The orbits of comets are very different to those of planets:
• Their orbits are highly elliptical (very stretched) or hyperbolic
  • This causes the speed of the comets to change significantly as their distance from the Sun changes
  • Not all comets orbit in the same plane as the planets and some don’t even orbit in the same direction

• As the comet approaches the sun, its speed increases
• As it moves further away from the sun, its speed decreases

The elliptical orbit of a comet