Elliptical Orbits (Cambridge (CIE) IGCSE Physics): Revision Note

Elliptical orbits

Extended Tier Only

• The orbits of planets, minor planets and comets are elliptical

  • An ellipse is a 'squashed' circle

• Planets and minor planets have slightly elliptical orbits

  • Their orbits are approximately circular, so the Sun is generally considered to be at the centre

• Comets have highly elliptical orbits

  • The Sun is not at the centre of the orbit, it is at one of the two foci of the ellipse

Comet speed

Extended Tier Only

• An object in an elliptical orbit around the Sun travels at a different speed depending on its distance from the Sun 

• Although these orbits are not circular, they are still stable

  • For a stable orbit, the radius must change if the comet's orbital speed changes

• As the comet approaches the Sun:

  • the radius of the orbit decreases

  • the orbital speed increases due to the Sun's strong gravitational pull (opens in a new tab)

• As the comet travels further away from the Sun:

  • the radius of the orbit increases

  • the orbital speed decreases due to a weaker gravitational pull from the Sun

Conservation of energy in elliptical orbits

• When an object moves in an elliptical orbit, energy must be conserved

• Throughout the orbit, gravitational potential energy is transferred to kinetic energy (opens in a new tab), and vice versa

  • When a comet travels closer to the Sun, it has greater kinetic energy

  • When a comet travels further from the Sun, it has greater gravitational potential energy

• As the comet approaches the Sun:

  • it loses gravitational potential energy and gains kinetic energy

  • the increase in kinetic energy causes it to speed up

• As the comet moves away from the Sun:

  • it gains gravitational potential energy and loses kinetic energy

  • the decrease in kinetic energy causes it to slow down