Explain why planets that are further from the Sun take longer to complete one orbit than planets that are closer to the Sun.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Planets that are further from the Sun experience a weaker gravitational pull from the Sun because the force decreases with the square of the distance. A weaker force means the planet’s orbital speed is lower. In addition, a planet that is further away has a larger orbit, so it must travel a greater distance in one revolution. The combination of a lower speed and a larger distance therefore makes the orbital period longer.
Examiner tips
- Use the word ‘weaker’ to show understanding of the inverse‑square law; mention lower speed and larger orbit; link both to a longer period; keep answer concise and to the point.
Common mistakes
- Confusing distance with speed; writing that the planet simply ‘moves slower’ without linking to the weaker force; forgetting to mention the larger orbit; using vague phrases like ‘takes longer’ without explaining why.
Mark scheme (4 marks)
- Gravitational force/pull from the Sun is weaker at greater distances
- A weaker gravitational force means the orbital speed of the planet is lower/slower
- A planet further from the Sun has a larger orbit / travels a greater distance in one orbit
- Both a lower speed and a greater distance to travel combine to make the orbital period longer
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Solar system; stability of orbital motions; satellites questions
- Explain why a planet in a circular orbit around the Sun remains at a constant di…
- Geostationary satellites are used for television broadcasting. Describe the orbi…
- A comet travels in a highly elliptical orbit around the Sun. As the comet moves …
- Explain why artificial satellites in low Earth orbit must be given an occasional…