Describe how our understanding of the Solar System has changed over time, from an Earth-centred model to a Sun-centred model, and state one piece of evidence that supports the Sun-centred model.

Eduqas A-Level Physics — 4.3 Orbits and the wider universe · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

For centuries, scientists believed that the Earth was at the centre of the Solar System. This view was eventually replaced by a model in which the Sun is at the centre.

Model answer (5 marks)

The early model placed the Earth at the centre of the Solar System (geocentric model). This view was later replaced by a model in which the Sun is at the centre (heliocentric model). The change was driven by new observations that the geocentric model could not explain, such as the retrograde motion of planets. One piece of evidence supporting the Sun‑centred model is the phases of Venus, which can only be seen if Venus orbits the Sun.

The phases of Venus show a full range from new to full, which is only possible if Venus is orbiting the Sun and not the Earth, confirming the heliocentric arrangement.

Examiner tips

  • Use the exact terms geocentric and heliocentric; mention retrograde motion as the key observation; state the evidence (phases of Venus) and explain why it fits the Sun‑centred model.
  • Keep the answer concise – one sentence per point – to match the 5‑mark structure.

Common mistakes

  • Confusing the terms geocentric/heliocentric; using vague phrases like ‘Sun‑based’ instead of heliocentric.
  • Failing to explain why the phases of Venus support the Sun‑centred model, e.g. by saying only that Venus has phases without linking to its orbit around the Sun.

Mark scheme (5 marks)

  1. Early model placed the Earth at the centre of the Solar System (geocentric model)
  2. The model was changed / replaced so that the Sun is at the centre (heliocentric model)
  3. The change in model was driven by new observations / evidence that the old model could not explain
  4. Correct identification of a piece of supporting evidence for the Sun-centred model — e.g. the phases of Venus
  5. Correct explanation of why that evidence supports the Sun-centred model

Key terms in this question

evidence

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