An astronomy group compares three models of the Solar System.
| Model | Description |
|---|---|
| P | The Earth is fixed at the centre. Mars moves on an epicycle, and every other object in the sky orbits the Earth. |
| Q | The Earth and the other planets orbit the Sun in perfect circles. Natural satellites may orbit planets. |
| R | The Earth and the other planets orbit the Sun in slightly elliptical paths. Natural satellites may orbit planets. |
The group considers these observations.
| Observation | Result |
|---|---|
| 1 | Mars sometimes appears to move backwards against the distant stars. |
| 2 | Four points of light remain close to Jupiter but change position around it from night to night. |
| 3 | Repeated measurements of planetary positions do not agree exactly with positions calculated using circular orbits. |
Explain which model is best supported by all three observations.
A new telescope produces a set of measurements that is not predicted by Model R.
Suggest how scientists should respond before accepting a replacement model.
80 exam-style questions on Edexcel GCSE Physics 7.1 The Solar System and gravity, covering 7.1.1 Weight and g on different bodies in space, 7.1.2 The Solar System, 7.1.3 Changing ideas about the Solar System, 7.1.4 Orbits of moons, planets, comets and satellites, and 7.1.5 Gravity and orbital motion. Each one has a worked solution and a mark scheme showing where the marks go.