Students should be able to describe the similarities and distinctions…

The idea

Sort these three by what they orbit and by how they got there. Planets orbit the Sun, moons orbit planets, and artificial satellites orbit the Earth or another planet after being launched by people. All three are held in orbit by gravity, all three follow roughly circular orbits, and none of them gives out its own light, so each is seen only by reflected sunlight. The differences worth writing are the object at the centre, the origin, natural or artificial, the size, and the orbital height or period. An artificial satellite is usually small, low and fast, taking hours to go round, while the Moon takes about a month. Size is a poor way of sorting the natural bodies, since some moons are larger than the smallest planet.

What earns the marks

• A similarity, such as all three being held in orbit by gravity or all being seen by reflected light.
• Planets orbiting the Sun while moons orbit planets.
• Artificial satellites having been launched rather than being natural.
• A difference in orbital period, orbital height or size, supported by the data given.

Common mistakes and how to put them right

• Saying that planets give out their own light, which only stars do.

• Sorting by size and concluding that every planet is bigger than every moon.

• Calling the Moon an artificial satellite, or the International Space Station a moon.

• Writing only differences, when a compare question needs similarities too.

Exam tips

• Answer compare questions in pairs: for each feature, say what each object does.
• Use the data in the table, because a comparison with numbers reaches the higher level.
• Say what the object orbits every time you name it, since that is what separates the three types.