7.5P Describe the orbits of moons, planets, comets and artificial satellites

The idea

An orbit is the curved path one body follows around another because of gravity. Moons orbit planets, planets orbit the Sun, and artificial satellites orbit Earth or other bodies. Most of these orbits are close to circular, so the body travels at a fairly steady speed and distance. Comets follow highly elongated elliptical orbits around the Sun: a comet moves fastest near the Sun, where gravity is strongest, and slowest when far away. A correct answer names the central body each object orbits and links the orbit shape to the object type, for example a planet circling the Sun on a near-circular path while a comet sweeps in on a long ellipse.

What earns the marks

• Names the central body that each object orbits, such as moons around planets and planets around the Sun.
• Identifies that planets and moons follow near-circular orbits while comets follow highly elongated elliptical orbits.
• States that a comet travels fastest near the Sun and slowest when far from the Sun.
• Describes an artificial satellite as an object made by humans that orbits Earth or another body.

Common mistakes and how to put them right

• Claims that planets orbit Earth.

  Put it right by stating that planets orbit the Sun and moons orbit planets.

• Treats a comet orbit as a circle.

  Put it right by describing the long elliptical path with the Sun near one focus.

• Says a comet moves fastest when farthest from the Sun.

  Put it right by placing the greatest speed near the Sun.

Exam tips

• Read the question stem to check which object is being asked about before choosing an option.
• Match each object to its central body: moon to planet, planet to Sun, satellite to Earth or another body.
• Reject any option that gives a comet a circular orbit or a constant speed throughout its orbit.