Earth and the Solar System

O Level / IGCSE Physics · Space physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The Solar System is one star, the Sun, with eight planets, their moons, dwarf planets such as Pluto, asteroids and comets all in orbit around it. The Sun holds most of the mass, so its gravitational attraction is the force that keeps everything in orbit.

The Earth orbits the Sun in about 365 days and spins on its tilted axis once in about 24 hours. The spin gives day and night; the tilt gives the seasons. The Moon orbits the Earth in about one month.

Gravitational field strength at a planet's surface depends on the mass of the planet, and it gets weaker further away. So planets further from the Sun feel a weaker pull, move more slowly and take longer to complete an orbit.

Rules to remember

Common mistake

Putting the orbital period into v = 2πr/T in days or years. Change it to seconds first: 1 day = 24 × 3600 = 86 400 s.

Worked example

Mars orbits the Sun at an average radius of 2.3 × 1011 m with an orbital period of 687 days. What is its average orbital speed?

  1. Period in seconds: T = 687 × 86 400 = 5.94 × 107 s.
  2. Distance round the orbit: 2πr = 2 × 3.14 × 2.3 × 1011 = 1.44 × 1012 m.
  3. v = 2πr ÷ T = (1.44 × 1012) ÷ (5.94 × 107) = 2.4 × 104 m/s.

Answer: About 2.4 × 104 m/s (24 km/s)

Practice questions

Try each one, then open the answer.

1. Which statement about the Earth is correct?

  1. A
    it orbits the Sun once in about 365 days and rotates on its axis once in 24 hours
  2. B
    it orbits the Sun once in 24 hours
  3. C
    the Moon orbits the Earth once in 365 days
  4. D
    the Sun orbits the Earth once a year
Show answer

Answer: A. One orbit of the Sun takes about a year (365 days); one rotation on its tilted axis takes about 24 hours, giving day and night. The Moon orbits the Earth in about a month.

2. How do the orbital speeds of the planets change with distance from the Sun?

  1. A
    they increase with distance
  2. B
    they decrease with distance
  3. C
    they are all the same
  4. D
    they depend only on the planet's mass
Show answer

Answer: B. The Sun's gravitational field weakens with distance, so more distant planets move more slowly and have much longer years.

3. Light from the Sun takes about 500 s to reach the Earth and about 43 minutes to reach Jupiter. About how many times further from the Sun is Jupiter than the Earth?

  1. A
    0.086
  2. B
    0.19
  3. C
    5.2
  4. D
    12
Show answer

Answer: C. 43 minutes = 43 × 60 = 2580 s. Light travels at a constant speed, so the ratio of distances = 2580 ÷ 500 ≈ 5.2. Using 43 without changing it to seconds gives 0.086; dividing the wrong way gives 0.19.

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