The solar system

The Sun is a star at the centre of our solar system. Eight planets orbit it, held by gravity, along with moons, dwarf planets, asteroids and comets.

Key facts

  • Planets in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
  • Earth spins once a day (day and night) and orbits the Sun once a year.
  • Seasons happen because Earth’s axis is tilted.
  • The Moon orbits Earth about every 27 days; we see its phases because half of it is always lit by the Sun.

Worked examples

Example 1

Why is it day in London while it’s night in Sydney?

  1. Only the half of Earth facing the Sun is in daylight.
  2. London and Sydney are on opposite sides, so one has day while the other has night.

Answer: Earth spins