Earth, Sun and Moon

Earth spins on its axis once a day, giving day and night, and orbits the Sun once a year. Its axis is tilted, which gives us seasons – opposite in the two hemispheres. The Moon orbits Earth about once a month; as it does, we see different amounts of its sunlit half, giving the phases. Sometimes the three line up to make eclipses.

Key facts

  • Day and night: Earth spins once every 24 hours. The Sun appears to move east → west.
  • A year: Earth orbits the Sun in about 365¼ days.
  • Seasons are caused by the tilt of Earth’s axis, not by our distance from the Sun.
  • Moon phases repeat about every 29.5 days: new → first quarter → full → last quarter → new.
  • Solar eclipse: the Moon blocks the Sun (at new moon). Lunar eclipse: Earth’s shadow covers the Moon (at full moon).
  • Earth turns 15° per hour, which sets time zones.

Worked examples

Example 1

It is summer in Cape Town in January. What season is it in London, and why?

  1. In January the southern hemisphere is tilted towards the Sun: higher Sun, longer days, summer.
  2. The northern hemisphere is tilted away: lower Sun, shorter days, winter.

Answer: winter – the hemispheres tilt opposite ways

Example 2

Tonight there is a first-quarter moon. What phase will it be in about a week, and in about two weeks?

  1. About a week (a quarter of the cycle) later the Moon is on the far side of Earth from the Sun: full moon.
  2. About another week later it is at last quarter.

Answer: full moon in a week; last quarter in two weeks

Example 3

It is 10:00 solar time at a place at 30°E. What is the solar time at 75°W?

  1. Difference in longitude 30 + 75 = 105°
  2. Time difference 105 ÷ 15 = 7 hours
  3. West is earlier 10:00 − 7 h = 03:00

Answer: 03:00