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Physics The Earth and space
Class 7 Intermediate About 12 min Premium

Long days and short days: the tilted Earth

Move the Earth round the Sun and see how long the day is in Kathmandu, how high the Sun climbs at noon and how long a stick's shadow is in June, September, December and March. Then imagine an Earth with no tilt.

Core concept

The Earth's axis is tilted, so as it goes round the Sun, Nepal leans towards the Sun in summer (long days, high Sun) and away from it in winter (short days, low Sun)

Topic: The Earth and space

The Earth spins on its axis once a day, giving day and night, and goes round the Sun once a year. Its axis is not straight up: it is tilted by 23.5°, and it always leans the same way in space. So in June the northern half of the Earth, where Nepal is, leans towards the Sun: the Sun climbs high in the sky and the days are long. In December our half leans away: the Sun stays low and the days are short. In March and September neither half leans towards the Sun, and day and night are equal all over the world. In this lab you follow Kathmandu, at 27.7° north of the equator, round the year.

Key words explained

What you will learn

  • Explain that the Earth's axis is tilted by 23.5° and always leans the same way.
  • Compare the length of the day and the height of the noon Sun in June and December.
  • Explain why a stick's noon shadow is short in summer and long in winter.
  • Explain why there would be no seasons if the Earth were not tilted.

Before you start

  • Know that the Earth spins once a day and goes round the Sun once a year.

Materials

  • A ball (the Earth) with a stick through it for the axis, and a lamp (the Sun)

Equipment

  • A 1 m stick to stand upright in the sun, a measuring tape

Procedure

  1. Put the Earth at 21 June and record the day length, the noon Sun and the shadow.
  2. Move the Earth to 22 December and record.
  3. Record 21 March and 23 September.
  4. Imagine an Earth with no tilt, and record June and December again.

How this simulation is simplified

  • Kathmandu is 27.7° north of the equator.
  • The Sun is treated as a point, and the bending of light by the air is ignored (so real days are a few minutes longer).
  • The shadow is for a 1 m stick standing upright on level ground at noon.
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