The Sun's height and the seasons

What the Activity is for

Angle of incidence

This is a teacher demonstration to show the large difference in the angle of incidence of the Sun's rays between midday on 21 December and midday on 21 June.

What to Prepare

• 1 OHP
• 1 large sheet (A3) of poster paper, mounted on stiff card
• these, or similar diagrams

What Happens During this Activity

Remind pupils of the daily path of the Sun through the sky (rising in the east, due south at noon, setting in the west). Then ask them what is the difference between the height attained at noon in summer and the height attained in winter.

Introduce the drawings which show the relationships between the incoming sunlight and the position of London on the globe in the middle of winter and in the middle of summer. The thick blue line represents the horizon of view. The diagrams show that in the middle of winter, the Sun only rises 14 ° above the horizon. However, in summer, it rises to an elevation of 67 ° above the horizon.

Teacher: What effect will this change in angle have?

Now model the effect.

• Use the OHP to project light towards the board.
• Place the poster sheet perpendicular to the beam about 50 cm from the lamp.
• Now turn the sheet so that it is tilted at an angle of 67 ° to the horizontal to model the second diagram. Measure the length of the rectangle of light on the paper.
• Now turn the paper so that it is at an angle of 14 ° to the horizontal as in the first diagram above. Measure the length of the rectangle of light on the sheet.
• Ask how much bigger one is than the other. (About 4 times)
• Now ask what this implies for the temperature the ground will reach? (It will be significantly cooler – but not by a factor of four, since the temperature of landmasses near the ocean is sustained by the body of energy contained in the sea. This is why the continents get a lot colder in winter.)