Sun and skin – a dangerous combination

Activity 2

Absorption of ultraviolet radiation by the atmosphere

Some of the ultraviolet radiation from the sun is absorbed by the Earth's atmosphere. Because of this, the time of day, the season and the altitude may all affect the amount of ultraviolet radiation our skin receives.

  1. Suggest the approximate time of day when the sun is in position A, position B, and position C.

  2. Draw a circle with a radius of 4 centimetres to represent the Earth.

    Draw a second circle around the first (this time with a radius of 4.5 centimetres), to represent the atmosphere around the Earth.

    Draw three lines from the same point on the Earth's surface to represent the sun's rays when it is in position A, position B and position C.

  3. Look at your diagram and determine the time of day that light from the sun travels through the least thickness of atmosphere.

  4. Why are people advised to keep out of sunlight near the middle of the day?

(Activity and illustration adapted with permission of the NSW Cancer Council.)

Teachers notes

  1. Position A of the sun approximates 12 noon, position B approximates mid-morning and position C approximates early morning. (You might point out to students that the diagram implies that the sun moves around the Earth and that this is incorrect. The diagram is a simple way of showing three different positions of the sun.)

  3. The light from the sun travels through the least thickness of atmosphere at 12 noon.

  4. At noon, sunlight travels through less atmosphere so less ultraviolet radiation is absorbed. People are advised to keep out of sunlight near the middle of the day because this is when more ultraviolet radiation strikes the Earth.

    You might like to discuss the position of the sun at your latitude at various times of the year. Students should understand that the time of year (as well as the time of day) has an influence on the thickness of atmosphere through which sunlight travels.
External sites are not endorsed by the Australian Academy of Science.
Posted March 1997.