How long is my shadow?
Stand a stick in the sun and move the sun from low in the morning sky to straight overhead. See how long the shadow is each time, and why shadows are long in the morning and evening and shortest at noon.
Core concept
Light travels in straight lines, so a stick's shadow is its height ÷ tan(the sun's height): the lower the sun, the longer the shadow, and with the sun straight overhead there is no shadow at all
Topic: Our planet and sky
Light travels in straight lines. When something blocks the sunlight, it leaves a dark patch behind it: its shadow: A dark shape made where an object blocks the light. All key words. In the morning the sun is low in the sky, so the light comes in at a slant and the shadow stretches out far. As the sun climbs, the shadow shrinks, and at noon it is at its shortest. If the sun were straight overhead (90°), there would be no shadow at all; this happens in the tropics on a few days a year. For a stick of height h and the sun at an angle A above the ground, the shadow is h ÷ tan A. Long ago, people told the time of day from shadows with sundials.
What you will learn
- Explain why a shadow forms.
- Describe how the length of a shadow changes with the sun's height.
- Work out a shadow's length as height ÷ tan(sun's height).
Before you start
- Know that light travels in straight lines.
Materials
- A stick or pencil, and a sunny place
- A measuring tape
Equipment
- None
Procedure
- Record the shadow of the 1.5 m stick with the sun at 30°.
- Raise the sun to 90° (straight overhead), and record.
- Lower the sun to 15°, as early in the morning, and record.
How this simulation is simplified
- The ground is flat and the stick stands straight up.
- Shadow = height ÷ tan(the sun's height above the horizon); none with the sun straight overhead.