Shadows and the Sun's Path
≈ 18 minA shadow forms when an opaque object blocks light. Because light travels in straight lines, a shadow always falls on the side of the object directly away from the light source.
As the Earth rotates, the Sun appears to move across the sky, so shadows change in two ways through the day:
| Time | Sun's position | Shadow |
|---|---|---|
| early morning | low, in the east | very long, pointing west |
| midday | highest in the sky | shortest |
| late afternoon | low, in the west | very long, pointing east |
A sundial is nothing more than this pattern turned into a clock.
Worked example — a shadow-stick investigation. Push a stick into the ground and mark the tip of its shadow every hour. A typical set of readings:
| Time | Shadow length |
|---|---|
| 08:00 | 3.6 m |
| 10:00 | 1.7 m |
| 12:00 | 0.9 m |
| 14:00 | 1.8 m |
| 16:00 | 3.5 m |
The shadow shrank to a minimum around midday and then grew again. The difference between the 08:00 and the 12:00 shadow is m.
Direction matters too, and it is specific to where we live. For almost all of South Africa the midday Sun stands in the northern half of the sky, so midday shadows point south. This is also why architects put the biggest windows of a South African house on the north side: that wall gets sunlight all year round.
At what time of day is a shadow shortest?
In almost all of South Africa, in which direction does a shadow point at midday?
In a shadow-stick investigation, the shadow measures 3.6 m at 08:00 and 0.9 m at 12:00. By how many metres did the shadow shorten?
A learner steps outside and notices that her shadow is very long and points towards the west. Roughly what time of day is it?