What it shows
The shadow of a fixed stick is long in the morning, shortest around midday and long again in the evening, and it swings from west to east as the Sun moves across the sky. Shadow length depends on how high the Sun is.
The science
Light travels in straight lines, so a shadow points directly away from the Sun. When the Sun is low in the sky in the morning or evening, the rays come in at a slant and the shadow is long. At midday the Sun is high, the rays come steeply down and the shadow is short. The Earth turning on its axis makes the Sun seem to move, so the shadow turns too, like the hand of a clock.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A straight stick or pencil about 30 cm long
- A sheet of chart paper and stones to hold it down
- A marker pen and a ruler
- A clock or a phone to note the time
- A sunny open spot such as a terrace or courtyard
Safety
For this project: Never look straight at the Sun, and work in the shade yourself between readings during the hottest hours.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the stick upright in a lump of clay or a bottle filled with sand in the middle of the chart paper.
Starting at about 8 am, draw along the shadow with the marker and write the time at its tip.
Repeat every hour until the evening, keeping the paper and stick in exactly the same place.
Measure the length of each shadow line with the ruler and write it next to the time.
Notice that the shortest shadow is around 12 to 1 pm and that the lines fan out like a clock.
Draw a line graph with time along the bottom and shadow length up the side.
Mark which direction the morning shadow pointed and which direction the evening shadow pointed.
Repeat on another day a few weeks later and compare the midday shadow lengths.
What you should see
A fan of shadow lines on the paper, longest at the ends of the day and shortest near midday, with the morning shadows pointing west and the evening shadows pointing east. The graph shows a U shape.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1When is the shadow shortest and why?
Around midday, because the Sun is highest in the sky and its rays fall most steeply.
2Which way does the shadow point in the morning?
Towards the west, away from the Sun rising in the east.
3Why does the shadow move?
Because the Earth turns, so the Sun appears to move across the sky.
4What is a sundial?
A clock that tells the time from the direction of a shadow.
Take it further
Measure the midday shadow once a month for a few months and see how it changes between winter and summer.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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