What it shows
The Sun’s image through a small hole is a circle whose size depends on the distance to the screen. Using similar triangles with the Sun’s distance gives its real diameter, about 1.4 million km.
The science
The pinhole makes an inverted image of the Sun. The Sun’s angular size is the same whether you look at the Sun or at its image, so the diameter of the image d divided by the distance L from the pinhole to the screen equals the Sun’s diameter D divided by its distance from the Earth, about 150 million km. For small angles D = (d / L) x 1.496 x 10 to the power 8 km. The angular diameter is about 0.53 degrees, so an image on a screen 1 m away is about 9 mm across.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A thick card with a neat 1 mm to 2 mm hole made with a pin
- A second white card as a screen
- A tape measure of 2 m
- A transparent ruler marked in millimetres
- A stand or books to hold the cards
- A calculator
- A notebook
Safety
For this project: Never look at the Sun, even through the hole, binoculars or a telescope. Only look at the image on the white card, with your back to the Sun. An adult must supervise.
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.
Step 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.
Make a tiny round hole in the thick card with a pin. A hole of 1 to 2 mm gives a clear image, but a large one gives a blurred image.
Hold the card with the hole facing the Sun and place the white screen behind it so that its shadow is on the screen and you can see the round bright spot.
Measure the distance L from the hole to the screen with the tape, then the diameter d of the bright disc with the ruler. Measure across the sharpest edge.
Repeat for L of 0.5 m, 1.0 m, 1.5 m and 2.0 m. A common mistake is to measure the diameter of a blurred edge, so average two diameters at right angles.
Subtract the hole’s diameter from each d, because the bright disc is the Sun’s image plus the width of the hole. Then calculate d / L for each reading. It should be nearly the same, about 0.0093.
Plot the corrected d against L and draw the best straight line through the origin. The slope is the Sun’s angular diameter in radians.
Multiply the slope by 150 million km to get the Sun’s diameter and compare with 1.39 million km.
Find how many times the Sun is bigger than the Earth by dividing by 12,742 km.
What you should see
A graph of image diameter against distance that is a straight line with a slope of about 0.009, giving a diameter of the Sun near 1.4 million km, about 109 times that of the Earth.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does the image get bigger when the screen is moved back?
The same angle spreads over a larger distance, so the image size is proportional to L.
02Why is the pinhole image round even through a square hole?
It is an image of the round Sun, and the hole is tiny.
03What is the angular diameter of the Sun?
About 0.53 degrees, nearly the same as that of the Moon.
04Why is looking at the Sun through the pinhole dangerous?
A look at the Sun, directly or through any aid, can damage the eyes. Only the image on the screen is safe to view.
Take it further
Use the same angle to explain why total solar eclipses are possible, since the Moon is 400 times smaller but 400 times nearer than the Sun.
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
Science projects
Earth, Space and Geography Models
Solar system, seasons, eclipses, volcanoes and the water cycle explained
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
Find out more about earth and space
Free, well-known places to explore the subject further. Ask a grown-up before you sign up for anything or type your name anywhere.