How it works
A pinhole projector uses a basic optical principle: light passing through a small hole forms an upside-down image of its source on a surface behind it. Each point on the Sun sends light through the hole in a straight line, and together those rays draw a small picture of the Sun.
On an ordinary day the image is just a bright circle. During a partial solar eclipse, it becomes a crescent, and during an annular eclipse, a ring. Because you only ever look at the projection, never at the Sun itself, it is one of the safest ways to watch.
Two-card projector
You need:
- two sheets of stiff white card or paper;
- a square of aluminum foil;
- tape, scissors and a pin or thin needle.
Steps:
- Cut a square hole about 2 to 3 centimeters wide in the middle of one card.
- Tape the foil over the hole, smoothing it flat.
- Poke one small, clean hole through the foil with the pin. A round hole gives a sharper image than one punched straight through card.
- Stand with your back to the Sun. Hold the foil card up so sunlight passes through the pinhole.
- Hold the second card behind it as a screen, in the shadow of the first card. Move it closer or farther until a small, sharp image of the Sun appears.
Box projector
A box gives a darker viewing area, so the image is easier to see.
- Take a long box, such as a cereal box or shipping tube. Longer is better.
- Tape a sheet of white paper to the inside of one end.
- At the other end, cut two holes side by side: one for the pinhole and one to look through.
- Cover the first hole with foil and make a pinhole in it.
- Stand with your back to the Sun and aim the pinhole end toward it. Look through the viewing hole at the white paper, where the image of the Sun appears.
How big will the image be?
The Sun spans about half a degree of sky. As a rule of thumb, the image diameter is about 1/108 of the distance from the pinhole to the screen.
| Distance from pinhole to screen | Approximate image diameter |
|---|---|
| 30 cm | About 3 mm |
| 1 m | About 9 mm |
| 2 m | About 2 cm |
| 5 m | About 5 cm |
A short box gives a tiny image. To get a larger one, project across a room or onto the ground from a balcony. A smaller hole makes a sharper but dimmer image; a bigger hole makes a brighter but blurrier one.
Projectors you already have
- Trees: gaps between overlapping leaves act as natural pinholes. During an eclipse, the ground under a leafy tree fills with crescents.
- Kitchen colander or perforated spoon: each hole projects its own small Sun.
- Crossed fingers: overlap the fingers of both hands in a waffle pattern and let the light fall on the ground.
- A card with holes: punch holes in a pattern, a letter or a word, and each hole will project the eclipse shape.
Safety reminders
- Keep your back to the Sun the whole time.
- Never look through the pinhole, a colander or your fingers at the Sun.
- Supervise children so they do not glance up between looks at the screen.
For direct viewing you need eclipse glasses that meet ISO 12312-2, as explained in the guide to solar eclipse eye safety. For dates of upcoming eclipses, see the eclipses page.