The  Transit of Venus was observed in Trinity College Cambridge by a large number of people. Several methods for viewing the Transit were available, the simplest and safest being the “reflected pinhole”, described below.
Eye Safety - do not look at the sun directly
The simplest and safest method to produce a clear image of the Transit is to use a ‘reflected pinhole’ which is just a small mirror, as shown here, blacked out with tape leaving a small square, say 5mm x 5mm (1/4” x 1/4”) (the smaller the square the sharper the image, but it will be less bright). If you reflect the sunlight through the window onto the wall of a darkened room then you get a perfect circle which is the disc of the sun. The mirror needs to be about 30 metres (30 yards) away from the wall to get a sharp image. You should use a smaller square (pinhole) if you want a shorter distance. Use some putty or plasticene to fix the mirror to a table, chair, wall or post - the mirror needs to be held still to prevent the image from wobbling.
The method has many advantages:
- cheap - all you need is any old (bit of) mirror and some tape.
- safe - observations are made in a darkened room, away from the sun (no sunburn!)
- perfect for mass viewing - many people can stand around and admire!
- easy to explain - no complex optics
- perfect for all solar observation (transits, sunspots and eclipses)
- really fun, safe and easy for children
Now all you have to hope for is a sunny day …
This is what I’m hoping to do tomorrow, but the forecast isn’t looking good. :(