Transit of Venus Viewing at Practically No Cost

5 05 2012


Subject: Transit viewing at practically no cost

I have read about a scheme that was used successfully to view an eclipse. A large wall was in complete shade and a notice invited passers-by who had a hand mirror to stand in the sunlight and reflect the light onto the wall. I do not know how far they would be from the wall but the reflection was close enough to see the progress stage of the eclipse. It is like re-inventing a pinhole camera and decreasing the effects of diffraction at the same time as the “source” is much larger than a pinhole.

My comments on the situation:

What is a pinhole camera? Basically it is a device for getting some light into a dark space with the proviso that the source of the light is small. Being a pinhole in a card does not change the nature of the sunlight so the source does not have to be a hole in something; it can be a small flat mirror or a larger one with most of the surface covered with black plastic adhesive tape and what’s more it would not even have to be circular as long as it is small – see next paragraph. You do not even need a box – you can use a darkened room which has a small conveniently-placed window or even a doorway for the projected beam and the screen can be a wall or a sheet of something with pale colouring inside the room. There is a drawback of course – you might need a distance of 20 to 30 metres from mirror to screen so you may have trouble finding a suitable place for the mirror as the Sun’s position changes unless you have an assistant with a larger flat mirror which puts another part into the light path. Once you find a suitable spot for the small mirror, direct the sunlight from the larger mirror to the smaller one. As time progresses, only the larger mirror would need to be adjusted by tilting it so I guess that the assistant would not be able to view the screen and would need to be relieved now and then to see the progress. The viewers would be inside the darkened room out of the light path.

Is it part of your experience during a partial eclipse [or at least during a partial phase] that you were under a tree, each small gap in the foliage, each with its own shape, produced, on plain ground, a useful picture of the partly covered Sun? In my youth, we had an old garage whose roof was recycled corrugated iron containing many (unoccupied) nail holes . In effect, the old garage was a batch of pinhole cameras. Every hole gave us a very good picture of the eclipse projected on the floor which most certainly was not white or even light coloured and the holes were only reasonably close to circular. This works for the tree because the size of the gap in the foliage is small compared with the distance from the gap to the ground. Here lies the reason that the small mirror in the scheme above does not even need to be round. The passers-by (above) would have had mostly rectangular mirrors. I have not yet tested this projection method so I cannot predict a good size of mirror but I expect that a diameter of 1.5 to two centimetres at 20 to 30 metres should be a starting point.

All sorts of variations are possible. e.g., masking a rear vision screen:
Remove top and bottom of a carton and put a light paper over one of the open parts. Face the other opening towards the mirror so that the box and screen are in the darkened room and viewing is from behind.

Bob Roeth NSAS




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