Yesterday I attended an IET talk by Mike Groom who is both an IET member and a member of the British Sundial Society. He told us about the history of solar time keeping, sun dials and heliochronometers. He also recounted the story of Erathosthenes who spotted that in Alexandria he could see the reflection of the sun in a well and used that for measuring the circumference of the earth.
I noted that the heliochronometers were typically turned to line the sun up on a target and then used a mechanical computer to adjust for mean time.
Putting this together with Erathosthene’s well, I came up with the following idea. The sunlight would shine down the darkened tube and the time could be calculated from the direction and elevation.
The idea is that you would point the tube at the sun and use the “targetting screen” to get it approprimately lined up. The computer would take over at this point and use an optical sensor in the end of the tube along with a compass and accelerometer to calculate the time. As for any solar time keeper, you’d need to enter the longditude and latitude to calibrate it.
On a side note, Erathosthenes is responsible for my first assembly language program on the Zx Spectrum. The Z80 Assember “Laser Genius” I used had code for the “Sieve of Erathosthene” as one of the examples, page 45 if you follow that link.