On Sat, 28 Aug 1999, Peter Abrahams wrote: > Used for timing meridian passage of the sun, accurate > to a few seconds. It was a simple device, a hollow 90 degree prism, 2 > sides silvered. At meridian, two images of the sun - one from glass side, > one reflected off both mirrors - would coincide.
I had to think about this for a minute, then it became clear - and very elegant! If I understood it right, the two narrower faces of the 45-90-45 prism are silvered, and the hypotenuse left clear and polished. Orienting the prism so the hypotenuse facess the Sun's Azimuth, you get one reflection from the double rear faces that tracks the Sun, *regardless* of the prism's orientation, and another from the hypotenuse, that moves at twice the Sun's rate. When they coincide, the prism is exactly on the solar Azimuth. Mount the prism accurately facing South one time, and you have a good indicator of Meridian crossing... Dave
