Msg: 6982 *Conference*
03-21-97 19:23:19
From: RON WIESEN
To : COMET _
Subj: REPLY TO MSG #6962 (CYBERLIFE)
It's like a sundial except it's a sun clock because it gives MEAN TIME like what your watch gives. Sundials give LOCAL TIME which leads or lags mean time by up to 16 minutes through the course of a year. With sundials, one needs a correction chart which allows one to mentally add or subtract a correction based on the date -- mental mathematics required. My sun clock is ANALLEMATIC and the brass tube (gnomon) has aperature slits that do the math for you. The shape of the slits can be thought of as an analog computer that runs on sunlight. Given solar declination which relates to day of the year, the slits pass a narrow beam of light to fall on the time arc at the correct MEAN TIME. The wide shadow cast by the tube falls on the time arc in a width of exactly 1 hour with the shadow edges at LOCAL TIME - one edge leading by 30 minutes and the other edge lagging by 30 minutes. So one can "interpolate" the LOCAL TIME from the shadow edges and one notes MEAN TIME precisely by the narrow light beam which is within the hour wide shadow. So much for the sun clock portion. The base "box" that the sun clock sits upon is the calendar portion. Looks innocent enough - just a rectagular base. East and West sides of the base are normal. North side has a little white placard sticking out and a "pin hole" in its otherwise normal surface. South side has a hyperbolic slit which is another analog computer shape. Only on November 21 is the midday path of the Sun such that slit and pin hole align. OK, truth is there is one other day of the year where alignment occurs but I can't prevent that and the mother-in-law won't be looking for a rainbow display on that date - so no big deal. Just inside the box on the North face is found: o a small glass prism (chunk of broken window pane) that refracts sunlight coming from the South and spreads a colored spectrum (rainbow) outside and onto the placard where it is seen. o the "real pin hole" of alignment. The diameter of this pin hole aperature is based on its distance from the slit on the South face and width of said slit, and the apparent angular diameter of the Sun (for the date of interest -- it varies a little being largest on January 1 because Earth is closest on New Years). Regarding the slit on the South face, it's long enough to accomodate the midday time span (50% of sunrise to sunset duration).