Dear Diallists, There were several mentions of the helical sundial in the BSS Bulletin some time ago, but I don't have the references to hand.
As far as I understand it, it is arranged on a polar axis (like the gnomon of an equatorial dial) and therefore the Sun moves around by 15 degrees every hour. If you mark two points where the equatorial plane intersects the helical surface at an E-W line, which are one half twist apart, then the shadow terminator will move between them during the 12 hours from 6 to 6 LAT at the equinox evenly (assuming a regular spiral) upwards or downwards, depending on the sense of the spiral. Half-way between them the flat of the spiral will lie N-S and the shadow terminator will be there at noon. You can therefore mark hour lines evenly across the spiral by dividing the half-twist into twelve equal parts. An easy way to make one to play with is to take a wide piece of elastic ribbon material and stretch it with a whole twist, supporting the two ends on a convenient frame (e.g. a wire coathanger suitably bent!) Then use the middle from 1/4 to 3/4 of the way up with the supported ends lying in the N-S plane and the axis pointing at the celestial pole. (That's not my original idea - I think the elastic was suggested in the BSS Bulletin but I'm afraid I do not remember whom to credit.) Now this is fine at the equinox, but the Sun's declination will throw it out at other times. I believe the effect is to separate the two shadows (on the two sides) by an amount depending on the width/pitch ratio, and if that *is* so then if both sides are graduated (the dark/light moves along one as the light/dark moves along the other) their average might be correct. Would anyone else like to comment? Andrew James N 51 04 W 01 18 >
