Patrick powers is correct in noting that the 0.5 degree width of the sun (corresponding to 2 minutes of time) creates a shadow penumbra that is virtually impossible to read to the second. But, a sharp edge can be achieved by a focusing dial, which creates an crisply defined image of the sun, 2 minutes of time wide, with no penumbra. If the leading, or trailing edge of this image is chosen as the indicator, then it would be possible to read it to a few seconds, and perhaps to 1 second, on a large dial scale. Alignment becomes the limiting factor for such a dial. A focusing dial that is accurate within 60 seconds all day long (and oftern within 30 seconds) can be seen at www.precisionsundials.com/renaissance.htm
Bill Gottesman Burlington, VT In a message dated 12/20/2001 5:03:43 AM Eastern Standard Time, [EMAIL PROTECTED] writes: > Subj: accuracy > Date: 12/20/2001 5:03:43 AM Eastern Standard Time > > Greetings to all > We all know that the atomic clock has the highest possible resolution, while > for sundials 2 to 1 minute seems to be the best achievement. I have a > feeling 1 second could be obtained considering what follows. ( I consider a > second ideal as one can feel it, I mean it is a timespan well related to the > human body, one second you live, the next you may be dead; it is also > somewhat > related to the heartbeat) > Consider a horizontal sundial whith the 15 degrees hour lines; it is > impossible to trace minute lines when keeping usual dimensions for the dial. > However, if we go far away from the base point of the gnomon, the portion of > the arc between 2 hours, becomes larger; so imagine we go to where 60 lines > can be drawn between two adjacent minute lines and we keep only the top part > of the line 11h59m59s and try to draw it on the ground related to a very > accurate positioning of the local meridian. These data allow also to > calculate the height of the gnomon tip. If both dimensions are feasable to > realise, we would have the desired accuracy of one second; of course the > positioning of the meridian is of paramount importance and probably the most > difficult to materialise. ( is this a nice subject for a university ?) Of > some important influence is the width of the gnomon. The indication of noon > time is never accurate because of this width, while the shadow cast for the > earlier hours, may be considered as a hairline and is thus more accurate, > which means that the morning part of the dial is more accurate if the shadow > hairline is taken into account for determination of the meridian. So in > fact, sundials should have a gap at noon corresponding to the width of the > gnomon, and the afternoon part should be calculated in function of the right > hairline shadow > related to a second parallel meridian. > Very interested to read your comments > Walter
