Hi Patrick, John C and the List, The problem of the shadow not properly filling the noon gap is the same umbra/penumbra related one which affects the reading accuracy of dials with 'solid' gnomons at all times of the day. This is usually ignored by dial makers but I know of one historical example where the maker seems to have offered a possible solution. An article by Vicki de Kleer (BSS Bulletin 19(iii), pp.116-117, September 2007) describes an 1890 brass horizontal dial made by the London maker Charles Baker for Sumburgh in the Shetland Islands at latitude 59 degrees 52 minutes N. The dial has a comprehensive Equation of Time table engraved around its periphery and also the two intriguing instructions: "AM Sub.[tract] 1 min" and "PM Add 1 min". It seems to me that these instructions are implying that the true time is being indicated slightly further into the penumbra area of the shadow than the observer might normally expect. With a dial so far north, the effect of 'shadow tapering' will be more prononounced than on most other dials. Does anyone have a different interpretation of these instructions, or know of other dials wich have similar ones? Regards, John Davis --------------------------------
Patrick Powers <[EMAIL PROTECTED]> wrote: > If you make the dial with a tublular gnomon, and you design the face so that the time is read from the center of the shadow instead of the edges of the gnomon's shadow, then you eliminate the Noon Gap. Do you think this is the best solution? < >From the point of view of an accurate dial, one that can be calculated and one that members of the public can read easily, I am certain that it is. I have designed two 12m (or so) diameter dials this way and, by a proper choice of gnomon diameter the shadow is very easy to read. Indeed, if you want a bit of fun, if you have Google Earth and care to type in 52 45' 16.28"N, 01 08' 12.33" W then you go directly to the second of these dials. It's in Barrow-on-Soar in Leicestershire, UK. The time can even be read from space (well, with good magnifcation it can ). Google Earth's imaging is so good that I have even thought of trying to calculate what day that photo was taken - though I haven't done so yet. (If anyone wants to try, the gnomon length was such that the shadow on the summer solstice just touches the outer diameter of the chapter ring which is just inside the 'mini-stonehenge' like stone circle). Regards Patrick --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
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