>That looks great using GE. When I diplay the raster in GE I get the impression that the whole dial is slightly turned to the East. Is that correct or is it an effect in GE? Thibaud<
Interesting you noticed that - I did too - slightly to my consternation at the time!. I first saw it when I moved the dial image to lie under the compass on the GE screen and when you do that it certainly looks like that. However the GE compass is not particularly accurate and it doesn't move as one goes from one part of a map to the other. A vertical line at the left hand edge of any global display or map might point true North but one at the right hand side will not. To try and check this out (as I did when I first saw that effect) you attempt to place a placemark (pin) precisely at the gnomon root and then at the North end of the 12 o'clock line, the displayed Longitude at the bottom of the screen stays the same at 01 08' 12.32"W. That and the fact that the dial is correct to within a minute or so convinced me that the alignment is close to N-S. The original determination of the N-S line was done by precalculating the shadow angles of a vertical pole for each minute of the day when I would be there and then noting the shadow positions on the ground at several (I managed to get six) times during the day when the sun was shining at one of the precalculated times. Triangulation then led to six points clustered close together well beyond what would later be the dial's chapter ring and I took a visual mid point of all six. In fact I think that the N-S orientation is probably better on this dial than is the alignment of the gnomon, because its fabricators made a jig by which the gnomon's securing bolts could be set in concrete before the gnomon was lowered onto them and fixed. Unfortunately the jig was designed to look symmetrical which ever way it was viewed and it was delivered without any information as to which way up it should be placed. I suspect that in the builder's haste to have the thing erected before the pre-arranged official opening day, the jig might have been placed upside down or not itself properly aligned N-S. At least that is my current thinking as to why the time error is about a minute. The other similar sized dial that I designed for Amble in Northumberland is (or was!) accuarate to a few seconds at some times of the day. Incidentally, the Amble dial also shows the effect you describe when you look at it on GE - go to 55 20' 05.87"N, 01 34' 53.78"W to see it. (The dial there is 17 or so meters South of a small 'amphitheatre') - but here again the dial shows solar time correct to within a few tens of seconds at the times of the day that I have checked it. Interesting, because when setting out true North (it had to be done in November!) I was only able to get one measurement for N-S in the two days that I was there and then only for less than a minute! Very lucky! Thanks for raising that interesting point. Patrick --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
