>Following on from Rolf Menzl's message, can anyone tell me if there is a >way of determining the declination of a wall RAPIDLY with a single reading >AT ANY TIME OF THE DAY ? >The method to find true north using a horizontal board with a vertical >gnomon, whereby measurements are made either side of noon and the positions >of shadows marked at the intersections of graduated circles, and then >measuring the angle between north and the wall, works fine, but requires a >whole day to perform. In Great Britain, which does not qualify as the >sunniest place on Earth, it is important to be able to make a single >reading at the moment the sun shines, which almost certainly will NOT be >conveniently at noon ! It is also quite likely that the sun will shine in >the morning and then, annoyingly, not in the afternoon, so a single >measurement would be most useful. >
To Peter, Rolf and Daniel Your method of using the sun to determine where south is will be inaccurate except on the four days in the year when the sun's shadow crosses the vertical axis of the analemma, because of the equation of time. My suggestion would be to ask one of your geological friends to borrow his Brunton compass, then subtract off the magnetic deviation from north found from an OS map. This will be accurate to about one or two degrees. This error will not affect the accuracy of your dial as much as the fact that it does not take account of the equation of time. If you really want an accurate declination of your wall I suggest that you measure it on a starry night. Make a rectangular sighting board. Place one edge against the wall and look towards the west up the other edge until a known star is occluded by the end of that edge. Note the exact GMT when this occurs. Repeat for a few other other known stars. Now use the facilities offered by a planetarium program like Redshift to determine the declination of those stars at tose times, and average the results. (Redshift takes account of the equation of time.) Malcolm McClure
