The direction of the sun measured against one edge of the octagonal stone pedestal using the angle finder and light slit
A Victorian sundial pedestal stands in the gardens of Riccarton estate outside Edinburgh. It once held a beautiful octagonal dial which is now in safe storage. A replacement dial is to go on show and it must be in a correct orientation. The gnomon on a sundial must point north. A wrong alignment will make the sundial inaccurate, and nothing on the 19th century pedestal actually marks the north.
Measuring the direction of the sun is a good reliable way of finding a true north-south line. The instrument in the photo has a light slit mounted on an angle finder against one edge of the pedestal and directed exactly towards the sun. Calculations using the measured angle of the sun then provide the angle of south. The calculations are the same as finding the declination of a wall for a wall sundial.
The measurements can be repeated for accuracy, and repeated again on a different edge of the pedestal. The results show the pedestal alignment is accurate to 1° for true north-south, which is remarkable. Two centuries ago it was a good installation and it is still good now, nothing appears to have changed since then. The new dial orientation is going to be accurate.