Advanced setup

For absolute precision, a horizontal sundial should have the angle between the gnomon and the horizontal exactly equal to the latitude of the place. In practice, so long as the difference is not more than 1 or 2°, the errors in time measurement are very small. In particular, unless you are confident that the plinth or surface on which the sundial is mounted is level to within ½°, there is no point in trying to compensate for these small differences.

If you are sure you have a level surface, you can compensate for differences in latitude using this method. What you are trying to do is to get the gnomon exactly parallel to the earth's axis, as it would have been if it had been mounted on a level surface at the latitude marked on the dial.

You do this by slightly tilting the dial, using a packing strip, which should be of wood, plastic or brass. Do not use any metal other than brass, since it may set up galvanic corrosion at the interface of the two metals. If you are to the North of the latitude stamped on the dial, you put a shim under the northern edge, and if you are to the south, you put a shim under the southern edge.

Please be careful to ensure that the edges of the dial-plate are lined up to north-south. Otherwise, when you put the shim under the northern (or southern) edge of the sundial, you will also move the gnomon sideways, so that it is no longer at right angles to the east-west line.

The baseplate is 17 cm. square, so a tilt of 1° would require one edge to be lifted by 2.97 mm..

To calculate the thickness required for other angular changes, you can interpolate. Each angular change of 1' requires a change in the thickness of approximately 0.05 mm. or 0.002in.

Do not be too concerned about getting the exact thickness, since without specialised equipment you will not be able to get the base which supports the gnomon exactly level. So, in practice, go to your nearest shop selling models (aircraft, boat kits etc) and see what they have in stock. 

If you are north of the latitude stamped on the dial, you need to put a shim under the northern edge of the dial-plate. For example, if your Spot-on Sundial is in Leicester, England at 52° 38N, you would need a London dial which is stamped with 51° 30N, so you want a shim which will produce an angular difference of 1° 08. The table above shows that we need a shim of 3.37 mm. (In practice, we would get either a brass strip of 4.1 mm. or a plastic one of 3.4 mm.)

If you live in Providence town. Mass. on Cape Cod, at 42°N 04', you will have the Spot-On Sundial model D (Milwaukee) centred on 43°N, so the correction to be made is 56'. From the table, this requires a shim of 2.97 - 4 x .05 = 2.77 mm or 0.109 ins. Because you are to the south of the latitude stamped on the dial, you need to put the shim under the southern edge of the dial-plate.