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 dialplate
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. (see note on suppliers
If you are north of the latitude stamped on the dial, you need
to put a shim under the northern edge of the dialplate. 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 dialplate.
Brass strip is supplied by K and S of Chicago, and widely distributed
in model shops in the USA and in many other countries. It is available
in thicknesses of 0.016, 0.025, 0.032, 0.064 and greater. (These
are equivalent to 0.41, 0.64, 0.81, and 1.63 mm). You can obviously
make up intermediate values such as 0.041 and 0.048 ins. by using
Plastic strip is available in a wider range of thicknesses, and
is equally suitable. In narrow thicknesses, it need not be unduly
obtrusive. In our local model store, you can buy plastic strip in
thicknesses of 1.6, 2.0, 2.7, 3.2, 3.4, 4.6 mm and greater.