Article of the month October

Foster-Lambert sundial 1

On a horizontal pole style sundial, the gnomon is fixed and points at the celestial pole.
On a horizontal analemmatic sundial, the gnomon points towards zenith and moves on a date scale.
In the figure, these gnomons are designated P and Z.

It is also possible to choose a gnomon exactly halfway between a pole style and a vertical. The figure shows this gnomon in red.
The resulting sundial is called a Foster-Lambert dial.
Its gnomon should also be placed on the correct date on the date scale.

It appears that in the Foster-Lambert sundial, the hour points lie on a circle and are all spaced 15 degrees apart.
The big advantage of this is that the hour scale may simply be rotated for any desired time adjustment.
Here is an example of a pattern for a horizontal implementation.

It is of course possible to design such a sundial for any plane, because that plane is equivalent to a horizontal plane in some other latitude and longitude.
The photograph shows a very nice vertical sundial on an east-declining surface for a latitude of 42.85 degrees. It reads civil summer time.

Particular to this dial is the use of a rotating device to set the gnomon to the correct date.
Even better is that at the same time, the hour scale rotates to take into account the equation of time.
In this way, this sundial indicates clock time.

Note. There exists a second Foster-Lambert sundial solution, where the gnomon is at right angles to the one in the dial described here.