Pocket Sundials
Today the idea of a pocket sundial might strike people as bizarre. However in earlier times before mechanical pocket watches became generally affordable the pocket sundial was very popular. It's accuracy was someone questionable but it could at least give a rough idea of the time.Since pocket sundials always included a compass they were popular wih travellers, especially those who needed to carry a compass anyway.
A pocket sundial could also be a status symbol. It is said that King Charles I used a silver model. Some examples are encrusted with gems and some are so small that they are unlikely to have had much practical value. Today some people still like to buy pocket sundials as curiosities.
How Does a Pocket Sundial Work?
The principle of a pocket sundial is exactly the same as that of its fixed counterpart: the sun shines on some sort of indicator - the gnomon - that casts a shadow onto a dial. The difference is that fixed sundials have to be carefully calibrated for both compass direction and location.Calibrating for direction is easy - simply use the enclosed compass. Compensating for location can be more tricky.
There are two basic forms of pocket compass. The simplest has a flip-up gnomon attached to the dial. This version has no calibration and becomes increasingly inaccurate the further the owner travels from home. This was clearly inappropriate for travelers or explorers.
Diptych
The more advanced pocket sundials were the diptych variety. Here the lid of the sundial lifts and the gnomon is a piece of string which runs between the dial and the lid. Changing the elevation of the string is effectively the same as changing the angle of the gnomon on a fixed sundial. This allows recalibration for different locations.Most examples of the diptych variety had a small number of holes in the lid section. The user would look up a list and choose the most appropriate hole for the gnomon depending on where they were currently located. As a form of calibration this was crude but normally good enough for most people.
Note that a diptych pocket sundial is not necessarily capable of calibration - some are built in this style yet with the string attached to a fixed position on the lid.