A diptych dial is a type of sundial that gains its name from the two hinged leaves which open like a book. As with other sundials, the diptych dial uses shadows cast by the sun to tell the time.
This is a modern replica of a scientific instrument made in the 17th/18th century.
How does it work?
When opened, the thread connecting the top leaf of the dial in its upright position to the base acts as the gnomon - the shadow-casting part of the dial. To use the dial to tell the time, it should be placed outside on a horizontal surface, and orientated so that the gnomon is pointing towards true north. This is done with the help of a compass which is embedded in the horizontal leaf.
Once the dial has been arranged in the correct manner, the shadow of the gnomon, cast by the sun, falls onto the main horizontal dial indicating the local time. As the sun travels through the day, the shadow moves across the dial indicating the passing of time.
The dimensions of the diptych are 50 x 80 x 20 mm
Instructions in Spanish, English, French and German
Obviously the sun has to be shining for this device to work! Also it tells solar time, which is not always the same as local clock time - think about how we change our clocks from winter to summer time.
The thread starts from a fixed position on the horizontal leaf. However there are a number of holes that have been created in the top leaf, which correspond to different latitudes. Grand Illusions is based at a latitude of approximately 52 degrees north, and the manufacturers have made these diptychs to correspond with our latitude. However if you want to adjust the diptych to a different latitude, this can be done. You will need to cut the existing thread, and introduce a new thread in the base, using a needle. Then pass the threaded needle through the correct hole in the top leaf, and create a knot, so that the thread is taut when the diptych is opened.