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Sundials, Gnomon

- A sundial works by using an indicator called a gnomon to cast a shadow onto a graduated surface. The origins of this system can be traced back to before the first century BC. Sundials were extremely important for over 1000 years until clocks and watches became accurate and freely available. There are two basic categories, fixed and portable. It is the fixed dial which can be free standing or secured on, or built into a building which is of interest in conservation terms. Many freestanding dials are listed, either as individual list entries or by virtue of the curtilage rule, and many of those fixed to buildings are specifically mentioned in list descriptions. They can be found almost anywhere on castles, cottages, churches and tombstones. Fixed sundials can be divided into two groups, horizontal dials which are drawn level with the horizon, or vertical dials which are drawn perpendicular to it. Early sundials were not particularly accurate because gnomons were set vertically or horizontally, which in theory would only tell the time accurately on the equator. In early medieval times accurate dials, in which the gnomons were inclined parallel to the earth's axis to take into account height and direction of the sun, and were capable of indicating hours of equal length appeared. During the Renaissance, the science of Gnomics or the Art of Dialling as it became known, was established as an important mathematical discipline.

Many sundials contain mottos and those fixed to churches often have the times of services marked on them, usually with a cross. Scratch dials were dials scored onto the walls of churches to indicate the times of services. Sundials, as well as being carved, were also probably painted onto limewashed surfaces. Gnomons were wood or metal and are therefore frequently missing on earlier sundials, and it is important to note that the gnomon would not always be contained within the dial but could be some distance away from it. Stained glass sundials were relatively common in the seventeenth century, and those that survive should be particularly treasured as practical combinations of science and art.