Most of us regard a sundial as an attractive ornament for a park or garden. Their effectiveness as time keepers is highly variable
That's unfortunate, because it is not at all difficult to ensure that your garden sundial will be an accurate timepiece, provided, of course that sun is shining.
But that will be covered in another article. For now, let's see what a sundial is, and what it is capable of.
We forget in this modern age that accurate, affordable watches and clocks have been around for much less than 200 years.
Before then, sundials were one of few ways to tell time with reasonable accuracy.
Shadow clocks dated at 1500BC are known from Egypt, but first dials appear to have been Babylonian. The Greeks adapted idea, Romans developed it further, and by about 100BC had perfected horizontal sundial (and placed it in their gardens).
Even in ancient days some people had schedules to keep, and both agriculture and religion required knowledge of seasons and movement of sun to determine planting and timing of ceremonies.
The sundial was an important means of providing that information, and considerable advances in mathematics, geometry and astronomy were made while it was perfected. The knowledge gained forms part of foundations of modern science.
Types of Sundial
There are four reasonably common types of sundial.
They all have two things in common. Each consists of a raised structure, called gnomon (silent "g") which casts a shadow onto a plate called dial. The dial is divided into hourly or shorter time divisions and may also show other information.The part of gnomon whose shadow indicates time on dial is called style.
The most abundant form is horizontal sundial, happily sitting on its pedestal or column and adding beauty and interest to home garden.
Related is equatorial sundial, with its dial oriented at same angle as latitude. It works slightly differently, and is easier to use when properly calibrated.
Thirdly, there is vertical sundial, ideally located on a wall facing due south in northern hemisphere, and north in southern hemisphere. The principle is much same, but sundial only occupies a semicircular area. Vertical sundials displayed time to public, and were used to correct unreliable public clocks.
And most elegant of all, portable sundial. George Washington had one - at that time pocket watches were most unreliable. Modern examples can be a work of art. They combine a compass with an adjustable dial. The dial is tilted to correspond to local latitude, and compass defines north. Pretty neat!