Sundials and Dialling

Sundial: an instrument for telling the time and/or date from the position of the Sun. More generally, it can give any function of the Sun's co-ordinates.

Dialling: the art and science of designing and constructing dials.

Diallist: one who designs or makes dials.

Figure 1 illustrates many of the parts of a sundial.  Although the dial pictured is a horizontal sundial, the terminology applies to many other types of dials.

Figure 1: The parts of a sundial - Northern & Southern Hemispheres.

Centre (of a dial): the point where all the hour lines, and a polar-pointing style, meet. This point does not always exist (e.g. on a polar dial or direct East or West dials). In simple horizontal and vertical dials, this point coincides with the root of a (thin) gnomon. In the case of a thick gnomon having two styles, there are two centres to the dial. The centre is often, but not always, the origin of the co-ordinate system used to describe the dial.

Declination (of the Sun): the angular distance of the Sun above or below the celestial equator. Its value follows an annual sine wave like curve, varying between 0° at the equinoxes and ±23.4° (approx.) at the solstices. It has positive values when the Sun is above the celestial equator (summer in the Northern hemisphere) and negative when below.

Declination Lines: lines on a dial showing the Sun's declination on a particular date. They are read by observing the shadow of a nodus.

Dial Plate (Face): the physical plate on which the hour lines and furniture lie. It (usually) supports the gnomon.

Furniture: all features on a dial plate other than the hour lines and their numerals are referred to as dial furniture. These may include declination lines, compass rose, Equation of Time graphs or tables, mottoes, etc. Other common furniture includes: date, maker's or benefactor's name, coats of arms, and latitude and (rarer) longitude.

Gnomon: the physical structure of a sundial which casts the shadow.

Hour Line: the line on a dial plate indicating the shadow position at a particular hour (includes fractional as well as whole hours).

Nodus: a point which casts a shadow to indicate the time and/or date on a dial face. It may take the form of a small sphere or a notch on a polar-pointing gnomon, or it may be the tip gnomon with an arbitrary (usually horizontal or vertical) orientation.

Noon Gap: the gap on the hour scale of a dial to account for the finite thickness of the gnomon.

Style: the line in space which generates the shadow edge used to indicate the time on the dial plate. Note that a gnomon with finite thickness will have two styles (one along each of the upper edges) which will each be operational for parts of every day. If the gnomon is the form of a long rod, the style will be the virtual line running along the centre of the rod and the dial is read by estimating the centre of the shadow.

Style Height: of a polar style is the angle that the style makes with the sub-style line.

Sub-Style: the line lying in the dial plane which is perpendicularly below (or behind for a vertical dial) the style.

Anyone can design and build a sundial!

Designing a sundial, even one that incorporates corrections for such things as a wide gnomon or longitude, is not very difficult.  If you enjoy mathematics or have a friend who can help, calculating the hour angles only takes a little time.  From this site you can download some spreadsheets that will assist you with these calculations. There is also free software available that will draw the sundial for you. In most cases you will have to transfer the design to the material you are building the sundial from. Some software also allows you to transfer the hour lines to a computer aided design or other illustrative software packages where you can add additional features to your sundial.

When you have chosen the sundial you wish to build and have determined the hour angles, you can construct a simple sundial from cardboard. This will allow to test your design and correct any problems before you build the final sundial. The cardboard sundial can be very simple and only include the hour lines and the gnomon.  Once you are satisfied with your design then it can be transferred to the material that will become your sundial. This material will depend upon what you are confident working with. Better yet - try something new! There are numerous photographs of sundials on the internet and from these you can get many ideas for building your own sundial.

If you enjoy woodworking, you can build a sundial using any type of wood you like. The hour lines and other dial furniture can be carved, engraved or inlaid. If you enjoy painting then you can use sign board, metal or even the surface of a wall as your dial plate canvas. Perhaps you enjoy working with metal, stained glass or stone. Once you have established the position of the hour lines and gnomon, how your sundial looks when it is finished is only limited to your imagination.

If you enjoy using computer aided design or other illustrative software packages then you have an opportunity to create designs that can be taken to particular manufacturing companies that can build your sundial or portions of it for you. This type of design can be used to engrave your sundial on a metal plate or to cut rubber resist that can be applied to stone prior to sandblasting your sundial into any type of stone you want. For intricate designs it is even possible to have the design laser engraved in stone or metal.  In any case, you have a lot of options.

The point is that there are so many ways to build a sundial that you are sure to find one that will be fun for you. And if you are careful at certain stages of its construction, you are sure to build a sundial that will work well and look great.

If you have any questions, just send me an email and I will try to answer them as best I can.

HAPPY DIALLING !!!