Novel equatorial sundial made by Roger Bunce

Background

Sundials and clocks go together naturally. Indeed, sundials were commonly used to set clocks to time until 1924 – the year the BBC started to broadcast a time signal. An important sundial development in the early years of the last century was the Pilkington & Gibbs Heliochronometer (UK Patent No 10,787, Date of Application, 8^th May 1906. This was reputed to be accurate to about one minute. Since then many other ingenious sundials have appeared in the patent literature.

Pilkington & Gibbs Heliochronometer c1906

An important feature of the PGH was that it indicated ‘Greenwich Mean Time’ (GMT), as distinct from ‘Local Apparent Time’ (LAT) which is the time shown by ‘ordinary’ sundials. The difference between these two time defining systems is significant. For example, in early November an ordinary sundial, located at Greenwich, would be about 16.5 minutes fast, and in mid February about 14 minutes slow, relative to an accurate clock which, in England, would indicate GMT (now termed ‘Coordinated Universal Time’ (UTC). The relationship between LAT and UTC is given by the Equation of Time (EoT). This can take three forms: a table, a graph, or a mathematical expression. Confusingly, any of these forms is normally referred to as the EoT. In addition, the time shown by an ordinary sundial differs from a similar dial located at Greenwich if it is physically moved east or west of Greenwich or if its latitude is changed. The PGH could also be set to correct for these changes in location.

The PGH was no doubt accurate. However, the price paid for accuracy was that it was not so easy to use as an ordinary sundial. One had to ensure that the month dial was set correctly and then physically rotate a disc so that the sun shone through a pinhole (‘Sight’) in order to project a spot of light centrally on an engraved line (‘Screen’). The time was then read off from a scale. I wanted to make a sundial which indicated UTC but which could be read more easily - rather like a clock.

My Sundial

My design is based on an equatorial sundial. The important features of this type of sundial are: The gnomon is parallel to the earths rotational axis, the dial plate is set at 90 degrees to the gnomon, and the dial plate is divided into equal radial divisions. As indicated earlier, in order for any sundial to function properly it has to be adjusted according to its latitude and longitudinal location. This together with other adjustments and features are as follows:

Orientation – As with any equatorial sundial, the gnomon is aligned north/south with the dial face ‘looking’ north (axial north rather than magnetic north). In other words, the person reading the time is facing south.

Latitude adjustment - The screws within the large supporting tube are slackened and the dial turned about the axis of the tube. This is only necessary when first locating the sundial or if it is moved to a different location.

Longitude adjustment - The is made by slackening the thumbscrew under the dial assembly and rotating the dial assembly about the gnomon axis. Again, this is a constant angle for a given location.

UTC or British Summer Time – This is selected by slackening the thumbscrew under the dial assembly and turning the dial forward or backwards an hour.

EoT correction – This is simply done by turning the month dial so that the correct month and week are adjacent the pointer. The month dial incorporates an EoT cam. The design calculations were done using MathCad.

Summer and Winter months – Like any other equatorial sundial, in the summer the sun shines on top of the dial plate and in winter below the dial plate, according to the dates of the spring and autumn equinoxes. For ease of reading during the winter, I wanted the shadow cast by the gnomon to be visible from above. To allow this, the main dial is provided with an ancillary dial with a non-graduated inclined surface (painted white). This ancillary dial can be turned axially relative to the main dial to prevent the suns rays being obscured by one end or other of the dial. This allows the time to be read during all 'sunshine' hours during the winter. Gnomon – One of the problems with some equatorial sundials is that the gnomon is susceptible to damage. The gnomon of this sundial is made from Nitinol (super elastic metal) which allows it to be bent through at least 90 degrees and recover immediately.

Motto – Sundials should have a motto! The motto on this dial is 'Brother Sun…' from the Canticle of the Sun, Saint Francis of Assisi c1225: ‘Be praised, my Lord, with all Thy creatures, above all Brother Sun who gives the day and lightens us therewith’.

Sundial Drawings

A more detailed description of the Sundial appears on http://modelengineeringwebsite.com together with a complete set of drawings for making the sundial. These are dimensioned in metric and are available free to down load. Prospective builders will need access to a lathe and small vertical milling machine, as well as the normal range of engineering workshop tools. The Sundial is primarily aimed at model engineers or horologists looking for 'something a bit different'.

Conclusion

This sundial was designed and made to explore new ideas and in this respect is considered a 'concept model'. As it stands, it is not suitable for continuous outside use, because the base is made from wood and the dials are paper. However, now that the design is proven, these can be made more durable. The dials would normally be engraved and the base comprise a brick, concrete or stone pillar. In addition, brass was used for most of the parts (for ease of machining), but in future stainless steel may be considered.

The Sundial is simple to make, easy to set up, easy to read during any 'sunshine hours' throughout the year, is accurate to within about two minutes and, most importantly, unlike 'ordinary sundials', it indicates UTC ('clock time') directly.

Page last updated 15 March 2010