A Brief History of the Heliochronometer Sundials

Introduction

The evolution of the heliochronometer is, largely, the story of the sundial and of its own unique development from this seemingly humble, though historically important, instrument. Man has always been pre-occupied with time. Not our 20th century pre-occuopation in terms of catching trains and planes and of being in time for meetings. But much more important pre-occupations to do with the very essence of life and of survival.

Prior to the advent of oil lamps, mankind's activities and achievements were, by and large, dictated by daylight. It was necessary, therefore, to guage how much of it was left for a given task or for a journey. It was even more important to have some method of measuring the passage of time so that the management and rationing of irrigation channels, so crucial to so much of the ancient world, could be equitably achieved. For at least two millenia, no timepiece other than the sundial was available to regulate these basic, life supporting, functions.

Ancient Egypt

Archaeologists have brought to light sundials that were in existence in Egypt around 1500 BC. In the three and a half thousand years since then, evolution has taken the form of infrequent but always significant leaps forward. Man has made sudden advances in his understanding of time and these have been acccompanied by advances in sundial sophistication and precision. It is, perhaps, significant that several of mankind's greatest mathematicians and scientists have played a part in this process.

The early Egyptians, cleverly, had already fixed the duration of a year as being 365 days. Within each of these, they were content to adopt a system of unequal hours - the period between sunrise and sunset being divided into twelve equal periods of time, regardless of whether it was a long summer day or a short winter one. Life was regulated by the sun. Regardless of for how long it was in the heavens, people rose at dawn, started work shortly thereafter, took a midday break when it was at it's zenith (and hottest) and then worked until the cool which accompanied it's setting.

As the great religions developed, so was woven into this pattern the times for prayers and other devotions. In those halcyon days without communications, it didn't matter, nor was it necessarily understood , that others around the globe to East or West were running similar lives but to a different daytime.

In those early centuries, the sundials were essentially horizontal bars, erected with a North/South alignment so that, at sunrise, the shadow came in from the West. It passed over the bar at midday and passed out to the East at sunset. A refinement of this has a shadow falling on, and therefore progressively descending/ascending steps. It is probably a device of this nature which receives double mention in the Old Testament. (II Kings XX and Isaiah XXXVIII).

560 BC

By 560 BC, the Greeks were playing a significant part in the development - and to them must go the credit for instituting an upright pole (or gnomon as it came to be called) whose shadow fell onto, and rotated within, a variety of concave hemispheres or cones. The Arabs, too, were influential at this time as were the Romans who joined in the development in the years immediately prior to Christianity.

It was in the Christian era that the next big advance occurred. It is not known to whom it is attributable, but it was discovered that by slanting the gnomon towards the celestial pole (ie. in the vicinity of the North Star) the swing of the shadow would follow repeatable patterns of the sun every day. Prior to this, the suns's seasonally different orbit (north in Summer, South in Winter) meant that the shadow of a vertical gnomon traced out different hours as the year progressed.

Now it was possible to compute where the hours would fall - regardless of whether the shadow was being made to fall on a horizontal or a vertical face. It was Ptolemy (139 - 191 AD) whose thesis explained this.

Copernicus

Surprisingly, however, equal hours were still not being generally adopted. It was not until after the Dark Ages, when these were introduced by Abu Hassan, an Arab who lived in the 13th Century. Then Copernicus (1473 - 1543) heralded Europe's first, though very major, contribution when he revolutionised the science of astronomy with his treatise that it was the Sun, not the Earth, that was the centre of our particular solar system. Once accepted, this watershed cleared the way for a much better undertsanding of solar time and, indeed, of its seasonal and other vagaries.

In the late 13th Century, clocks started to trickle onto the world stage but it was not for a further three hundred years that mechanical timekeeping was sufficiently widespread to threaten the pre-eminence of sundials. With the 18th Century advent of watches, however, and the regular time which they recorded, it became clear that, if sundials were to be retained as practical timekeepers, they too must reflect regular time. The vagaries of the sun's apparent movement would have to be eradicated.

One of these regular seasonal variations was called the Equation of Time. It was observed that there was a difference between clock and sun time of up to nearly 15 minutes depending on an annual cycle. The Equation of Time was quantified and Tables were produced which enabled one to apply a correction, by the simple expedient of adding or subtracting the relevant number of minutes, to achieve clock time. Such Tables are frequently found engraved on better quality sundials to this day.

Longitude

There was also the problem of positional errors - particularily those associated with longitude. At a given moment, sundials separated in longitude by just a few miles will reflect different times. As could have been observed in the early days of the Egyptian Horizontal Bars, sundials to the East will be in advance of those to the West - purely by virtue of the fact that the sun gets to them first. Thus, at a given moment, a sundial in Dover would reflect a markedly different time to one in Penzance.

The mechanical timekeepers, of course, showed the same time and, furthermore, had established, acrosss the globe, Standard Time Zones - in each of which all clocks and watches kept the same time. Each Standard Time Zone covered a longitude band of approximately 15 degrees but was sensibly adjusted to embrace national geographical boundaries.

Thus the British Isles all keep one Standard Time whilst most of Western Europe keeps one which is an hour in advance. To allow for all these niceties, it was necessary for the sundial to have a means, without disturbing the gnomon, of setting precise angular offsets to the dial.

The Heliochronometer

Dialists, grappling with all these challenges, produced the Heliochronometer. From it's first inception, it had been realised that the use of light through a small slit or hole could be more accurate than that of a shadow - which can be indistinct. The Dial could be elevated in order to cater for the latitude and mechanical gearing could allow the necessary rotational adjustments to cope with longitude and Standard Time.

A series of methods were developed to make the necessary allowances for the Equation of Time and, in all cases, the result sought was a direct reading of Standard Time. 18th and 19th Century heliochronometers set the standard for a line of beautiful instruments, usually heavy, ornate, precisely machined and with levelling screws to ensure correct setting up and alignment.

Although never lending itself to quantity production, a version of the heliochronometer, predominately crafted in bronze, flourished in the UK in the early years of this century. Several examples can still be found both here and on the Continent. It seems to have been the increasing price of bronze after the First World War that spelt the death knell of this talented venture.

Building on the experiences and accumulated knowledge of this rich past, John Gunning has now designed the latest in a historical, but rare, line of Heliochronometer. It is both well timed and well suited to shepherd us into the next Millenium.