Telling Time by the Sun

Exceptional

This is a universal ring sundial made of brass (1) © Timm Delfs

This is a horizontal sundial made of stone with a brass gnomon (2) © Timm Delfs

The Samrat Yantra of Maharajah Sawai Jai Singh II of Jaipur in Delhi (3) © Wikipedia

This vertical sundial in alumnium was custom-made for its precise location (4) © Timm Delfs

The Glashütte based manufacturer Nomos have come up with a very simple

Sundials have a hard time with watch collectors and enthusiasts. There are no moving parts inside; nothing mechanical that might be viewed through a sapphire case back. In addition, most of these people think that sundials are inaccurate and old-fashioned. But what would Abraham-Louis Breguet have done without the help of a sundial?

Timm Delfs

At the time of some of the most ingenious inventions in complicated watchmaking, there was no telephone time signal and no radio that would tell the exact time once every hour. But there were high-precision sundials that were able to indicate the exact time at a rate that no mechanical clock was able to maintain. Even at the advent of the railway, time at the different stations had to be synchronised using precision sundials. Tables with the figures of the equation of time and the knowledge of one’s longitude helped to convert solar time into civil time.

The earth is clockwork

Normally, sundials do not need any moving parts because they are mounted onto the biggest clockwork system in the world: the earth itself on its orbit around the sun. This ensures that all sundials are synchronised in a particular way, something that definitely cannot be said about the mechanical watch. The earth’s rotation around its axis determines the solar day, whose length undergoes slight variations during one year. These are mainly due to the inclination of the earth’s axis in relation to the plane in which it moves around the sun and can total up to a maximum of 16 minutes. A second factor is the elliptical shape of the earth’s orbit, which causes it to travel at an inconstant speed.

Philipp Matthäus Hahn (1739-1790) had already invented a sundial that enabled him to read solar time with great precision. His invention, a mechanical sundial with a small aperture, had to be rotated around an axis so that a ray of light was projected through the pinhole onto a curved line the shape of the figure 8, called analemma. This display was linked to a mechanism whose gear-train moved a pair of hands over a conventional clock-dial. This kind of sundial was later called a heliochronometer and became standard equipment for railway stations in England. Well-known manufacturers were Pilkington & Gibbs and Negretti & Zambra. Often the correction of the equation of time was incorporated so that civil time could be read directly.

As old as human culture

The history of sundials goes back thousands of years. The first known sundial, dated circa 1500 BC, was discovered in Egypt. With the advent of cities and trade it became necessary to subdivide the day into smaller units in order to be able to organise daily life. All the great civilisations from the Babylonians to the Romans knew some way or other to tell the time by the position of the sun. That we divide the day into 24 units has its roots in Babylonian culture which used a sexagesimal number system. Day and night were divided into six subdivisions (six being considered a perfect number as the sum of its divisors is equal to its double) then 12. The French Revolution succeeded in replacing units of distance, volume and weight with the decimal system but was forced to capitulate before units that were embedded too deeply in people’s minds.

The concept of dividing the day into 24 units has remained, though not without posing certain problems. The Romans still counted temporal hours, meaning that they counted 12 hours from sunrise to sunset and again from sunset to sunrise, no matter if it was winter or summer. This meant that the length of their hours was continually changing. Representing this would later create a problem when the first mechanical clocks were introduced in the 13th century. The Japanese retained temporal hours until well into the 19th century, creating mechanical clocks with variable going rates that had to be adjusted on a daily basis. Refined mechanisms changed from day-mode to night-mode automatically.

Modern sundials

In the 13th and 14th century it was the church that promoted new inventions in timekeeping. In the monasteries the first mechanical clocks were used to help follow the strict timetable even at night-time. The sundial, however, never really disappeared. It continued to be the reference until the telegraph and later the radio made it easier to synchronise clocks and watches around the world. Even today the sundial has its reason to be. Apart from being an embellishment for a garden or a house, it is an astronomical instrument that helps enormously to understand our planet and its course around the sun. There are still some manufacturers left that surprise the market with innovations nobody would have expected. The Bernhardt sundial, for example, features a dial whose division suggests that it can be read to the single minute, which of course it can. An unusually thick gnomon with a curved shape casts a broad shadow whose left edge represents the hour hand. Its special shape automatically compensates for the equation of time, the continuously varying difference between solar and civil time. It is therefore one of the few sundials that directly indicates civil time.

Similarly refined is the Helios Subsolaris, a sundial that not only gives a minute-precise reading of the time, but also the date and the position of the subsolar point, the place on the earth’s surface that is exactly below the sun. It works with a small concave mirror that projects a light-spot onto a translucent screen shaped like the surface of the earth. Carlo Heller, the engineer behind the Helios brand, is also responsible for a globe-shaped sundial showing the actual illumination of our planet and the time by a system of light conductors, inspired by laser technology. Recently, Helios presented a portable world-timer sundial, Icarus, which can tell the time in any time zone regardless of whether the user is north or south of the equator, as the rotating dial represents both hemispheres. The watch industry hasn’t shown much interest in sundials so far, despite the fact that making them better-known could help explain some of the complicated watches featuring equation of time mechanisms. The Glashütte based manufacturer Nomos is one of the companies to have come up with a very simple "shepherd’s dial", a stainless steel ring that can be worn around the neck and which makes an attractive piece of jewellery for men.

While sundials may have many advantages over mechanical clocks and watches, one considerable drawback remains: they only work in direct sunlight. In this context it is interesting to note that paradoxically one of the nations most interested in sundials are the English. ■

(1) This is a universal ring sundial made of brass
It is portable and can be used all around the globe. These instruments were popular in the 18th century.

(2) This is a horizontal sundial made of stone with a brass gnomon
This type of sundials is very popular in English gardens and parks.

(3) The Samrat Yantra of Maharajah Sawai Jai Singh II of Jaipur in Delhi
Singh erected several of these phantastic gardens containing enormous sundials one could walk onto in the 18th century.

(4) This vertical sundial in alumnium was custom-made for its precise location
The figures in the shape of an 8 allow for a correct reading of legal time taking into consideration the equation of time.

(5) The Glashütte based manufacturer Nomos have come up with a very simple "shepherd’s dial"
The portable instrument is based on the principle of the shepherd’s dial, a device that measures time by the hight of the sun. It is not very acurate but fun to wear.

(6) The German engineer Carlo Heller has devised various sundials in a contemporary design
This is his portable world-timer. It shows true solar time as well as civil time of any chosen time zone, including daylight-saving time.

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