Understand the origin of time measurement

What is a sundial?

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Thursday, 05 March 2009 16:30

Sundials are perhaps the most ancient of scientific instruments, and the earliest known form of time-keeping. They probably started life as poles in the ground with the direction and length of the shadow giving an approximate time of day. However, by the middle of the second millennium BC there were already fixed and portable versions of more accurate sundials being produced in the Middle East, Egypt and China. These early sundials relied on the height of the sun in the sky to indicate the time by the length of the shadow it produced.

This fabulous site is a treatise on Sundials, Calendars, Armillaries etc. I have copied the pages on sundials here for two reasons. One, I found the site easy to get lost in. Two, academic sites tend to suddenly disappear. I do encourage you to go to this site, however, and read more, in depth, with bibiographies and larger pictures.

An ivory diptych sundial.

An ivory diptych dial made in Nuremberg by Johann Gebhart in 1556. Nuremberg was famous for its ivory sundials during the sixteenth and seventeenth centuries. From the Whipple Collection. .

More sophisticated forms were developed by the Greeks and Romans - these were normally bowl-shaped dials with vertical or horizontal gnomons (shadow-casters) and hour-lines marked in the hollow of the bowl. The Romans also produced new forms of portable dial, such as the pillar dial and the ring dial; like all the others, they depended on the altitude of the sun to tell the time. Unlike dials which depend on the direction of the sun, these instruments were relatively easy to make. However, neither vertical nor horizontal gnomons would satisfy the requirements of a direction dial because the shadow they cast at a particular time of day varies in direction from season to season. It was not until around the first century A.D. that it was demonstrated that a gnomon set parallel to the Earth's axis would cast a shadow in the same direction at the same time every day of the year. Ptolemy discusses such sundials in his Analemma. But even after that time such direction dials only slowly superseded the altitude dials, and it was not until the twelfth century that they became common.

Portable direction dials did not appear until much later, since it was not until the advent of the magnetic compass that such dials could be correctly oriented. While altitude dials were simply turned towards the sun, and fixed direction dials could be set on the meridian permanently, it was impossible to use a portable direction dial if the direction of South could not be found immediately. Compasses were first added to sundials round about the end of the fourteenth century and after that time direction dials proliferated. Altitude dials such as ring dials and pillar dials remained in common use, but the numbers of direction dials were much higher.

During the mediaeval and Renaissance periods various more elaborate designs appeared. These were either intended to be more accurate (such as the Regiomontanus dial, the navicula and the quadrans vetus) or were intended to encompass more forms of sundials (such as the astronomical compendium and the diptych dial). Sundials from this period are often particularly ornate, having been made for wealthy patrons or rich merchants. Few of the cheap, wooden versions used by the lower classes have survived.
Sundials and Calendar Reform

A form of sundial was the earliest means of checking the accuracy of calendars. Chinese astronomers would check the positions of the solstices and equinoxes by observation of a meridian line sundial. This consisted simply of a line which was drawn in the local meridian (North-South line) with an obelisk set at the South end of the line. Positions were marked along the line to indicate where the tip of the obelisk's shadow should be at the equinoxes and the solstices. By this means, astronomers could determine where adjustments were required to the calendar to keep it in step with the motion of the sun.

A slightly different form of meridian line was developed in Italy during the fifteenth century. This time, instead of using the shadow of an obelisk, the sun was allowed to shine through the roof of a building onto a meridian line on the floor inside, and the point of light was used as the indicator. Such a meridian was set up in Florence Cathedral in about 1460 when Paolo Toscanella pierced the Brunelleschi dome and marked a meridian on the floor of the nave. This meridian was only marked out for the period immediately around the summer solstice (by a graduated marble slab) and so could only be used for checking whether the solstice coincided with the correct date in June.

It was partly study of the movement of the sun on the Florentine meridian line which indicated the urgent need for calendar reform in the sixteenth century. Ignazio Danti made use of the line when carrying out his calculations, but found it too inaccurate for his purposes. He established a further two meridians in the church of Santa Maria Novella and, in 1576, oversaw the construction of the meridian indicator in the Cathedral of San Petronio in Bologna. This indicator was replaced in the following century by the famous astronomer, Giovanni Domenico Cassini, who made use of it in continuing to monitor the accuracy of the new Gregorian calendar.

Sundials and Poetry

A poetic motto on a sundial

The motto around the edge of this sundial reads' Wen ich campast recht sol weissen so richt mich night bei eissen' (translation - 'if the compass is to show the way correctly, don't place it near iron'). This is a common motto on Nuremberg sundials, particularly those made by the Tucher family. This dial was made by Hans Tucher around 1600.
The practice of adding poetical mottoes to sundials did not flourish until the early modern period. However, there were numerous mentions of sundials in poetry during the earlier history of the instrument. Two famous examples come from Chaucer's Canterbury Tales and from the Shakespeare play As You Like It.

In the Shipman's Tale of the Canterbury Tales the narrator tells how a rich merchant is cheated by a monk, first by seducing the merchant's wife, and then by stealing his money. The monk invites the wife to dinner with the words, 'And lat us dyne as soone as that ye may; for by my chilyndre it is pryme of day'. The cylinder referred to is a pillar sundial, a form of altitude dial first made by the Romans and common among the merchant classes for many centuries.

In Act 2, scene VII of As You Like It, Jacques gives an account of his meeting with the fool. During this encounter the fool

drew a dial from his poke,
and looking on it with lack-lustre eye,
says very wisely "It is ten o'clock"

It is not clear what sort of dial is meant in this passage, but it shows the commonplace nature of sundials during the Renaissance period.
Ptolemy on Sundials

A mural sundial

A mural sundial for planetary (unequal) hours, for use at a latitude of 48 degrees.

The Analemma is Ptolemy's work relevant to sundials. The problem it addresses in general terms is that of finding, through geometrical constructions on a plane, those arcs and angles by which a specific point may be determined on the celestial sphere. This is useful in the design of sundials, in which the position of the sun in the sky (that is, on the celestial sphere) needs to be related to the line traced out by the shadow of the gnomon (the pointer of the sundial) on the plane of the sundial itself. More specifically, Ptolemy's Analemma seeks to derive the position of the sun in spherical co-ordinates given certain data such as the hour of day and the geographical latitude.

Ptolemy's advance over previous methods was to employ a more elegant and convenient system of spherical co-ordinates with which to identify points on the celestial sphere, and to employ a graphical technique ('nomographic', in modern terminology), for generating the necessary plane geometrical constructions, that was far more convenient than working through the problem using trigonometry.
Sacrobosco and Sundials
The quadrans vetus or old quadrant, so called to distinguish it from the quadrans novus or astrolabe-quadrant devised by Profatius (Jacob ben Machir ibn Tibbon; d. 1305), was an instrument which could be used to solve problems in surveying and, from observations of the sun, to find the time. As such its description was often included in early-modern treatises on dialling. The picture shown here comes from Ein wolgegrundes kunstreichs sumarii büchlein aller sonner uhr (Strasbourg, 1539).

An image of a quadrans vetus

Sacrobosco's Tractatus de quadrante describes the construction and use of the quadrans vetus. He discusses two forms of the instrument, both of which could be used to find the time in unequal hours. The simpler version required the use of tables from which the noon altitude of the sun could be derived for any day of the year. Sacobosco appears to have been working with a particular set of tables, derived from the Humeniz Tables compiled in Paris in 1239. The more sophisticated form of quadrant had, as in the picture, a scale co-ordinating the astronomical and civil calendars which indicated the noon solar altitudes, and which could be placed according to the observer's latitude; this substituted for the tabulated data.

The Tractatus de quadrante derives in some measure from earlier medieval treatises, in particular the Astrolabium of pseudo-Messehallah. But there appears to be no direct precedent for an account of the construction of the instrument. Thus, while Sacrobosco's treatise was inadequate in this respect, and was superceded by better treatments, in particular the Quadrans vetus of Johannes Anglicus, written in the 1260s, he may have been the first to devote a work to this instrument.
Last Updated on Thursday, 05 March 2009 16:55