Gnomonics
Sundials
Savile's statutes obliged his professors of astronomy to teach 'the whole science ... of gnomonics' along with optics, geography, and navigation. The relationship of these fields may not seem obvious to professors of these disciplines today, but their connection was clear in 1619. Gnomonics is the art or science of constructing sundials – thus called because dials show the time of day by the shadow cast by the ‘gnomon’, a pin or triangle raised above the surface of the dial. Keeping time therefore required knowledge of astronomy (since it was the relative movement of the sun which caused the shadows to move), of optics (since the shadow was cast by light), and of geography (since the length of the shadow varied with latitude as well as longitude and the seasons of the year, just as the altitude of the heavenly bodies allowed navigators to determine latitude at sea). Gnomonics was also included in the ninth of Vitruvius's Ten Books on Architecture, because a thorough understanding of the movement of the sun across the heavens at different latitudes and seasons of the year was crucial to keeping buildings warm and illuminated in the winter but cool in the summer.
More concrete manifestations of growing interest in Oxford in this branch of practical mathematics are the great variety of sundials scattered across the university and its colleges in the sixteenth, seventeenth and eighteenth centuries. At one level, these instruments were exercises in scholarship, in learning how to emulate antiquity. At another, they were evidence of modern technical proficiency rivalling and even surpassing that of antiquity. At a more practical level, they offered timekeeping instruments far more reliable over the long term than the mechanical clocks of the era. The statutes of the era strive to depict the university as a well-oiled machine. In an era in which the rhythms of the University were increasingly governed by bells rung mechanically, the sundials needed to regulate this clockwork helped to maintain the smooth functioning of the university.
Coincidentally, Oxford's collection of historic sundials was massively increased in 1924 with the donation of scientific instruments by Lewis Evans, which founded the Museum of the History of Science. The centrality of dialing to Evans's interests is manifest in the portrait, now hanging in the Museum, in which he holds one of his prime examples. Sundials accordingly loom large within the hundreds of items contained in the Lewis Evans Collection. Oxford therefore provides unusually rich resources for considering the ways in which this branch of applied mathematics was integrated into seventeenth-century academic culture.
Images 1-3. The intellectual associations of gnomonics are nowhere better illustrated in Oxford than by the splendid sundial over the entrance to the Codrington Library in All Souls College (images 1-2). Dating from 1658, this dial was designed by Christopher Wren at the point in which he transferred from All Souls (where he had been a fellow since 1653) to become Professor of Astronomy at Gresham College, London, in 1657. In 1661, however, he was recalled to Oxford as Savilian Professor of Astronomy, where he resided again until Charles II appointed him Surveyor of Works in 1669. The presence of the dial on the Codrington may seem anomalous, since the Library was only erected between 1716 and 1720, in the last years of Wren’s life, and not fully furnished until 1751. But as Loggan’s engraving of 1675 shows (image 3), this sundial was originally placed on the south wall of the college chapel, the pinnacles and buttresses of which were replicated by Hawksmoor in designing the Codrington. A photograph reproduced in Howard Colvin and J.S.G. Simmons, All Souls: An Oxford College and its Buildings (1989), p. 73, also shows the dial still on the chapel in 1870. It was removed one year later and placed atop the library in 1877.
Image 1: southern facade, Codrington Library, All Souls College, Oxford; photo by Andrew Shiva, 15 April 2014; Wikimedia Commons; CC BY-SA 4.0.
Image 2: Photo by Robin Stevens, 20 February 2010. Licence: CC BY-NC-ND.
Image 3: David Loggan, Oxonia illustrata, plate XXIII, detail.
Resources for understanding the basic principles of gnomonics are abundantly available online.
(1) Introductory videos from the Museo Galileo in Florence provide brief overviews of
- The history of the sundial, from ancient Egypt to Renaissance Europe;
- Basic principles of sundial construction; and
- The Museo Galileo's own monumental sundial built in 2007.
(2) Other introductory materials include
- How sundials work: a more detailed account, from the British Sundial Society;
- Sundials in Renaissance Europe: further materials from History and Philosophy of Science in Cambridge (1999); and
- Sundials on the Internet: a collection of further resources.
Credit: Howard Hotson (October / December 2016)