Astronomical Images : Ptolemy using his rulers

William Cuningham

Astronomical Images

William Cuningham (1531 ' post 1586) was a Norwich-born physician, educated at Corpus Christi College, Cambridge. His magnum opus, The Cosmographical Glasse, was published in 1559 by John Day (1522-1584). In it, Cuningham discussed many aspects of practical mathematics and surveying in particular. He promoted the use of instruments including the quadrant and Ptolemy's rulers. The text and images combined elements derived from the works of Peter Apian and Oronce Fine, as well as those of the English mathematician, Robert Recorde. The author gratefully recognised Day's skill and expense in producing the many images in the work. Cuningham also produced a series of almanacs and prognostications during his career, for which he was heavily criticised in William Fulke's Antiprognosticon (London, 1560). This figure shows Ptolemy's rulers in use. The rulers were also known as the parallactic instrument. Like the quadrant and the astrolabe, the rulers had come down from the Platonic tradition of astronomy as an instrument for measuring altitudes and zenith distances. The rulers received less attention in later writings than the other two instruments, although Copernicus described the use of the instrument and Tycho Brahe also used it. The figure may be considered as a hybrid image, since it features elements from both the geometric tradition (for example, lettered points) and realistic depiction (for example, features of the landscape and animal subjects). Borrowing from the geometric tradition also contributed to the success of the key (to the right of the image), in which points and lines of the instrument are labelled with letters and pairs of letters. More information on hybrid images can be found in the 'further reading'. While Ptolemy's rulers performed the same function as instruments like the quadrant, there was an important difference in its construction: the rulers were composed of straight rods. This meant that makers did not face the difficulty of graduating curved scales.