Astronomical Images : Heart-shaped projection of the Earth

William Cuningham

Astronomical Images

<p style='text-align: justify;'>William Cuningham (1531 ' post 1586) was a Norwich-born physician, educated at Corpus Christi College, Cambridge. His <i>magnum opus</i>, <i>The Cosmographical Glasse</i>, 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 <i>Antiprognosticon</i> (London, 1560). This image demonstrates the mathematical construction of a heart-shaped or 'cordiform' map. Representation of the spherical Earth in two-dimensions presents a particular mathematical challenge that cannot be achieved without some distortion or loss of proportion. In response to this challenge, cartographers proposed a range of different projections in order to translate the points of a sphere onto a plane. The cordiform projection was one such approach that was developed in the early sixteenth century and popularised particularly by Oronce Fine, whose cordiform map (<i>Recens et integra orbis descriptio</i>) of 1534 was reproduced extensively in sixteenth-century travel and cosmographical works. The cordiform projection was essentially a reworking and extension of a technique described by Ptolemy in his <i>Geography</i>. This system preserves distances along both the zero-degree meridian line (represented as a straight vertical line) and along the parallels representing latitude (mapped as arcs of concentric circles centred around the North Pole). Consequently, this system preserves distances across the map, permitting distances between two places to be accurately measured using a pair of compasses. As a consequence of the preservation of distance along the parallels, the meridian lines that intersect them become curved, demonstrating a change in curvature that becomes more and more obvious the further the projection extends. Once the projection extends to 360 degrees, representing the entire globe, the furthermost meridian describes a heart-shape. In the third book of <i>The Cosmographical Glasse</i>, Cuningham describes the process for constructing a cordiform map. This figure shows the net of gradations underlying this projection. Point K represents the pole of the Earth, with vertical line KRML representing the zero-meridian. The parallels and meridian lines divide the globe into 10 degree intervals. The equator is the parallel marked by a double line and passing through M; other significant circles including the tropics and Arctic and Antarctic circles are similarly marked by a double line, to distinguish them from other parallels. Having described the construction of this net, Cuningham then instructs the reader to add 'the face of the Earth', according to the latitude and longitude values presented in the fifth book of his treatise.</p>

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