Johann Gabriel Doppelmayr's map of the world c.1720 based on astronomical observations, was in advance of its time in scientific content. Here, Rodney Shirley, author of Mapping of the World, Early Printed Maps of the British Isles 1477-1650 and its' recent sequel Printed Maps of the British Isles 1650-1750, examines this unusual map and shows in a table how accurate were Doppelmayr's calculations for latitude and longitude.

SOME COLLECTORS, including myself, find that many maps of the eighteenth century lack the interest and appeal of those from earlier times. In the case of world maps, there were few new geographical discoveries to record until the voyages of Captain Cook in the 1770s, and compared to many maps of the previous century the decorative artistry of the borders and cartouches tended to be stultified or lacking altogether.

Nevertheless, great changes were taking place in the scientific concepts underlying the mapping of the world. The unusual world map by Johann Gabriel Doppelmayr which was published from Nuremberg in about 1720 displays in unique form the astronomically-determined findings of latitude and longitude recorded by more than fifty astronomers and scientists of the time. Born in 1677 (although some authorities say 1671), Doppelmayr was educated at Altdorf and Halle in Germany and made lunar observations his speciality. He became Professor of Mathematics at Nuremberg where he remained for fifty years. During this time he was associated with the better-known geographer and publisher, J. B. Homann, contributing the introduction or Einleitllng zur Geographie to Homann's Grosser Atlas of 1714. [1] Several globes, both celestial and terrestrial, arc known of which bear his name [2]. Doppelmayr died in 1750.



Doppelmayr's map consists of two large hemispheres showing the world in outline. The long title starts Basis Geographiae Recentioris Astronomica (the most recent astronomical founda­tions of geography). Tables above and below the hemispheres contain text setting out the latitude and longitude of 138 locations where observations have been made astronomically to determine their positions. The names of the scientists who made the observations are given in each case with a symbol indicating whether the method of observation was by an eclipse of the sun, of the moon, or of the satellites of Jupiter. Each location noted in the text is marked on the map according to its observed position, providing an invaluable record of geographical adv­ancement for the time. In the spandrels and corners are depictions of putti engaged in making astronomical measurements and on the right-hand hemisphere is a large decorative cartouche with explanatory text.

The fixing of latitude - the angular elevation of a place north or south of the equator - had been practised for many centuries but the accurate determination of longitude remained an unsolved problem. It was well known that longitude could be calculated if only the difference in time between an original point and the observer could be known. Taking the circumference of the earth as 360 degrees corresponding to one revolution of 24 hours or 1440 minutes, it was easy to calculate that each degree must represent 4 minutes of time. Thus, with a degree of longitude at the equator being about 67 miles, to fix a position to within a mile, demanded the measurement of a time difference accurate to within 3½ seconds. But until the 1770s, when Harrison's masterlv naval chronometer became available, there were no timepieces for maritime purposes approaching this fineness ot accuracy. Navigation at sea continued to be fraught with error, leading to wild mis-estimations of position, false landfalls and unnecessary ship-wrecks year after year.
In the previous centuries, observations of solar and lunar eclipses had allowed longitude to be determined but the occasions were infrequent and, with the forms of telescopes available up to about 1700, the measurements were often in error. In the early part of the seventeenth century Galileo had shown that more precise observations of the eclipses of the satellites of Jupiter could allow the determination of mean time in two different places at once. His demonstrations were not fully appreciated at the time, and it was only through the active sponsorship of the French Academic Royale des Sciences, founded in 1666, that the problem of determining longitude in this way was tackled afresh. The eminent scientist Jean­Dominique Cassini was recruited from Italy and from the 1670s onwards teams of scientists under his direction were despatched to remote parts of the world to make exact observations, principally of the eclipses of the satellites of Jupiter, and hence the longitude. Together with Calculations of latitude, the results were all translated on to a large circular map of the world eight metres in diameter on the floor of the Paris Observatory. In 1696 Cassini's son Jacques arranged for the Parisian publisher Nolin to print a circular world map on a smaller scale, based on forty-three observations up to that time. In the early eighteenth century this circular map was redrawn (without any further locations being added) and was more widely disseminated by Halma and then by Van der Aa in the Netherlands [3]



The roll-call of astronomers who arc named on Doppelmayr's map, with its much larger number of observations, is impressive. Some, such as Tycho Brahe and Regiomontanus predate Cassini's initiative and their calculations are based on lunar or solar eclipses. In the late sixteenth century Brahe worked from the Danish observatory at Uraniborg on the island of Hven (now Swedish island of Ven), where the young Willem J. Blaeu was an apprentice. When he was invited to Prague by the German Emperor Rudolf II Brache calculated the latitude of that city as 50° .14' .30" and its longitude as 34° 55' 00" His findings are very close to the figures of 500 05' 00" for latitude and the equivalent of 340 37' 30" for longitude accepted today. The calculations for Vienna made even earlier in the 1470s by Regiomontanus are again of commendable accuracy.

A selected listing of observations taken from those recorded on Doppelmayr's map is given in Table I. These include places as far apart as Lima, Mexico City and Quebec on the western hemisphere and Goa, Malacca and Peking in the Far East. Two locations in Tartary many miles north of Peking, 'Seringua' and 'Kokotam', were visited by Jesuits but cannot readily be located on present-day maps. However. as a result of researches at the Royal Geographical Society and in the Science Museum library, Francis Herbert has suggested that Seringua is probably the modern town of Novoselenginsk on the Selenga river - the orientals perhaps having trouble distinguishing 'I' and 'r'. 'Kokotarn has passed through several name changes and transliterations. among them Kalgan. Wanchuan, and Chang­chia-k'ou; it is now known, according to the Pinyin system, as Zhangjiakou.

The English astronomer, Edmond Halley. provided data for Oxford, and in 1676-77 undertook an expedition to the remote island of St Helena to observe eclipses of the sun, of the moon, and of the transit of Mercury across the sun. From these observations he was able to calculate the position of the island more exactly. Halley's contemporaries Wright and Flamsteed were also contributors.  From France, in addition to Cassini, appear the distinguished names of De la Hire, Picard, Feuillce and Varin; Riccioli and Grimaldi from Italy; Keppler and Kircher from Germany.