Editing Gerardus Mercator (section)

===Duisburg, 1552–1594===
In 1552, Mercator, aged 40, moved from Leuven to [[Duisburg]] in the [[United Duchies of Jülich-Cleves-Berg|Duchy of Cleves]] (in modern-day Germany),  where he spent the rest of his life. He never gave his reasons for the move but several factors may have been involved: not having been born in Brabant he could never be a full citizen of Leuven; Catholic intolerance of religious dissidents in the Low Countries was becoming ever more aggressive and a man suspected of heresy once would never be trusted; the Erasmian constitution and the religious tolerance of Cleves must have appeared attractive; there was to be a new university in Duisburg and teachers would be required.{{sfn|Crane|2003|loc=Chapter 16|p=173}} He was not alone; over the years to come many more would flee from the oppressive Catholicism of Brabant and Flanders to tolerant cities such as Duisburg.{{efn|Other refugees in Duisburg included Johannes Oeste (or Otho), Georg Cassander and Cornelius Wouters. See {{harvnb|Crane|2003|loc= Chapter 19|p=191}} }}

The peaceful town of Duisburg, untroubled by political and religious unrest, was the perfect place for the flowering of his talent. Mercator quickly established himself as a man of standing in the town: an intellectual of note, a publisher of maps, and a maker of instruments and globes.{{efn|name=sales}} Mercator never accepted the privileges and voting rights of a [[Bourgeoisie|burgher]] for they came with military responsibilities which conflicted with his pacifist and neutral stance. Nevertheless, he was on good terms with the wealthier citizens and a close friend of Walter Ghim, the twelve-times mayor and Mercator's future biographer.{{sfn|Crane|2003|loc=Chapter 17|p=178}}

Mercator was welcomed by [[William, Duke of Jülich-Cleves-Berg|Duke Wilhelm]] who appointed him as court [[Cosmography|cosmographer]]. There is no precise definition of this term other than that it certainly comprehends the disciplines of geography and astronomy but at that time it would also include astrology and chronology (as a history of the world from the creation). All of these were among Mercator's accomplishments but his patron's first call on his services was as a mundane surveyor of the disputed boundary between the Duke's territory of the [[County of Mark]] and the [[Duchy of Westphalia]].{{sfn|Crane|2003|loc=Chapter 19|p=194}}

[[File:Baldewein astronomical clock 1561 Kassel.jpg|thumb|upright=0.6|right|[[Astronomical clock]] with rotating globes]]
Around this time Mercator also received and executed a very special order for the Holy Roman Emperor, Charles V: a pair of small globes, the inner ("fist-size") Earth was made of wood and the outer celestial sphere was made of blown crystal glass engraved with diamond and inlaid with gold.{{sfn|Ghim|1595}} He presented them to the emperor in Brussels who awarded him the title ''Imperatoris domesticus'' (a member of the Imperial household). The globes are lost but Mercator describes them in [[q:Gerardus Mercator#melanchthon01|a letter]] to [[Philip Melanchthon]]{{efn|Melanchthon is a significant correspondent of Mercator since he was one of the founders of Lutheranism, second only to Luther.}} in which he declares that the globes were rotated on the top of an [[astronomical clock]] made for Charles V by [[Juanelo Turriano]] (Janellus).{{efn|name=www.wilhelmkruecken.de}} The clock was provided with eight dials which showed the positions of the moon, stars and planets. The illustration shows a similar clock made by the German craftsman {{ill|Ebert Baldewein|de|lt=Baldewein}} at roughly the same time.

[[File:Mercator theory of magnetism.svg|left|thumb|upright=0.8|Mercator's theory of magnetism]]
Earlier, Mercator had also presented [[Charles V, Holy Roman Emperor|Charles V]] with an important pamphlet on the use of globes and instruments and his latest (incorrect) ideas on magnetism: ''Declaratio insigniorum utilitatum quae sunt in globo terrestri : coelesti, et annulo astronomico'' (A description of the most important applications of the terrestrial and celestial globes and the astronomical ring).{{sfn|Mercator|1554}} The first section is prefaced by Mercator's ideas on magnetism, the central thesis being that magnetic compasses are attracted to a single pole (not a dipole) along great circles through that pole. He then shows how to calculate the position of the pole if the deviation is known at two known positions (Leuven and Corvo in the Azores): he finds that it must be at latitude 73°2' and longitude 169°34'. Remarkably, he also calculates the longitude difference between the pole and an arbitrary position: he had solved the longitude problem—if his theory had been correct. Further comments on magnetism may be found in an earlier letter to Perrenot{{efn|Mercator's [[:q:gerardus Mercator#perrenot|letter to Perrenot]] is available on wikiquote (along with an introduction by Harradon).}} and on the later world map.{{efn|For magnetism on the world map see [[Mercator 1569 world map#Prime meridian and magnetic pole]]. Note that he calculated two possible positions.}} In the Hogenberg portrait (at the head of the article) his dividers are set on the position of the magnetic pole.

[[File:1595 Europa Mercator.jpg|thumb|upright=1.5|An updated version of the 1554 map of Europe as it appears in the 1595 atlas]]
In 1554, Mercator published the long-awaited [[#europe1554|wall map of Europe]], dedicating it to his friend, now Cardinal, Antoine Perrenot. He had worked at it for more than twelve years, collecting, comparing, collating and rationalising a vast amount of data and the result was a map of unprecedented detail and accuracy.{{efn|For a description see {{harvnb|Crane|2003|loc= Chapter 17}} }} It "attracted more praise from scholars everywhere than any similar geographical work which has ever been brought out."{{sfn|Ghim|1595}} It also sold in large quantities for much of the rest of the century, with a second edition in 1572 and a third edition in the atlas of 1595.{{efn|For example, Plantin alone sold 400 copies of the map of Europe in 1566, twelve years later.}}

The proposed university in Duisburg failed to materialise because the papal licence to found the university was delayed twelve years and by then Duke Wilhelm had lost interest. It was another 90 years before Duisburg had its university.{{sfn|Taylor|2004|p=139}} On the other hand, no papal permit was required to establish the [[:de:Akademisches Gymnasium|Akademisches Gymnasium]] where, in 1559, Mercator was invited to teach mathematics with cosmography.{{sfn|Crane|2003|loc=Chapter 19|p=194}} One year later, in 1560, he secured the appointment of his friend Jan Vermeulen ([[Molanus]]) as rector and then blessed Vermeulen's marriage to his daughter Emerantia. His sons were now growing to manhood and he encouraged them to embark on his own profession. Arnold, the eldest, had produced his first map (of Iceland) in 1558 and would later take over the day-to-day running of Mercator's enterprises.{{sfn|Crane|2003|loc=Chapter 19|p=193}} Bartholemew, his second son, showed great academic promise and in 1562 (aged 22) he took over the teaching of his father's three-year-long lecture course—after Mercator had taught it once only! Much to Mercator's grief, Bartholemew died young, in 1568 (aged 28).{{sfn|Crane|2003|loc=Chapter 22|p=222}} [[Rumold Mercator|Rumold]], the third son, would spend a large part of his life in London's publishing houses providing for Mercator a vital link to the new discoveries of the Elizabethan age. In 1587 Rumold returned to Duisburg and later, in 1594, it fell to his lot to publish Mercator's works posthumously.{{sfn|Crane|2003|loc= Epilogue|p= 320}}

[[File:Mercator Britain copied by Ortelius.jpg|thumb|upright=1.5|[[Abraham Ortelius]] copy of the 1564 map of Britain]]
In 1564, Mercator published his [[#britain1564|map of Britain]], a map of greatly improved accuracy which far surpassed any of his previous representations. The circumstances were unusual. It is the only map without a dedicatee and in the text engraved on the map, he pointedly denies responsibility for the map's authorship and claims that he is merely engraving and printing it for a "very good friend". The identity of neither the author nor the friend has been established but it has been suggested that the map was created by a Scottish Catholic priest called John Elder who smuggled it to French clergy known to Antoine Perrenot, Mercator's friend.{{sfn|Crane|2003|loc=Chapter 19|pp=189–205}}{{sfn|Taylor|2004|loc= Chapter 14|pp=156–166}}{{sfn|Barber|1998|pp=43—77}}{{sfn|Woodward|1987|loc=Chapter 54}} Mercator's reticence shows that he was clearly aware of the political nature of the pro-Catholic map which showed all the Catholic religious foundations and omitted those created by Protestant [[Henry VIII of England|Henry VIII]]; moreover, it was engraved with text demeaning the history of England and praising that of Catholic Ireland and Scotland. It was invaluable as an accurate guide for the planned Catholic invasion of England by Philip II of Spain.

[[File:Atlas Cosmographicae (Mercator) 129.jpg|thumb|left|upright=1.5|Lotharingia (Lorraine) as it appeared in the 1595 atlas]]
As soon as the map of Britain was published, Mercator was invited to undertake the surveying and [[#lorraine1564|mapping of Lorraine]] ([[Lotharingia]]). This was a new venture for him in the sense that never before had he collected the raw data for a new regional map. He was then 52, already an old man by the norms of that century, and he may well have had reservations about the undertaking. Accompanied by his son Bartholemew, Mercator meticulously triangulated his way around the forests, hills and steep-sided valleys of Lorraine, difficult terrain as different from the Low Countries as anything could be. He never committed anything to paper, but he may have confided in his friend Ghim, who would later write: "The journey through Lorraine gravely imperiled his life and so weakened him that he came very near to a serious breakdown and mental derangement as a result of his terrifying experiences."{{sfn|Ghim|1595}} Mercator returned home to convalesce, leaving Bartholemew to complete the survey. No map was published at the time, but Mercator did provide a single drawn copy for the Duke and later he would incorporate this map into his atlas.{{sfn|Crane|2003|loc=Chapter 21|p=212}}

[[File:Mercator Chronologia title page 1569.jpg|thumb|upright=1.5|''Chronologia'' title page{{efn|For the title see {{harvnb|Mercator|1569}}. The text over the illustration is ''in via virtuti nulla est via'', meaning ''no way is impassable to virtue'' (Ovid).}}]]
The trip to Lorraine in 1564 was a setback for his health, but he soon recovered and embarked on his greatest project yet, a project which would extend far beyond his cartographic interests. The first element was the ''Chronologia'',{{sfn|Mercator|1569}} a list of all significant events since the beginning of the world compiled from his literal reading of the Bible and no less than 123 other authors of genealogies and histories of every empire that had ever existed.{{sfn|Crane|2003|loc=Chapters 23|pp=223–228}}{{sfn|Taylor|2004|loc=Chapter 17|p=185}} Mercator was the first to link historical dates of solar and lunar eclipses to [[Julian day|Julian dates]] calculated mathematically from his knowledge of the motions of the sun, moon and Earth. He then fixed the dates of other events in Babylonian, Greek, Hebrew and Roman calendars relative to the eclipses that they recorded. The time origin was fixed from the genealogies of the Bible as 3,965 years before the birth of Christ.{{sfn|Mercator|1569|p=262}} This huge volume (400 pages) was greeted with acclaim by scholars throughout Europe and Mercator himself considered it to be his greatest achievement up to that time. On the other hand, the [[Catholic Church]] placed the work on the [[Index Librorum Prohibitorum]] (List of Prohibited Books) because Mercator included the deeds of [[Martin Luther]]. Had he published such a work in Louvain, he would again be laying himself open to charges of heresy.{{efn| name=Chronologia}}

The Chronologia developed into an even wider project, the ''Cosmographia'', a description of the whole Universe. Mercator's outline was (1) the creation of the world; (2) the description of the heavens (astronomy and astrology); (3) the description of the earth comprising modern geography, the geography of Ptolemy and the geography of the ancients; (4) genealogy and history of the states; and (5) chronology. Of these the chronology had already been accomplished, the account of the creation and the modern maps would appear in the atlas of 1595, his edition of Ptolemy appeared in 1578 but the ancient geography and the description of the heavens never appeared.{{sfn|Crane|2003|loc=Chapters 22 and 23|pp=217–228}}{{sfn|Taylor|2004|loc=Chapter 17|p=185}}

[[File:Mercator 1569.png|thumb|upright=1.5|left|The 1569 Mercator map of the world [[Mercator 1569 world map|(Higher-resolution images)]]]]
As the Chronologia was going to press in 1569, Mercator also published what was to become his most famous map: [[Mercator 1569 world map|''Nova et Aucta Orbis Terrae Descriptio ad Usum Navigantium Emendate Accommodata'']] ('A new and more complete representation of the terrestrial globe properly adapted for use in navigation').{{sfn|Crane|2003|loc=Chapter 24}}{{sfn|Taylor|2004|loc=Chapter 21}} As mariners had started to explore the oceans in the [[Age of Discovery]] the problem of accurate navigation had become more pressing. Their locations could be a hundred miles out after a long voyage because a course of constant direction at sea (a [[rhumb line]]) did not correspond to a straight line on their chart. Mercator's solution was to make the scale of his chart increase with latitude in a very special way, such that the rhumb lines became straight lines on his new world map. Exactly how he arrived at the required solution is not recorded in any of his own written works, but modern scholars{{sfn|Gaspar|Leitão|2016}} suggest that he used the tables of rhumbs devised by [[Pedro Nunes]].{{efn|{{harvnb|De Graeve|2012a}} has shown that Mercator's library contained a copy of the Theory of the Loxodrome by Pedro Nuñez. This was published in 1566, three years before Mercator completed the map on the new projection.}} The large size of what was a wall map meant that it did not find favour for use on board ship but, within a hundred years of its creation, the [[Mercator projection]] became the standard for marine charts throughout the world and continues to be so used to the present day. On the other hand, the projection is clearly unsuitable as a description of the land masses on account of its manifest distortion at high latitudes and its use is now deprecated: other projections are more suitable.{{efn|See the discussion at [[Mercator projection#Uses|Mercator projection]].}} Although several hundred copies of the map were produced{{efn|name=sales}} it soon became out of date as new discoveries showed the extent of Mercator's inaccuracies (of poorly known lands) and speculations (for example, on the arctic and the southern continent).{{efn|For further comments on the inaccuracies of the 1569 map see [[Mercator 1569 world map#Geography]]. For a discussion of the southern continent in particular see {{harvnb|Zuber|2011}} }}

Around this time, the [[marshal]]l of Jülich approached Mercator and asked him to prepare a set of European regional maps which would serve for a grand tour by his patron's son, the [[John William, Duke of Jülich-Cleves-Berg|crown prince Johannes]]. This remarkable collection has been preserved and is now held in the [[British Library]] under the title [[#europe1570|Atlas of Europe]] (although Mercator never used such a title). Many of the pages were assembled from dissected Mercator maps and in addition there are thirty maps from the ''[[Theatrum Orbis Terrarum]]'' of Abraham Ortelius.{{efn|A facsimile of the Mercator contributions in the Atlas of Europe has been published by {{harvnb|Watelet|1997}} }}

[[File:Africa North 1578, Gerardus Mercator (4154597-recto).jpg|thumb|upright=1.5|right|Mauretania in the 1578 Ptolemy]] 
Apart from a revision of the map of Europe in 1572, there would be no more large wall maps and Mercator began to address the other tasks that he had outlined in the Cosmographia. The first of these was a new definitive version of [[#ptolemy|Ptolemy's maps]].{{sfn|Crane|2003|loc=Chapter 26}} That he should wish to do so may seem strange given that, at the same time, he was planning very different modern maps and other mapmakers, such as his friend [[Abraham Ortelius]], had forsaken Ptolemy completely. It was essentially an act of reverence by one scholar for another, a final epitaph for the Ptolemy who had inspired Mercator's love of geography early in his life. He compared the great many editions of the Ptolemy's written Geographia, which described his two projections and listed the latitude and longitude of some 8000 places, as well as the many different versions of the printed maps which had appeared over the previous one hundred years, all with errors and accretions. Once again, this self-imposed diligence delayed publication and the 28 maps of Ptolemy appeared in 1578, after an interval of almost ten years. It was accepted by scholars as the "last word", literally and metaphorically, in a chapter of geography which was closed for good.{{sfn|Crane|2003|loc=Chapter 26}}{{sfn|Taylor|2004|loc=Chapter 19}}

 Mercator now turned to the modern maps, as author but no longer engraver: the practicalities of the production of maps and globes had been passed to his sons and grandsons. In 1585 he issued a [[#atlas1585|collection of 51 maps]] covering France, the Low Countries and Germany. Other maps may have followed in good order had not the misfortunes of life intervened: his wife Barbara died in 1586 and his eldest son Arnold died the following year so that only Rumold and the sons of Arnold were left to carry forward his business. In addition, the time he had available for cartography was reduced by a burst of writing on philosophy and theology: a substantial written work on the Harmonisation{{efn|For an (online) exegesis of the Harmonisation see {{Harvnb|Jonge|1990}} }} of the Gospels{{sfn|Mercator|1592}} as well as commentaries on the epistle of St. Paul and the book of Ezekiel.

In 1589, at the age of 77, Mercator had a new lease of life. He took a new wife, Gertrude Vierlings, the wealthy widow of a former mayor of Duisburg (and at the same time he arranged the marriage of Rumold to her daughter). A [[#atlas1589|second collection of 22 maps]] was published covering Italy, Greece and the [[Balkans]]. This volume has a noteworthy preface for it includes mention of Atlas as a mythical king of [[Mauretania]]. "I have set this man Atlas," explained Mercator, "so notable for his erudition, humaneness, and wisdom as a model for my imitation."{{sfn|Crane|2003|loc=Chapter 30|p=308}} A year later, Mercator had a stroke which left him greatly incapacitated. He struggled with the assistance of his family trying to complete the remaining maps, the ongoing theological publications and a new treatise on the Creation of the World. This last work, which he did succeed in finishing, was the climax of his life's activities, the work which, in his own opinion, surpassed all his other endeavours and provided a framework and rationale for the [[#atlas1595|complete atlas]]. It was also his last work in a literal sense, for he died after two further strokes in 1594.{{sfn|Crane|2003|loc=Chapter 31|p=318}}