Background: Gerardus Mercator (; 5 March 1512 - 2 December 1594) was a 16th-century German-Flemish cartographer, geographer and cosmographer. He was renowned for creating the 1569 world map based on a new projection which represented sailing courses of constant bearing (rhumb lines) as straight lines--an innovation that is still employed in nautical charts. Mercator was one of the founders of the Netherlandish school of cartography and is widely considered as the most notable representative of the school in its golden age (approximately 1570s-1670s).
Context: Mercator never committed any of his prison experiences to paper; all he would say was that he had suffered an "unjust persecution". For the rest of his time in Leuven his religious thoughts were kept to himself and he turned back to his work. His brush with the Inquisition did not affect his relationship with the court and Nicholas Perrenot recommended him to the emperor as a maker of superb instruments. The outcome was an Imperial order for globes, compasses, astrolabe and astronomical rings. They were ready in 1545 and the Emperor granted the royal seal of approval to his workshop. Sadly they were soon destroyed in the course of the Emperor's military ventures and Mercator had to construct a second set, now lost. He also returned to his work on a large up-to-date and highly detailed wall map of Europe which was, he had already claimed on his 1538 world map, very well advanced. It proved to be a vast task and he, perfectionist that he was, seemed unable to cut short his ever-expanding researches and publish: as a result it was to be another ten years before the map appeared. In 1547 Mercator was visited by the young (nineteen year old) John Dee who, on completion of his undergraduate studies in Cambridge (1547), "went beyond the seas to speak and confer with some learned men". Dee and Mercator were both passionately interested in the same topics and they quickly established a close rapport which lasted throughout their lives. In 1548 Dee returned to Leuven (Louvain in Dee's text) and registered as a student: for three years he was constantly in Mercator's company. Apart from a possible short visit to Duisberg in 1562 the two men did not meet but they corresponded frequently and by good fortune a number of their letters are preserved. Dee took maps, globes and astronomical instruments back to England and in return furnished Mercator with the latest English texts and new geographical knowledge arising from the English explorations of the world. Forty years later they were still cooperating, Dee using Mercator's maps to convince the English court to finance Martin Frobisher's expeditions and Mercator still avidly seeking information of new territories. The final success in Leuven was the 1551 celestial globe, the partner of his terrestrial globe of 1541. The records of the Plantin Press show that several hundred pairs of globes were sold before the end of the century despite their high price--in 1570 they sold at 25 carolus guilders for a pair, equivalent to 2500 euro in modern currency. Celestial globes were a necessary adjunct to the intellectual life of rich patrons and academics alike, for both astronomical and astrological studies, two subjects which were strongly entwined in the sixteenth century. Twenty two pairs are still in existence. CANNOTANSWER
Question: When did he arrive in Leuven?

Answer:

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