Seeing that the use of large Gothic characters reduced available space, Gerardus Mercator was the first to do away with them on his globes and maps. He replaced them with cursive Italian script, in which the characters are joined. He also introduced a hierarchy of fonts to denote the type of information being conveyed.

This new calligraphic convention had the dual advantage of elegance and economy of space, making it possible to put more words on a globe. And the ranking of information that it established is still a basic rule in cartography today.

En rouge le premier méridien de Mercator, en vert le méridien de Greenwich. © UNIL

The idea of a prime or zero meridian, serving as a reference for the other longitude lines on Earth, reflected geophysical considerations. Based on his investigation of magnetism, Gerardus Mercator set his prime meridian on Fuerteventura in the Canary Islands, 14.1° to the west of the Greenwich meridian used universally nowadays.

© Laurent Dubois BCUL

Fabrice Ducrest © UNIL.

Two magnetised needles were encased in the stands at the time the terrestrial and celestial globes were produced, but they have since been lost. They were intended to help determine longitude, using the globe and the needle’s declination in relation to the north. This procedure was apparently already known to Sébastien Cabot three decades earlier.

While the first compasses were invented in China around 1040, they were not imported into Europe until about the 12th century. At the time it was believed that the magnetised needle pointed towards the North Star, an idea that was challenged from the 13th century. In 1546 Gerardus Mercator demonstrated from measurements of declination that the place the magnetised needle pointed to could not be in the sky and had to be on Earth.

In 1600 William Gilbert proved that “Magnus magnes ipse est globus terrestris”, meaning the Earth itself is a great magnet. This was the first property generally attributed to our planet, 87 years before Isaac Newton explained gravity.

Gerardus Mercator’s interest in terrestrial magnetism made him a forerunner in this area. Noting that the magnetic north pole did not correspond to the geographic one, he posited that there might exist a Magnetic Island, Magnetu[m] insula, lying near the North Pole. He may have borrowed this idea from the Carta marina of the Swedish writer Olaus Magnus. Was he hoping to use it as a simplified solution to the longitude problem ?

According to historians it was between 1538 and 1541 that Mercator anticipated the crucial importance of terrestrial magnetism for maritime navigation. This realisation represented a huge step forward in this area and prompted him to situate the magnetic north pole on the Earth and leave the geographic north pole in the heavens, on the world’s rotational axis.

As we can see in the arctic region of the terrestrial globe’s gores, Mercator clearly placed a rocky magnetic island at a point on the calotte that is separate from the geographic north pole. This enabled him to allow for the angle of magnetic declination and thus explain why the north indicated by the stars and the direction given by a compass differed.

La Petite Ourse appelée vulgairement la Petite Casserole et l’Etoile Polaire (83°N 20°W). © UNIL

La jambe, le nombril et l’épaule du Pégase (20°N 27°W). © UNIL

Gerardus Mercator tried to improve his terrestrial globe by adding stars (as Gemma Frisius did) that could serve as reference points for travellers. The terrestrial globe shows 29 stars in several different areas: in the seas, in the middle of the deserts of Africa, on the plains of America and Asia and near the North Pole. Their size and brightness were meant to help travellers get their bearings.

Mercator may have wanted to help sailors or travellers to find their way more easily during the night, an aim that the compass set into the horizon ring also suggests. These features thus combined the terrestrial and celestial globes in one, supposedly making it less cumbersome than taking both on a journey. It seems unlikely, however, that even the terrestrial globe alone would have been a travelling companion.

From each wind rose 32 such lines fan out in all directions, showing the routes that ships could theoretically take on the high seas using the compass. Each rhumb line on Mercator’s terrestrial globe joins two points by crossing the intervening meridians at the same angle, creating the curve that a ship would have followed sailing on an unchanging course.

Drawing rhumb lines on a spherical surface is no simple matter. Although Mercator inherited the knowledge and skills needed to do so from his mentor, Gemma Frisius, he succeeded in transposing on the 3-dimensinal surface of his terrestrial globe the network of straight lines which had, until then, been incorrectly represented. He drew them on the gores in such a way that they became rhumb lines when laid on the sphere. He managed to do this superbly four years after the Portuguese geometer Pedro Nunez had imperfectly formulated the convergence of the meridians on a spherical surface, around 1537.

Perhaps wanting the terrestrial globe to be used as an onboard navigational instrument, Mercator came up with a crucial innovation for mariners even though the idea of using a globe on a ship was not really plausible for practical reasons that one can easily imagine. In 1569 he proposed a revolutionary solution that could at last be used: a planisphere, i.e. a projection of his terrestrial globe on a two-dimensional surface. This stroke of genius brought together portolan maps, based on the courses and durations of known sea voyages, and the maps of the Greeks, based on latitude and longitude.

In this case rhumb lines became straight trajectories on a Mercator projection. Mercator left no explanation of the procedure he used to achieve this conversion and it would not be until 1695 that the rhumb line equation was really worked out.

Ptolemy’s view revolutionised geographic knowledge when the world was first represented by Europeans and still inspired scholars during the Age of Discovery (c. 1418-1565). Apparently, though, it was the way Portuguese mariners’ discoveries in India had been made to fit into the Ptolemaic view by earlier globemakers that dissatisfied Mercator and prompted him to design a terrestrial globe of his own.

Indeed, the Greek scientist’s overriding and nearly tyrannical conception was gradually discredited in some circles by new observations, particularly those of sailors and explorers. Mercator, while still revering Ptolemy’s ideas, was among those who would temper the classical view with a more pragmatic approach. His terrestrial globe included the latest information reported by mariners as well as accounts of travellers like Marco Polo. In this respect he can be considered the founder of modern geography and even, as some observers already called him during his lifetime, “the modern-day Ptolemy”.

On Mercator’s terrestrial globe the Mediterranean basin is far better represented than by Ptolemy. Similarly, the features of Africa correspond more faithfully to reality as regards both landforms and nomenclature. Drawing on better sources, Mercator also improved the representation of Scandinavia, the Far East and the North Pole.

Gerardus Mercator undoubtedly mastered the series production of globes better than anyone else. In his time, globemaking was fraught with difficulties ranging from drawing the gores to engraving them on the copper plates to making the globe stay balanced in all positions and while rotating it to adjusting the printed gores so that the lines of all the circles met.

A veteran engraver, Mercator innovated in the gores’ application by creating polar calottes separately (the vertical gores themselves extended only to the 70th parallels instead of continuing all the way to the north and south poles). This not only consolidated the vertical gores but also made it easier to lay them out on the sphere and to read information in the polar regions.

Mercator also introduced a special design for the stands which remained the same subsequently. Finally, by varnishing the globes he enhanced the colours while protecting the surfaces.