Mercator distanced himself in a number of ways from his master Gemma Frisius. For experts such as Elly Dekker (1994), he apparently tried to reconcile the opinions of Claudius Ptolemy and Nicolaus Copernicus.

For example, in the 16th century, the constellation Lyra was traditionally shown as a bird or as a combination of a bird and a sort of violin. These representations stem from the Arab influence in the circulation of Ptolemy’s Almageste. Seeking greater accuracy, Mercator replaced the bird-violin image with a Vultur cadens, a musical instrument of Greek origin that was unknown in the Arab world. The Greek version of the Almageste actually described Lyra as being composed of a “shell” (i.e. a tortoise shell, like the one used by Hermes to make the very first lyre) with horns and a crossbar.

A number of human figures are dressed on Mercator’s globe whereas Frisius shows them naked.

Mercator’s cartographic aspirations can also be seen in the sizeable nomenclature of individual constellations and stars. The constellations are identified by their Latin and Greek names with a transliteration in Arabic (or what is meant to be Arabic). Such efforts point up Mercator’s erudition. From this point of view his celestial globe is the most comprehensive of any made in the 16th century. Here are a few of Mercator’s additions :

• Ceginus (Gamma Bootis, Phi Bootis),
• Incalurus (Mu Bootis),
• Saclateni (Êta Aurigae),
• Angetenar (Tau Eridani),
• Acarmar (Thêta Eridani),
• Alphart (Alpha Hydrae),
• Markeb (Kappa Puppis, Kappa Velorum)
• etc.

The influence of Nicolas Copernicus can be seen in the precession of the equinoxes, i.e. the gradual change in the Earth’s rotational axis. Mercator applied the new Copernican view to calculate stars’ positions, which in this theory referred to the year 1550.

Calculating to correct for precession was the biggest astronomical challenge faced by users of Ptolemaic star positions. A catalogue that defines stars’ coordinates based on a slowly changing system of reference always indicates the positions for a given moment in time, called the epoch. To calculate a star’s position at a later epoch based on existing catalogues, astronomers had to increase the star’s ecliptic longitude by a constant value as determined using the well-known theory of precession.

In the 16th century, a number of theories of precession coexisted. The values that were used to correct for precession, i.e. by increasing the longitude in relation to the one at the time of Ptolemy, could vary enormously.

In this area Mercator once again distanced himself from his master Gemma Frisius. Comparing the two geographers’ globes, we can for instance see a difference in the longitude of the star Regulus. The precession correction of Frisius’ globe (1537) is 19°40’, whereas the one on Mercator’s globe (1551) is 20°55’. By modern standards, a difference of 1°15’ would mean a difference of epoch corresponding to almost 90 years. This contrasts with the 14 years which actually separated the two globes’ publication. It is worth noting that celestial globe designers always opt for an epoch close the date their creations go into production.

There are variations between the globe published by Gemma Frisius (1537) and the one published by Mercator (1551), for example in the representation of Aires and Gemini. This suggests that Mercator drew on different sources to indicate stars’ positions. Since his globe was published later, he no doubt believed that these positions were more appropriate.

For centuries the positions of stars were taken from the star catalogue in Claudius Ptolemy‘s Almageste and it was not until the late 1500s that the new observations of Tycho Brahe would be more accurate.

In the 16th century men of science like Gemma Frisius and his pupil, Gerardus Mercator, referred to star catalogues that all had one thing in common : the stars’ positions were described using a system of coordinates whose fundamental plane was the elliptic. On a celestial globe like the one produced by Frisius in 1537, the meridians were generally drawn from the north elliptic pole to the south elliptic pole and not, as in the case of Mercator’s terrestrial globe, from the north equatorial pole to the south equatorial pole.

Mercator’s celestial globe is one of the few examples that differs from the usage of its day by using a system of equatorial coordinates that sets the German-Flemish globemaker apart from other contemporaries. This was no small feat as Mercator applied his system to over a thousand stars. Experts suppose he had an effective method for converting the coordinates, suggesting that he used a universal astrolabe.

Diagram of equatorial coordinates

In this system, the Earth is at the centre. Projected onto the celestial sphere, the Earth’s equator becomes the celestial equator (blue circle). The same holds for the Earth’s north and south poles. The elliptic (yellow circle) is the plane of the Earth’s orbit around the Sun. The hour circle, or meridian of the given star, is the large circle passing through the poles and the star itself. The intersection of the elliptic and the celestial equator sets two points. The one pointing to the constellation Pisces is called the vernal point, from which the right ascension (horizontal red line) leads along the celestial equator. The declination is determined by the position of the star on the meridian (vertical red line).

© Autiwa, source : Wikipedia.

Past historians, influenced by Fiorini (1899) and Stevenson (1921), contended that Mercator did not have the same talent in astronomy as he did in geography, even though his celestial globe could still be considered a remarkable scientific work. Yet more recent studies, especially by Elly Dekker, have shown that Mercator’s celestial globe presented considerable improvements compared with earlier spheres, in particular the one designed by Gemma Frisius.

The astrological information on Mercator’s celestial globe stemmed from sources that were translated during his time. For example, he used De supplemento, an almanac by Girolamo Cardano first published in Milan in 1538 and reprinted in Nuremberg in 1543.

Mercator also knew Tetrabiblos by Claudius Ptolemy, a four-volume mathematical study translated and published in Nuremberg in 1535 by Joachim Camerarius. Like all his contemporaries Mercator also practised astrology, which he promoted by producing his celestial globe.

Besides the up-to-date information that Mercator presented on the nature of the stars, their positions were set in accordance with the brand-new theory of the precession of the equinoxes published by Nicolas Copernicus in 1543 in his De revolutionibus orbium coelestium, a book which laid the bases of heliocentrism. Mercator was thus the first globemaker to tap into this theory.