In his representation of the constellations and their nomenclature, Gerardus Mercator produced the most comprehensive celestial globe of the 16th century. These representations included figures from literary tradition inspired by Greek sources. In this respect, the German-Flemish cartographer sometimes seems less of an astronomer than a man of the Renaissance who selected his sources with a critical eye.

Each constellation is indicated with its Latin and Greek names, with an added transliteration of its Arab name (or what was meant to be Arabic in the 16th century). Mercator must have consulted several sources for his nomenclature and, on the face of it, proceeded in encyclopaedic fashion. His knowledge of astronomy came from books, not from observations.

Mercator’s Pleiades

Modern representation of the Pleiades. Source : stellarmap.com

Pleiades

The Pleiades cluster has been known since Antiquity. Some of its stars can be seen with the naked eye. Only four of them are described in Ptolemy’s star catalogue and its offshoots. The Mercator’s globe presents seven stars.

Portfolio of constellations

Northern hemisphere

1. Ursa Minor

2. Ursa major, Calisto

3. Draco, Aben

4. Cepheus

5. Bubulcus

6. Corona Cnosia vulgo septentrionalis

7. Hercules, Algethi Nessus Vitruvio, ignotum idolum Arato

8. Lyra postea Alohore id est Vultur cadens dicta

9. Avis Cygnus, Adigege

10. Cassiopeia

11. Perseus, Cheleub

12. Auriga, Erichthonius

13. Aquila, Vultur volans, Alcair

14. Delphinus

15. Sagitta, Telum

16. Serpemtarius, Alangue

17. Anguis, Serpens

18. Equus

19. Pegasus, Equus Gorgoneus, Alpharaz

20. Andromeda

21. Triangulus

22. Canicula, Algomeisa, Alsehere

23. Cincinnus, Caesaries, Berenicis crinis, Trica

24. Aries

25. Taurus

26. Gemini : Apollo quibusdam Castor, Anhelar Chaldaeis – Hercules quibusdam Pollux, Abrachaleus Chald.

27. Cancer

28. Leo, Alezet

29. Virgo, Erigone

Northern hemisphere

Close

Southern hemisphere

30. Libra

31. Scorpius, Nepa Ciceroni, Alatrab

32. Sagittarius, Crotus, quibusdam Chiron

33. Capricornus, Pan, Algedi

34. Aquarius, Ganimedes

35. Pisces

36. Cetus, Pistrix, Balena

37. Orion, Algeuze

38. Flumen, Eridanus, quibusdam Nilus

39. Lepus (listée une seconde fois, en tant que 51e constellation, sous le nom de “Leptus” par J. van Raemdonck)

40. Canis major

41. Argo navis

42. Hydrus, Asvia

43. Crater, Vas, Patera

44. Corvus

45. Centaurus, Chiron

46. Fera, Lupus – 20 étoiles

47. Ara, Thuribulum, Lar, Sacrarium – 7 étoiles

48. Corona australis, Arato

49. Piscis meridionalis

50. Antinous

Southern hemisphere

Close

• to find out the “quantity of day”, i.e. the latitude and times at which the Sun rises and sets ;
• to recognise the stars in the sky, even without the least prior knowledge ;
• to provide the wherewithal to measure how high a star or the Sun stands above the horizon and the Sun’s distance below the horizon ;
• to indicate the time it takes for a star to complete its course above the horizon, from the time it rises to the time it sets ;
• to determine the meridian of a place ;
• to find the latitude of a country ;
• to situate the sign and degree of the Zodiac where the Sun is located ;
• to determine the time of year when a star will pass the meridian at midnight ;
• to determine the time when a star will reach the meridian ;
• to determine when twilight will fall.

Mercator’s celestial globe is surrounded by a brass meridian capped at the North Pole by a time dial (absent on the Lausanne copy) and a horizon ring indicating the main religious holidays and their dates along with the twelve signs of the Zodiac for astrological forecasts.

The sphere is covered with twelve gores extending to the 70th parallels, with each polar region capped by a round calotte. The celestial gores are aligned with their equatorial coordinates rather than the coordinates of their ecliptic. They meet at the celestial poles, which thus coincide with the axis of the globe’s base.

A large part of the area around the South Pole is blank, which is no surprise since it could not be seen from the latitudes of Europe. Thus, stars with a declination exceeding an absolute value of 66°30’ are missing. The constellations are named in Latin and Greek with an Arabic transliteration

Mercator’s globe shows a precession correction of 20°55’, in line with the theory of Nicolaus Copernicus. The equator and ecliptic are graduated with the degrees numbered on them by tens. The prime meridian runs just next to the tail of Pisces, shown below the wing of Pegasus.

The horizon ring, or rational horizon, is divided width-wise into two halves :

• the inner half, from inside to outside, indicates the ring’s division into degrees, the signs of the Zodiac, the days and months of the Roman calendar, the main religious holidays and the main winds in the different seasons ;
• the outer half indicates astrological forecasts, a trace of Arab astrology which still had its followers in the 16th century. Historians suppose that in including them Mercator must have made a concession to popular beliefs.

© Royal Library of Belgium, reproduced with permission

The relative shapes and dimensions of individual stars are shown in six different sizes along with the nebulae. A list of models is given near the top of the globe, above the constellation Gemini.

Apart from the Milky Way and a large number of stars not belonging to symbolic asterisms, Mercator included nearly all of Ptolemy’s 1022 stars (according to J. van Raemdonck there are 934) spread across 51 constellations, compared with the 48 constellations commonly referred to since Antiquity. Among the additional representations we can find :

• Antinous, made up of twelve stars, situated in the southern hemisphere below the feet of Orion ;
• Lepus, made up of twelve stars, situated in the southern hemisphere below the feet of Orion ;
• Cincinnus (Caesaries, Berenicis crinis, Trica), made up of one star and two nebulae and located in the northern hemisphere under the tail of Ursa Major (the Plough). Coma Bernecis (Berenice’s Hair) has been on the official list of constellations since 1930 ;
• Canicula (Almogelsa, Alschere, Procyon) is situated in the northern hemisphere below the tail of Cancer, its correct position, contrary to the location in the southern hemisphere assigned to it later by Jocodus Hondius.

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.

There seem to be a number of complementary reasons behind illustrations of sea monsters or other exotic creatures in the field of cartography. For mapmakers the idea was:

• to liven up the image of the world with decorative features,
• to fill empty spaces with information,
• to draw attention to the vitality of the oceans and the variety of creatures living in them,
• to suggest that the oceans and uncharted lands could be sources of danger,
• to serve as the graphic memory of literature on the subject,
• to provide points of reference in a geography of the marvellous,
• to draw attention to the artistic talent of the cartographer, who could then sell his work at a higher price, depending on the quality and quantity of the illustrations.

Sea and land creatures

For historians, three representations of sea monsters are predominant in Renaissance cartography: Olaus Magnus‘s Carta Marina (1539); the Monstra Marina & Terrestria map (1544) by Sebastian Münster, partly inspired by Olaus Magnus; and the Islandia map by Abraham Ortelius, published in Theatrum Orbis Terrarum (1586) and inspired by the two previous works.

Gerardus Mercator was particularly influenced by Magnus’ Carta marina, which contained the largest, most varied and most comprehensive collection of sea monsters of the day.

Mercator’s terrestrial globe is illustrated with eleven sea monsters, seven of which were taken from Magnus’s Carta marina and two others from Gonzalo Fernandez de Oviedo‘s Historia general de las Indias (1535), a book that represented the very first images of animals in America. Mercator no doubt used both Magnus and Oviedo to demonstrate his ability to combine the most recent sources in a nutshell. In addition to the sea monsters he included one terrestrial animal.

The monsters in cartography provides a detailed roundup of Mercator’s sources and inspirations for the twelve sea monsters and opossum.