Gerard Mercator (1512-1594) is best known for the first use, in 1569, of the map projection bearing his name. This was the first projection on which any given compass bearing could be plotted as a straight line, thereby greatly aiding navigation at sea. Mercator was a publisher of maps and atlases, but he is only known to have produced one pair of globes: a terrestrial globe in 1541 and a matching celestial globe in 1551. These globes were produced while Mercator was in Louvain where he had lived since entering the University of Louvain in 1530. Surviving examples of the Mercator globes are rare and the pair at the Harvard Map Collection are the only known matched pair in America.
You can pick a constellation from any of the three drop-down lists:
To see an image of the part of the globe containing the selected constellation, click on the button below the drop-down lists. If a constellation depicted by Mercator is selected, the image will include that constellation, and, in some cases, nearby constellations. If a newer constellation is selected, then the image will show the part of the globe where that constellation would have been depicted, had Mercator drawn all 88 modern constellations. In some cases, particularly for southern constellations, the area shown will be a part of the globe where no constellations were shown by Mercator. Mercator used uncharted areas of the sky to draw the globe's cartouches, and these areas will be show when you ask to see the position of some of the modern constellations.
- The list of 50 constellation names used by Mercator on the globe.
- The list of the modern names for the 50 constellations depicted by Mercator. Most of the names shown by Mercator are the same as the names used today, but where they differ, this second list shows the modern equivalent.
- The full list of 88 modern constellation names. This includes both constellations depicted by Mercator and those that have been recognized after Mercator.
A number of interesting features of the Mercator celestial globe can be seen while exploring the images of the globe:
- Mercator depicted 50 constellations on his celestial globe. This includes the 48 traditional (Ptolemaic) constellations plus two others which, though still from classical times, are later additions to the standard set:
Coma Berenices - 3rd century BC, honoring Queen Berenice of Egypt, who, as legend has it, cut off her hair as an offering for the safe return of her husband from the wars.
Antinous - 2nd century AD, honoring Emperor Hadrian's beloved companion, who drowned in the Nile and was subsequently deified by Hadrian.
- There are 88 modern constellations, agreed upon by the International Astronomical Union in 1928. These include 47 of the 48 described by Ptolemy, Coma Berenices, and 40 others. Three of the 40 were defined from stars that in classical times had been viewed as part of the large southern constellation Argo Navis (the one Ptolemaic constellation that is no longer recognized.) The 37 other post-classical constellations date mostly from the 17th and 18th centuries. A large number of these newer constellations, approximately 25 of them, are in the southern hemisphere, in parts of the sky not visible to the Greeks and Romans, or to astronomers in Northern Europe until travel to the southern latitudes became practicable.
- The globe shows both constellations and individual stars. Different symbols (displayed in a key next to Ursa Major) depict six different levels of brightness. This follows the star catalogue in Ptolemy's Almagest, where each star's brightness was listed according to a six-point scale.
- The globe is drawn from an "outside" perspective. In other words, if the stars are imagined as being positioned on a celestial sphere, you are looking at the outside of the sphere with a celestial globe. In terms of left-and-right, this is the opposite view from what we see on earth looking up at the sky. For example, on a summer night, looking at the southern sky, you can see Sagittarius, and, to its right, Scorpius. On Mercator's globe, when held at the corresponding orientation, you will see Scorpius to the left of Sagittarius. In classical times, the figures represented by constellations were thought of as facing in towards the Earth. Consequently, with the outside perspective of the globe, the figures are drawn from the back. This outside perspective was not an invention of the globe-makers of Mercator's era. The celestial globe seen atop Atlas's shoulders in the Atlante Farnese statue, dating circa 150 AD, reveals this same outside view of the celestial sphere.
- Globes are typically mounted in their stands by a rod that passes through the globe's northern and southern poles, perpendicular to the equatorial plane. With a celestial globe, there is a choice between using the plane of the celestial equator (the extension of the Earth's equator) or of the ecliptic (the path in the sky along which the sun moves in its annual motion) as the plane to which the north-south rod is perpendicular. Mercator chose the celestial equator, and so the north-south rod passes through the north and south celestial poles. (The north celestial pole is where we locate the north star, which is the fixed point around which the stars appear to rotate during each 24 hours.) Unlike Mercator, most celestial globe makers in the 16th and 17th century chose the ecliptic, and on their globes the north-south rod passes through the north and south ecliptic poles.
- Mercator dedicated the celestial globe to George of Austria, the Prince-Bishop of Liège between 1544 and 1557. The dedication can be seen in the primary cartouche, in the southern hemisphere of the globe. (The southern sky, much less well known in 1551 than the northern sky, meant that the globe had an amount of empty space in the south, suitable for the placement of cartouches.)
- Ptolemy listed 1,022 stars in his catalogue in the Almagest. Mercator shows all of these individual stars. Look, for example, at the constellation Ara, for which Ptolemy catalogued 7 stars; you will be able to count the 7 individual stars drawn by Mercator. Mercator's information about the stars (location and brightness) often reflects more recent astronomical observation than what was provided by Ptolemy. It is interesting to note that though manuscript copies of the Almagest existed, the first printed version appeared in 1515, only 36 years before Mercator produced his celestial globe.
- In addition to the constellations and stars, Mercator draws the position of the Milky Way. This is clearly seen near the outstretched arm of Cassiopeia, where Mercator has also labeled this feature as 'Via lactea'.