Geomagnetism and the Cartography of Juan de la Cosa:
A New Perspective on the Greater Antilles in the Age of Discovery

Aldo Alvarez

On the second voyage of Christopher Columbus to the New World in 1493, he named as his cartographer Juan de la Cosa who had proven his map-making abilities during their first voyage. The Admiral appreciated the personal qualities that he observed of the maestre and owner of the Santa María and decided to confide in him this veiled and important task. The cartographic virtuoso Juan de la Cosa left us a map of the known world in 1500 that is preserved and exhibited at the Naval Museum of Madrid. It is the only known map of a witness of the first and second voyages of Columbus that includes a rendering of Cuba, Hispaniola, Jamaica and Puerto Rico as perceived by the instruments and knowledge of their time. The observations made by Cosa in this region between October 12, 1492 and March 10, 1496, are recorded on this testimonial projection.

Figure 1. The Atlantic and the New World region from the World Map of Juan de la Cosa of 1500, from facsimile of original located at the Naval Museum of Madrid

The Earth produces a physical manifestation of its dynamic nature known as its magnetic field, or earth produced magnetism – geomagnetism – that varies continuously over the years. This magnetism has a direct effect on one of the most important navigational instruments, the magnetic compass needle. Depending on one’s position over the planet’s surface, a variation occurs between the magnetic north to which the compass points and the true geographic north. Present-day geographical positioning systems automatically correct for this variation, but the explorers at the beginning of the age of discoveries were presented with a navigational predicament that took many years to quantify. Research conducted by Ricardo Cerezo Martínez,1 ex-director of the Naval Museum of Madrid, relative to the magnetic declinations in the Atlantic and the Caribbean during the age of discoveries, provides valuable information in helping to understand the perception of the region by the explorers of the era. The ignorance of the adventurous cartographer Juan de la Cosa about the magnitude of the impact of the magnetic field of the earth on the compass needle (magnetic declination) resulted in a representation of his observed geography that differs with modern era cartography. Frequently, interpretation of historic events is based on present-era map representations and not on the geography as perceived by the witnesses of the period. By knowing the magnetic declinations of the era for the Antilles, the map of Juan de la Cosa can be used to provide a new geographic perspective that may help in answering controversial issues.

After taking into consideration the magnitude of this phenomenon for the period, a new understanding of the explored region can be obtained by using his map. This cartographic information combined with the historic record can be used in interpreting events of the past. As an example, one contentious event, the discovery site of Puerto Rico by Columbus during his second voyage, has been studied over the years with inconclusive results. Many alternatives to this paradox have been proposed over the years, but unfortunately most are of a subjective nature lacking convincing evidence.  Juan de la Cosa, a witness of this event, can communicate, via his cartography, helpful information for deciphering the geography of this moment in history. Considering that the whereabouts of the Admiral’s Diario de a bordo, his day to day on-board log, is unknown,2 a primary witness of the first trips of discovery can enhance the understanding of the historical events of the era. This magnificent piece of cartography can also embellish the understanding of the writings of Christopher Columbus, Diego Alvarez Chanca, Michele de Cuneo, Bartolomé de Las Casas, Fernando Columbus, Peter Martyr, Juan Ponce de Leon and others. By knowing the physical geography of the area as observed by its discoverers, a more precise understanding of the events can be obtained.

Magnetic declinations for the year 1500 are presented in addition to various nautical charts, comparing the map of Juan de la Cosa with the present. These include various samples of the Spanish cartography of the period, such as the map of Captain Juan Escalante de Mendoza of 1575 of the west coast of Puerto Rico and maps from 1700 and 1737 of the area. This information may provide clues in deciphering other geographic mysteries of the age of discovery.

Juan de la Cosa

The Spanish cartographer, pilot and explorer, Juan de la Cosa, participated in the first voyage of Christopher Columbus to the New World as maestre of the nao Santa Maria.3 During this voyage, they explored portions of the Bahamas, Cuba and Hispaniola from October 12, 1492, when they reached Guanahaní an island in the Bahamas archipelago, until departing Cabo de las Flechas, on the north coast of Hispaniola on January 16, 1493 and returning to Spain.4  On the Admiral’s second voyage to the New World in 1493, he named as his cartographer Juan de la Cosa who had proven his mapmaking abilities during their first voyage. During this second voyage they explored the Lesser and Greater Antilles from November 3, 1493 5 until they returned to Spain from La Isabela, on the north coast of Hispaniola on March 10, 1496.6 Later, he returned to the New World in 1499 as chief pilot for Alonso de Ojeda in whose expedition he made his first of several voyages to Tierra Firme (north coast of South America).7 Here he explored in 1499, from the Boca de la Serpiente (Serpent’s Mouth, the southern entrance to the gulf of Paria near the mouth of the Orinico River) to Cabo de la Vela in the Guajira Peninsula.

In 1508, Cosa’s status as the most experienced navigator in the West Indies was acknowledged by being named by King Fernando to participate along with Amerigo Vespucci, Vicente Yañez Pinzón, and Juan Diaz de Solis in the Junta de Burgos. He contributed not only his geographical knowledge but also his technical skill as a pilot and cartographer. The office of Piloto Mayor in the Casa de Contratación and the proposal of the creation of a master map of the Spanish discoveries (the Padrón Real) were initiated by this junta.8 Native Indians eventually killed him with poisoned arrows in Urabá in 1510.9

In 1500 the cartographer Juan de la Cosa produced a map of the known world that is exquisitely preserved and exhibited at the Naval Museum of Madrid (Figure 1). Cosa’s beautifully drawn  manuscript was compiled upon his return to Spain in mid 1500. The date of the map has been debated, but studies using X-ray and ultraviolet-ray analysis indicate that the inscribed date of 1500 is accurate.10 The original parchment measures 183 by 96 centimeters (72 by 38 inches) showing both the Old and New Worlds. At the west end of the map, occupying unknown lands is an image of the Christ Child on the shoulders of Saint Christopher. Beneath this image is an inscription: “Joan de la Cosa la fizo en el Puerto de Sa Ma en anno de 1500” (“Was made in the port of Santa Maria in the year of 1500 by Juan de la Cosa”).11 It is a very important historical document as it is the only known map of an eyewitness of the first and second voyages of Columbus that includes a rendering of the Antilles as perceived by the instruments and knowledge of their time. In addition, it includes a depiction of the geography of the north coast of Tierra Firme (South America) as a result of his voyage of 1499.

Geomagnetism and the Map of Juan de la Cosa of 1500

The earth produces a magnetic field, called the Magnetosphere that surrounds and protects the planet. Although its existence had been theorized for many years, beginning with William Gilbert in 1600,12 it was first discovered in 1958. Scientists at first thought that it was doughnut shaped and symmetrical. Later on, sensitive instruments aboard space vehicles found it to be teardrop shaped.13 This field protects us from the solar wind, the stream of electrically charged particles that surges outward from the sun’s surface and interacts with the Earth’s magnetic field as they approach Earth. The particles are deflected toward both of the Earth’s poles so that only a small portion of them enters the atmosphere. During periods of enhanced solar activity, the quantity of solar particle penetration increases.

Earth’s core has remained molten due to heat from ongoing radioactive decay. Circulation patterns of the fluid molten iron of the outer core of the planet (densities average 10.7 g/cm3) versus the slow movement of the overlying mantle (densities average 4.5 g/cm3) and the influence of Earth’s rotation, produce a dynamo effect that generates electrical currents that in turn induce a magnetic field.14 The poles of this magnetic field do not coincide with true north or south, the axis of rotation of the Earth. In mid-2002, the average position of the modeled north magnetic dip pole (according to the International Geomagnetic Reference field (IGRF)-2000 geomagnetic model) was 81.5° N, and 111.4° W, in the Canadian Arctic Ocean. This is approximately a distance of 950 kilometers from the true geographic north pole. The geomagnetic field can be quantified as total intensity, vertical intensity, horizontal intensity, and inclination and declination. The total intensity is the magnetic strength, which ranges from about 23 microteslas or 2.3 x 10-4 nanoteslas around Sao Paolo, Brazil to 67 microteslas near the south magnetic pole near Antarctica.15 The angle of horizontal intensity with respect to the north geographic pole is the declination, sometimes called variation by mariners.

This Earth-created magnetism, or geomagnetism, is very dynamic and is continuously changing. Variations of the magnetic field are caused by the buffeting of the Earth’s magnetosphere by the variable solar wind which in turn influences the Earth’s ionosphere and surface magnetic field. On occasion the resulting geomagnetic storms result in colorful displays known as aurora borealis and australis. The resulting phenomenon creates varying spatial patterns of magnetism with irregular distribution that influence the magnetized needle of the nautical compass. Depending on location on the Earth’s surface, this magnetic field will cause a deviation in the direction of the compass needle relative to true geographic north. The horizontal angle formed between the direction of the true geographic north and the magnetic north at any place is called magnetic declination. A declination toward the east is defined as positive and toward the west as negative.

In recent years isogonic charts (maps with isogonal lines or lines of equal magnetic declination) for earlier epochs have become available. These are based on models derived from archeomagnetical observations which are readings during the last few decades by archaeologers and geophysicists.16 The measurements of weak residual magnetism in several sites located around the globe such as fire kilns in historic settlements and dated sediment layers and lava flows have been published by Daly and le Goff 17 and by Constable.18 Unfortunately, none of the resulting isogons produced by the models for the epoch of 1500 covers the Caribbean region in any detail.

During his first voyage, Columbus on the night of September 13, 1492 and passing one hundred leagues west of the Azores longitude, noticed an unusual shift of the compass needle as it moved from positive to negative magnetic declination. He wrote in his log, “…las agujas de marear que fasta entonces nordesteaban, noruesteaban una cuarta de viento todo entero…” (“...the compass needles that up to this point had a variation toward the northeast, changed one fourth to the northwest, all at once...”).19 The compass needle had shifted from northeast variation to a northwest variation, “una cuarta” or 11˚ 15’.20   At this point in time, the small positive (toward the northeast) variation of the compass needle that navigators in that part of the world had experienced for centuries took on a whole different meaning. He had crossed a point on the Earth’s surface of zero magnetic declination (an agonic line). Columbus had observed a physical manifestation of the planet that with the passage of time would be known as the phenomenon of magnetic declination.

Columbus again reported this occurrence during his return to Spain from his first voyage and later on subsequent voyages. He used these events to approximate the longitude of his location referenced as one hundred leagues west of the Azores.21 During his third voyage to the New World in 1497 when passing near Trinidad toward Hispaniola, Columbus again passed an area of zero magnetic declination as the compass needle shifted from positive to negative. On the night of August 16, 1498 he observed the following, “…en este viaje nunca le ha noreuesteado, hasta noche, que noruesteava más de una cuarta y media, y algunas agujas noruesteavan medio viento, que son dos cuartas; y esto fue todo de golpe, anoche…” (“...on this trip it had no northwest variation, until last night, when it varied to the northwest by one fourth and a half, and some needles half a wind, that are two one fourths; and that occurred all at once, last night...”).22 That night the explorers witnessed a shift from positive to negative that varied from cuarta y medio to medio viento or from 16˚53’ to 22˚30’.23 Crude measurements of the magnitude of the magnetic declination of the compass needle began in the next century. Experienced navigators, such as Juan Escalante de Mendoza and others, applied empirical corrections over the years, but the problem generally persisted until Halley published his magnetic charts, Atlantic Chart and World Chart in 1701 and 1702. 2

In Table 1, magnetic declinations are presented for the region of the Antilles and for the area from Cadiz to the Canary Islands as estimated by Ricardo Cerezo Martínez for the year 1500.25 These values illustrate various significant realities in reference to Juan de la Cosa’s Map. The magnetic declination east of the Canary Islands, toward the coast of Spain is approximately five degrees positive (toward the northeast) for the period. This was an area frequently traveled by the navigators of the time and in these areas they were accustomed to experiencing positive magnetic declinations. These variations with Polaris (true North), were attributed to imperfections of the instrument. Since the variation was always in the same direction (positive), the difference was accepted as a natural phenomenon and a correction was implemented without knowledge of the cause.26 

Region Magnetic Declination (In degrees)

Cadiz  to Great Canary  Is.       +5
Greater Antilles                       -11
Guadalupe to Jamaica             -11
Puerto Rico to Jamaica           -11
Trinidad to Cabo Vela             -3
Trinidad to Santo Domingo      -8
Trinidad to Puerto Rico           -8
Trinidad to Guadalupe             -9

Table 1:  Magnetic Declinations in the Caribbean circa 1500. Values  as shown by Ricardo Cerezo Martínez in La cartografía náutica Española en los siglos XIV, XV, y XVI, p. 105.

In his map of 1500 (Figure 1) Juan de la Cosa has a meridian drawn perpendicular to the equator near the Azores. He believed that at this location the orientation of the compass needle was pointing directly toward true north. Ricardo Cerezo explains, “Juan de la Cosa cometió el error propio de quien ignoraba la existencia de la declinación magnética al trazar el meridiano de la Azores perpendicular al Ecuador porque creía que en él la orientación de las agujas de marear debía de ser la del norte verdadero. Tal idea persistió durante muchas decadas en el bagaje de conocimientos náuticos. Con el trazado de este meridiano expresó Juan de la Cosa que los rumbos en su carta eran una trancripción exacta de las lecturas obtenidas en la aguja…” (“Juan de la Cosa committed the error typical of somebody who ignores the existence of magnetic declination, by drawing the meridian at the Azores perpendicular to the Equator, here he believed the compass needle pointed to true north. This idea persisted for many decades in the baggage of nautical knowledge. With the drawing of this meridian Juan de la Cosa expressed that the directions on his map were an exact transcription of the observations obtained by his compass needle.”)27 Juan de la Cosa transcribed his map west of the Azores, using the magnetic north of his compass as true geographic north. The ignorance of the explorers of the New World of the impact of the magnetic field of the earth on the compass needle resulted in a representation of their observed geography of the Antilles and the north coast of South America that differs with modern era cartography as modern maps are mostly transcribed with meridians pointing toward true geographic north.

Frequently, interpretations of historic events are based on modern map representations and not on the geography as perceived by the witnesses of the period. By knowing the magnetic declinations of the era for the Antilles, the map of Juan de la Cosa can provide a new geographic perspective that may help to interpret past events. The understanding of the writings related to that epoch by historians can be enhanced by knowing the perception of the physical geography of the area by the participants of the events, those that actually lived that moment in history. As an example, in the next section, the map of Juan de la Cosa of 1500 will be used as a tool to analyze the possible location of landing sites in Puerto Rico by Christopher Columbus in 1493. 

The Discovery of Puerto Rico

On November 19, 1493, Christopher Columbus and his armada of seventeen ships with over twelve hundred men and women landed on the island of Puerto Rico.28  The recent finding of the fleet’s payroll log in the Archivo de Simáncas near Valladolid indicates that the armada was composed of five naos and twelve carabelas.29 Starting on this date, the Admiral visited for two days a site on the island whose location has been and continues to be a source of much controversy. Unfortunately, the geographic location of his landing site is not precisely indicated or explained in any known historic document. Distinguished historians, geographers, poets and politicians have offered dozens of opinions of the possible sites, such as Guayanilla, San Juan, Rincón, Aguadilla, Aguada, Añasco, Mayagüez, Boquerón and El Combate but regionalist emotions tend to dominate the opinions, obscuring the objective analysis.30 Celebrations are held every year in the various municipalities that claim the distinction on the anniversary of the discovery. Distinguished personalities are invited, and the festivities proceed with parades, fireworks, and all the formalities of the occasion.

Statues and plazas in honor of the great Admiral abound. The rivalry and the differences of opinion between the inhabitants of the towns that claim the distinction have become so bitter that on occasion they have resulted in acts of violence.31 Recently, politicians have noticed that political opportunities can be gained by becoming an expert historian or geographer and some have claimed absolute knowledge of the truth which by chance tends to favor their district. Some have even attempted to have history legislated by conveniently contriving to have the legislature formulate a law declaring their town the official site.32

Samuel Morrison, in his Pulitzer Prize-winning book of 1942, Admiral of the Ocean Sea, was of the opinion that Columbus had sailed the southern part of the island and had anchored in Boqueron Bay near the southwestern tip of the island located in the municipality of Cabo Rojo.33 The distinguished Spanish historian, Martín Fernández de Navarrete exposed in 1825 that the event had occurred in the port of Mayagüez. In 1998 some members of the Academy of Puerto Rican History expressed the opinion that the event took place someplace within Aguadilla Bay (includes both Aguadilla and Aguada).34 The controversy continues. 

Figure 2. The Antilles from the Map of Juan de la Cosa of 1500. The heavy parallel line is the Tropic of Cancer which is drawn perpendicular to the compass rose North (magnetic north). This is an additional indication that directions were transcribed with the magnetic north as true geographic North.

The map of Juan de la Cosa can be useful in deciphering this paradox. This expression by a witness can be used to reduce the number of possible landing sites when combined with other historical documents, thus, simplifying the overall analysis. After diminishing the alternatives, further synthesis of evidence can then be applied toward a solution to this mystery of history. Various chroniclers of the epoch left writings with succinct pieces of information that by themselves are ambiguous but when combined with Cosa’s map can be of great significance. The Italian chronicler, Pietro Martire d’Angheira (Peter Martyr) in his Décadas del Nuevo Mundo, in reference to the landing site wrote the following: “…En su ultimo cabo de occidente solo para hacer aguada…”  ( “…On (near) the last cape of the west (coast) only to get water…”)35 The Spanish historian Bartolomé de Las Casas, in his Historia de Las Indias, in his interpretation of the Diario de Colón, wrote of the site, “…En una bahía de ella, al Poniente…” (“…In one of her bays on the west coast…”).36 The son of Christopher Columbus, Fernando Columbus wrote, “…En un Puerto de aquélla, hacia el Occidente, fondeó la Armada…” (“…The Armada anchored in a port on the west (coast)…”).37  

The chroniclers do not mention the specific location of the landing site, but they mention something in common: the landing occurred on the west coast of the island. Although this may seem to be trivial, it is of utmost importance to the solution of this historical dilemma. They are stating that the landing site was not on the east, north, or south coast of the island. All locations on these coasts can be eliminated as possibilities. But what was the west coast of the island of Puerto Rico according to the perception of its discoverers? How did they define it? Is it the same as our modern interpretation? The map of Juan de la Cosa, drawn by a witness of this event, can assist in finding the answer. To facilitate the study of this question, the portion of the map of Juan de la Cosa that contains the island of Puerto Rico is amplified in Figure 3. 

Figure 3. The eastern portion of the island of Hispaniola and the island of Puerto Rico from the Map of Juan de la Cosa.

On the upper left of Figure 3 is the eastern portion of the island of Hispaniola, the present day Dominican Republic. Toward the south, on the southeastern tip of this island is another small island, the island of Saona, named by Columbus in honor of his friend Michele de Cuneo of Saona.38 The most eastern northern tip, Cabo Engaño, is clearly visible as is the Gulf of Samaná, or as Columbus called it, Golfo de Las Flechas north of this tip. Toward the east of Hispaniola is the island of Puerto Rico or San Juan Bautista as originally named by Columbus. The thick line that passes through the south coast of the island of Puerto Rico is the Tropic of Cancer. Since the time of Erathosthenes, 240 B.C., the equinox line in the northern hemisphere has been known to be located at approximately 23˚ N. Modern measurements place it at 23˚30’ with some minutes of variation over the years. 3

Juan de la Cosa knew that the Tropic of Cancer was at 23˚ N, which implies that the latitudinal measurements of the explorers during the second voyage of Columbus placed the island of Puerto Rico at the 23˚ north parallel. The southern coast of Puerto Rico where the Tropic of Cancer line is drawn is actually at latitude 18˚ N, indicating a five-degree error. In his letter dated January 30, 1494 to the King and Queen of Spain, describing his second voyage up to that time, the Admiral places the new colony of La Isabela at 26˚ N latitude, which in reality is located at 20˚ N,  representing a six-degree error.40

In between Cabo Engaño and Puerto Rico are three small islands located in what is now called Mona Channel (see Figure 4). The largest of the three is Mona Island, and on its northwestern corner is Monito. Toward the northeast of Mona is the island Desecheo. On the north coast of the island of Puerto Rico can be seen various distinct points. From east to west, the large bay is San Juan Bay. Continuing westward one can see the rest of the north coast up to a cape that is called today Punta Borinquen. At the cape that is today known as Punta Higüero  the north coast ends and the west coast begins. The peninsula of Punta Higüero, jutting out toward the northwest, points directly in the direction of Desecheo Island, precisely as it is represented by modern cartography (see Figure 5). Punta Higüero has been known with various names, such as Punta de San German, Punta de San Francisco and others, over the centuries.41 The southern tip of the west coast is Cabo Rojo. Toward the east on the southern coast is Guayanilla, one of the reputed landing sites. Figure 5 shows a modern era representation of the island of Puerto Rico.

Figure 4. Silhouette of the island of Puerto Rico, as drawn from the Map of Juan de la Cosa of 1500 without Tropic of Cancer line (tracing of silhouette done by author).



Figure 5. Modern era representation of the island of Puerto Rico.

The Explorer’s Perception

The following illustration, Figure 6, shows a nautical map for the year 2000 of the northwestern section of Puerto Rico with the compass rose indicating a magnetic declination of 11˚ 45’W (negative) for the area.42 Magnetic declinations for future years can be estimated by using the annual increase on the map (six minutes per year). Thus, for example, if one were interested in the magnetic declination of the area for the year 2002, this could be easily computed resulting in a magnetic declination of 11˚ 57’ toward the west (negative). Navigators apply this correction to the reading of the compass in order to approximate true geographic north.

Figure 6. On the left - Northwestern Puerto Rico from Chart 25640 NOAA April-2000. On the right - the compass rose from the same chart enlarged - with magnetic declination for the year 2000 given as 11˚45’W.

In order to better visualize the perspective of Juan de la Cosa and the explorers of the era, Figure 7 illustrates the northwestern portion of Puerto Rico illustrated in Chart 25640 after being rotated 11˚ clockwise (the magnetic declination for the year 1500, of approximately -11 degrees from Table 1 for this area of the Caribbean). After the applied rotation, one can perceive the northwestern area of the island of Puerto Rico as we know it today, but with the magnetic north taken as true geographic north, as it was transcribed by Juan de La Cosa. Considering that the cartography was transcribed using magnetic north as true geographic north, one can visualize a different perspective relative to what can be defined as the north and west coasts of the island. 

The modern era map transcribed with true geographic north illustrates the west coast as extending from Punta Borinquen to Cabo Rojo. However, after rotating clockwise the magnitude of the magnetic declination, a new definition is perceived. The west coast now commences at Punta Higüero extending to Cabo Rojo, with Aguadilla Bay and Punta Borinquen now located on the north coast, exactly as it is represented in the map of Juan de la Cosa.

The discoverers were not the only ones with this perception of the west coast of the island. This notion persisted for many years as evidenced by various subsequent cartographic maps. Three other cartographic representations of the era are presented to provide validation of this observation. As a first example, Figure 8 presents a nautical map by Francisco Fernández de Valdelomar of 1737 illustrating the northwestern portion of the island compared with a modern nautical map of the same area.43 In this comparison, one can see the similarity of the Fernández map of 1737 with the modern map that has been rotated clockwise 11°. One can discern on the 1737 map that the compass rose is pointing toward the magnetic north not the true geographic north. The segment of the coast from Punta Higüero south toward Punta Cadena is almost identical in orientation. The directions on the 1737 map have also been directly transcribed from the nautical compass needle just as in the Cosa map. The cape that is named Punta de San Francisco is the same as the present day Punta Higüero. Here the west coast ends and the north coast begins from this point eastward beginning with what is named Bahía de la Aguada, or the present day Aguadilla Bay. Using the magnetic north as if it were the true north, the map of 1737 is drawn very similar in appearance to Chart 25640 of April-2000 after being rotated 11˚ clockwise. Both the 1737 map and the April-2000 map indicate that the north coast of the island of Puerto Rico commences at Punta Higüero, exactly as shown in the Juan de la Cosa map. This cartographic example illustrates that still in the year 1737 some Spanish cartographers were transcribing their maps with the magnetic north as true geographic north. 

Figure 7. Chart 25640 NOAA April-2000 rotated clockwise 11˚ compared with the northwestern area of Puerto Rico from the map of Juan de la Cosa. Punta Borinquen and Aguadilla Bay are now perceived as being located on the north coast on both maps.



Figure 8. Map of Fernando Fernández of Valdelomar of 1737(left) compared with NOAA Chart 25640- April 2000 rotated 11˚ clockwise comparing the northwestern corner of Puerto Rico (right). Footnote of the Fernández map states, <<Plano de la Bahía de Aguada, Bahía de Añasco y Puerto de Mayagüez por Francisco Fernández de Valdelomar 1737 – Archivo General de Puerto Rico, Archivo 1, gaveta 4, no. 436.>>



Figure 9. Map of 1700 titled <<Aguada del norte de la isla de Puerto Rico, donde van a hacerla las flotas>>, by an unknown cartographer explicitly locating the Bay of Aguada (present day Aguadilla Bay) on the north coast of Puerto Rico. Footnote states <<Aguada del Norte de la Ysla de Puerto Rico donde van a hacerla las flotas, Mapa ca. 1700.>> Museo Naval de Madrid.

In Figure 9, a map is shown that defines explicitly where the north coast of the island ends and the west coast begins as perceived in 1700. 44 The map of 1700 of Aguada del Norte De La Ysla de Puerto Rico, by an unknown cartographer, clearly names the Bay of Aguada (present day Aguadilla Bay) in 1700 as being perceived as being located on the north coast of the island. On the extreme right of this map, which uses a perspective view looking from north to south, can be seen Pta de Sa Fran that refers to Punta de San Francisco or present day Punta Higüero. According to this nautical map, from this location the north coast commences from west to east, and the west coast from north to south, exactly as in Juan de la Cosa’s map of 1500.

The great Spanish navigator Captain Juan Escalante de Mendoza wrote his epic work, Itinerario de Navegación de Los Mares y Tierras Occidentales, in 1575, a publication in three volumes describing the nautical science of the era, including specifics on piloting, astronomy, cartography, naval construction, fleet management, naval artillery, meteorology, etc. It is one of the most complete works of its nature, a true nautical bible of its time. Juan Escalante de Mendoza’s treatise was so detailed, that for various years it was declared a state secret by King Phillip II and its publication forbidden.45 The Captain, after a five-year period navigating and reporting on all Spanish ports of the New World for his majesty, left nautical charts of the various ports he visited. The nautical chart presented in Figure 10 is of the west coast of Puerto Rico drawn in a perspective view from west to east, as if arriving from Hispaniola, detailing the west coast in 1575.46

Figure 10. Map of the Western Coast of Puerto Rico by Juan Escalante de Mendoza in 1575. There are no spaces between words. The title reads “Baia de s german y guadianilla”. “Bay of San German and Guadianilla”. The bay of San German was the first port of the island located in what is now known as Añasco Bay. The west coast is defined as from Punta de s german to C. Rojo(Cabo Rojo).

In his book Escalante de Mendoza describes all the ports existing in Puerto Rico in 1575.  The nautical chart of the west coast of the island illustrated in Figure 10 defines in addition to other valuable information, precisely what the Spanish considered as the west coast: from Punta de S German (San German) southward toward Punta C. Rojo (Cabo Rojo).  Not only is it defined in his map, but he clearly describes the west coast of the island in his book, “El segundo puerto se nombra San Germán, que está en el remate que la isla hace de la banda del oeste, y cuasi en los mismos dieciocho grados, en el cual hay muy poco, o cuasi ninguna población, más pudiese muy bien en él, tomar agua y las demás cosas para la navegación.” (“The second port is named San German, that is at the end [conclusion] of the west coast, and almost at eighteen degrees, in it there is very little population, but you can very well get water and all other things for navigation.”).47 The Captain clearly defines the end of the west coast as being Punta San German, present day Punta Higüero.  On the map he also defines the distance from one cape to the other, “Desde la punta de sgerman a Cabo Rojo hay ocho leguas” (“From the point of San German to Cabo Rojo there are eight leagues. This is equivalent to twenty-four present day nautical miles, which is precisely the distance between these two capes.48

Conclusion

Depending on location, Earth’s magnetic field creates an effect on the nautical compass needle that creates an angle variation relative to true geographic north called magnetic declination. This phenomenon is dynamic and continuously changing. The magnitude of the magnetic declination west of the Azores was unknown to the first European explorers that visited the New World. The Spanish explorer, cartographer and participant of the first and second voyages of Christopher Columbus, Juan de la Cosa, left us a magnificent world map in 1500 that includes the only known cartographic testimonial of the Antilles and the north coast of Tierra Firme by a witness of this epoch. He transcribed the New World portion of his map with the magnetic north indicated by his compass as true geographic north. This resulted in a representation of their observed geography that differs with modern-era cartography, one that most frequently uses true geographic north in its map representations.

In knowing the approximate values of the magnetic declinations for the New World in the early days of discovery, the map of Juan de la Cosa of 1500 can be used to provide a new perspective that may help in clarifying geographic mysteries. The understanding of historical writings by chroniclers of the period can be better interpreted by taking into consideration the perception of the physical geography of the area by the explorers of the epoch. As an example, the map of Juan de la Cosa of 1500 was used to assist in determining the geographic location of the landing site of Christopher Columbus in Puerto Rico in 1493. An amplification of the segment of Cosa’s map was made in order to facilitate its study and this portion was rotated clockwise by the number of degrees of the magnetic declination of the area in order to better visualize the perception of its discoverers. This map was then used in combination with the writings of the chroniclers Peter Martyr, Bartolomé de Las Casas, and Fernando Columbus to reduce the number of possible landing sites. Although these authors do not indicate the exact location of the landing site on the island, they do concur on the fact that the event occurred on the west coast.

The map of Juan de la Cosa is then used to establish the definition of the west coast of the island according to the perspective of its discoverers. In this manner, all other sites on the north, east and south coasts of the island can be eliminated from the list of possible landing sites, simplifying the analysis. Three other Spanish nautical charts were then presented to validate the definition of the west coast of the island for the epoch: Juan Escalante de Mendoza’s map of 1575, Fernando Fernández de Valdelomar’s map of 1737, and the map of 1700 by an unknown author defining Aguadilla Bay as being located on the north coast. All agree in the definition of the west coast of the island during that era as being precisely located between Punta Higüero in the north toward Punta Cabo Rojo in the south. 

In reality, the map of Juan de la Cosa combined with the historical sources cited has assisted in eliminating from the list of possible landing sites, various traditional locations. The spatial referencing of the west coast in his map as a result of the magnetic declination of the area and his transcribing the directions of his observations using the magnetic north of the compass as true geographic north has given us the perception of the west coast of the island of Puerto Rico by its discoverers. Cosa’s map has illustrated that all sites located on the east, north, and south of the island on his cartography are not a possible location. The historic chroniclers, Peter Martyr, Bartolomé de Las Casas and Fernando Columbus indicate it as such. On the south coast, the site of Guayanilla is clearly not a possibility. On the north coast, three possible landing sites can be discarded: San Juan, Aguadilla and Aguada. These much publicized traditional locations, Guayanilla, Aguadilla and Aguada can now be eliminated as plausible landing sites by the cartographic evidence of a witness. The elucidation of this mystery is now reduced to four possible sites: Rincón, Añasco, Mayagüez or Cabo Rojo. Other methods of analysis can now be applied, such as geodesic and cartographic, in combination with other historic writings of the witnesses of the event, such as Dr. Diego Alvarez Chanca, Michele de Cuneo, Juan Ponce de León and others in order to obtain an objective clarification of this paradox of history.

1 Ricardo Cerezo Martínez, La cartografía náutica Española en los siglos XIV, XV, y XVI (Madrid:Consejo Superior de  Investigaciones Científicas, 1994).
2
Robert H. Fuson, “The Diario de Colón: A Legacy of Poor Transcription, Translation, and Interpretation,” Terrae Incognitae 15 (1983): 59-75.
3
Antonio Vascano, Ensayo biográfico del celebre navegante y consumado cosmógrafo Juan de la Cosa (Madrid, 1892), p.13. 
4
Bartolomé de Las Casas, Historia de las Indias (Mexico City: Fondo de Cultura Económica, 1951) 1:302.
5
The Armada of seventeen naos and carabelas reached the island of Dominica on November 3, 1493. Consuelo Varela, Juan Gil, Cristóbal Colón. Textos y documentos completos (Madrid: Alianza, 1992), p. 236.
6
   Cerezo Martínez, La cartografía náutica Española en los siglos XIV, XV, y XV, p. 96.
7
   Silvio A. Bedini, Christopher Columbus and the age of exploration, an encyclopedia (New York, 1998), p. 213.
8    
Ibid, p. 213-215.
9
Urabá is located on the north coast of present day Columbia, near the city of San Sebastian near the Isthmus of Panama.
10
Cerezo Martínez, La cartografía náutica Española en los siglos XIV, XV, y XVI, p. 112.  
11
Vascano, Ensayo biográfico del celebre navegante y consumado cosmógrafo Juan de la Cosa, p. 11.
12
William Gilbert, On the Magnet, first English ed., (London,1900). 
13
  Robert W. Christopherson, Geosystems an introduction to physical geography, third edition, 1997, p. 323.
14
  Ibid.
15
Chris M. Goulet, Magnetic declination, www.cam.org/~gouletc/decl_fag.html, September 3, 2002.
16
L. Hongre, G. Hulot, A. Khokhlov, An analysis of the geomagnetic field over the past 2000 years, Physics of the Earth and Planetary Interiors, 106 (1998): 311-335.
17
L. Daly, M. le Goff, “An updated and homogeneous world secular variation database: 1. smoothing of the archaeomagnetic results,” Physics of the Earth and Planetary Interiors 93 (1996): 159-190.
18
C.G. Constable, C.L. Johnson, S. P. Lund, “Global geomagnetic field models for the past 3000 years: transient or permanent flux lobes?,” Philosophical Transactions of the Royal Society of London, ser. A., 2000, 358, pp. 991-1008.
19
Cerezo Martínez, La cartografía náutica Española en los siglos XIV, XV, y XVI, p.12.
20
Aldo Alvarez, Paradoja de la historia: el descubrimiento de Puerto Rico (San Juan, 2000), p. 45.     
21
Martín Fernández de Navarrete, Colección de los viajes y descubrimientos que hicieron por mar los españoles desde fines del siglo XV (Buenos Aires, 1945),  vol.1, p.378.
22
Las Casas, Historia de las Indias, 1: chap. cxlviii.
23
The 360° of the sphere is divided by the 32 directions of the rose on the Spanish compass and are known as cuartas.  A medio viento is two cuartas or 22°30’.
24
Cerezo Martínez, La cartografía náutica Española en los siglos XIV, XV, y XVI, p. 15.
25
Ibid, p. 105.
26
Ibid, p. 12.
27
Ibid, p. 20.
28   
Alvarez, Paradoja de la historia: el descubrimiento de Puerto Rico, p. 131. Columbus’s childhood friend Michele de Cuneo accompanied the Admiral on his second voyage. Cuneo wrote a letter to the Italian noble Hieronnymo Annari dated October 15, 1495, describing the second voyage. In his letter he indicates the date of departure from the island as November 21, 1493. The physician of  the fleet Dr. Diego Alvarez Chanca also wrote a letter describing the second voyage but to the Cabildo or mayor of Sevilla in which he indicates that the fleet anchored two days on the island. Columbus landed on the island November 19, 1493  and left November 21, 1493 at dawn.  
29
J. Varela, M. Carrera, M. León, Segundo viaje de Colón (Valladolid: Instituto de Estudios de Ibero America y Portugal, 1998), p.16.
30
Alvarez, Paradoja de la historia: el descubrimiento de Puerto Rico, p. 18-19.
31
Ibid, p.17.
32
El Nuevo Día, December 8, 2000.  At the end of November, 2000, the mayor of Aguada, Yuyo Román, convinced the governor, Pedro Rosello, to submit a project to the state legislature that would declare the municipality of  Aguada as the official Columbus landing site. Both were non-returning politicians, because they had lost the elections earlier that month. But, their political party still controlled both the house and senate until December 31, 2000. On the sixth extraordinary session of the legislature after the elections of the year 2000, the bill was submitted with little publicity.  It eventually passed the house on party lines but was eventually defeated in the senate as a majority of its members considered that history should not be legislated.
33
Samuel Eliot Morison, Admiral of the ocean sea: A life of Christopher Columbus (New York, 1942).
34
Osiris Delgado, Adam Szaszdi, Ricardo Alegría, Sobre el lugar del descubrimiento de Puerto Rico (San Juan : Academia Puertorriqueña de la Historia, 1998),  p. 4.
35
Pedro Martir, Décadas del Nuevo Mundo, Editoral José Porrúa e hijos, México, 1964, p. 111.
36
Delgado, et al, Sobre el lugar del descubrimiento de Puerto Rico, p. 4.
37
Hernando Colón, Vida del almirante don Cristóbal Colón (México: Fondo de Cultura Económica, 1947), p. 150.
38
José Gonzalez Ginorio, Descubrimiento de Puerto Rico (San Juan, 1936),  p. 89.
39
John Noble Wilford, The mapmakers (New York, 2000),  p. 25.
40
Varela and Gil, Cristóbal Colón. Textos y documentos completos, p. 249.
41
  Aurelio Tió, Nuevas fuentes para la historia de Puerto Rico (Barcelona: Ediciones Universidad Interamericana, 1961),  p. 194-211.
42
Nautical Chart 25640, National Oceanic and Atmospheric Administration, for April 2000.
43
Delgado et al, Sobre el lugar del descubrimiento de Puerto Rico, appendix plate 2.
44
Ibid, appendix plate 1.
45
Juan Escalante de Mendoza, Itinerario de navegación de los mares y tierras occidentales, 1575 ( Museo Naval de Madrid, 1985),  p.2.
46
Aurelio Tió, Dr. Diego Alvarez Chanca (Barcelona, 1966),  p.171.
47
Escalante de Mendoza, Itinerario de navegación de los mares y tierras occidentales, 1575,  p. 165.
48
Alvarez, Paradoja de la historia: el descubrimiento de Puerto Rico, p. 125

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