|João de Castro|
|Governor and Viceroy of Portuguese India|
|Monarch||John III of Portugal|
|Preceded by||Martim Afonso de Sousa|
|Succeeded by||Garcia de Sá|
|Born||27 February 1500
Lisbon, Kingdom of Portugal
|Died||6 June 1548 (aged 48)
Goa, Portuguese India
João de Castro (7 February 1500 – 6 June 1548) was a Portuguese nobleman and fourth viceroy of Portuguese India. He was called Castro Forte ("Stronghold" or "Strong Castle) by poet Luís de Camões. Castro was the son of Álvaro de Castro, civil governor of Lisbon. His wife was Leonor de Coutinho.
A younger son, and destined therefore for the church, he became at an early age a brilliant humanist, and studied mathematics under Pedro Nunes, in company with Louis, Duke of Beja, son of king Manuel I of Portugal, with whom he contracted a lifelong friendship. At eighteen he went to Tangier, where he was dubbed knight by Dom Duarte de Menezes the governor, and there he remained several years.
Voyages to India and the expedition to Egypt
In 1535 he accompanied Dom Louis to the siege of Tunis, where he had the honor of refusing knighthood and reward at the hands of the emperor Charles V. Returning to Lisbon, he received from the king the small commandership of São Paulo de Salvaterra in 1538.
Soon after this he left for India in company with his uncle Garcia de Noronha, and on his arrival at Goa went off for the relief of Diu. In 1540 he served on an expedition to Suez under Estêvão da Gama (the son of Vasco da Gama and them viceroy of Portuguese India), by whom his son, Álvaro de Castro, a child of thirteen, was knighted, out of compliment to him. Died D. Garcia, succeeded him in the government D. Estêvão da Gama, and D. João de Castro was found with him in the expedition to the Red Sea. D. Estêvão da Gama went with 12 large galleons and carracks, and 60 galleys, on 31 December 1540, being D. João de Castro captain of a galleon. This expedition to Suez was truly remarkable, and João de Castro made a detailed roadmap of it, with maps, calculations, pictures and detailed notes from the coasts of the Arabian Peninsula as of those of the countries of today Somalia, Eritrea, Ethiopia, Sudan and Egypt, as far to Suez and to several ports in the shores of the Sinai Peninsula, all included in the Roteiro do Mar Roxo, which offered to the Prince Louis. Eight months later he returned to Goa on 21 August, having acquired by the experiences made during the expedition, the name of the philosopher. "I pay great attention to eclipses of the moon," he writes, as also to longitudes and latitudes, fishes, seaweeds, currents, winds,the colour of the Red Sea, and every detail that might concern the art of navigation, to the delight of his friends Pedro Nunes and Prince Louis, who had furnished him with special instruments and other assistance for his voyage.
D. João de Castro called for the need of the coordination between the observation and reason, in the case of the navigation: "This science of navigation is poorly distributed among the men, or because they act like idiots, which for a long time and continuous exercise they reach many particulars, though with all their works are never to gain authority in their office, or those who have no experience, but with much learning and great practice in the science of mathematics, reached the shadow of this art but not the true science." It is in overcoming this divorce that rooted the ways of science, which reaches an understanding with no epilogue, an open knowledge, in constant motion approach, not compatible with the spirit of the system. João de Castro tell us that the knowledge of science is "come to the truth", that science is "closer to the truth and not the absolute truth": "We need to consider how much we owe for teaching us how not to ignore the fact that we can be closer to the truth, as are human things that men can learn (...). Not only to be expected by virtue of reaching the truth, but also get close to her."
Returning to Portugal, João de Castro was named commander of a fleet, in 1543, to clear the Atlantic European seas of pirates; and in 1545 he was sent, with six sail, to India, to assist Martim Afonso de Sousa, who had been dismissed of the viceroyalty. Seconded by his sons (one of whom, Fernão, was killed before Diu) and by João Mascarenhas, João de Castro achieved such popularity by the overthrow of Mahmud, king of Gujarat, by the relief of Diu, and by the defeat of the great army of the Adil Khan, that he could contract a very large loan with the Goa merchants. These deeds were followed by the capture of Broach, by the complete subjugation of Malacca, and by the passage of António Moniz into Ceylon; and, in 1547, by the appointment as viceroy by king John III of Portugal.
After the victory of his Armada in the refief of Diu, he besought the King not to prolong his term of office beyond the ordinary three years, and to allow him to return to the Serra de Sintra, and in his will he says: "I have near Cintra a quinta, called the Quinta of the King's Fountain, which I made, and to which I am greatly devoted because I made it and because it is in a country where my father and ancestors were born". After his victory over the overwhelming odds of Mahmud and of the Adil Khan, D. João de Castro set about rebuilding Diu, and to obtain money sent an appeal to the citizens of Goa with some hairs of his beard in pawn since it was impossible to send the bones of his son, as he had first intended, his death being but recent. The citizens of Goa responded nobly to the appeal, and when the Governor returned to Goa in the spring of 1547. they received him with great rejoicing. His triumph ceremony has been often described in the chronicles and tapestry.
He addressed the chief officials and magistrates of Goa: "I am not asham'd, gentlemen, to tell you that the Vice Roy of India wants in this sickness those conveniences the meanest souldier finds in the Hospitals. I came to Serve not to traffick in the East, I would to your selves have pawn'd the bones of my Son and did pawn the hairs of my beard to assure you I had no other plate or hangings in the house to buy me a hen, for in the fleets I set forth the souldiers fed upon the Governour's salary before the King's pay, and 'tis no wonder for the father of so many children to be poor. I request of you during the time of this sickness to order me out of the king's revenue a proportionable maintenance and to appoint a person of your own who may provide me a moderate allowance." 
He did not live long to fill this charge, dying in the arms of his friend, Saint Francis Xavier, on 6 June 1548. He was buried at Goa, but his remains were afterwards exhumed and conveyed to Portugal, to be reinterred under a splendid monument in the convent of Benfica. The chronicler Diogo do Couto ends his portrait of the Viceroy thus: "And for his great charity, temperance, disinterestedness, exceeding love of God, and other qualities of a good Christian, it may be affirmed that he will be receiving in glory the prize and guerdon of all his trouble and toil." And for the author Aubrey Fitz Gerald Bell: "by his energy, vigour of thought and action, by his splendid character, humane and resolute, he closed the most brilliant half-century of Portugal's history with a key of gold."
The Terrestrial Magnetism in the Roteiro from Lisbon to Goa: the experiences of João de Castro
The ancient Greeks had discovered that a dark stone metal could attract or repel objects of iron. Was the origin of the study of magnetism. At the time of the great sailing, the navigators could not find a ship at sea by the two coordinates, latitude and longitude, the determination of this required a clock on board to indicate the exact time at the meridian of reference, and the astronomical determination of longitude gave unacceptable errors. During the trip to India, D. João de Castro has carried out a series of experiments that succeeded in detecting the phenomena, in particular related to magnetism and the magnetic needle on board. It should be assumed that such knowledge comes from Pedro Nunes, of course the direct inspiration of all the observations he has done in his travels. When on 5 August 1538, D. João de Castro decided to determine the latitude of Mozambique, found the cause that dictated the astonishing uneasiness of needles; noted the deviation of the needle, discovering it 128 years before Guillaume Dennis (1666) of Nieppe, which is recorded in history of sailing as if he were the first to know about this phenomenon. His observation near Baçaim, on 22 December 1538, of a magnetic phenomenon, for which there were variations of the needle because of the proximity of certain rocks, confirmed four centuries later, called local attraction. D. João de Castro refuted the theory that the variation of magnetic declination is not formed by geographic meridians.
To the words of Pedro Nunes expressed in De Crepusculis, which refers the things he had discovered, meditating and investigating, that nowhere he had read before and wouldn't have credit if they were not shown, we can associate the words of D. João de Castro in its Roadmap (Roteiro) of Lisbon to Goa: "in this written script many things that will seem strange and impossible, which I wrote fearfully, not because of them were not very certificate, but for fear that I had to out of the common opinion. "
The comments made by João de Castro are the most important record of values of magnetic declination in the Atlantic and Indian oceans, in the sixteenth century, and useful for the study of terrestrial magnetism. He made 43 determination of magnetic declination through rigorous measurements of geomagnetic declination over the entire circum-Africa route. The instrument used by him was the Bussola de Variacão, also developed by Felipe Guillen a decade earlier in Seville. João de Castro undertook many observations and can in a way be considered as one of the discoverers of crustal magnetism. He discovered spatial variations of Declination in that Bay of Bombay (near Baçaim), which he attributed to the disturbing effects of underwater rock masses (this is near where the large basaltic and rather strongly magnetized Deccan traps outcrop). In the 1890s, G. Hellman, quoted by Chapman and Bartels (1940), considered Castro to be the most important representative of scientific maritime investigations of the time, and the method he tested was universally introduced on ships and was used until the end of the sixteenth century. It was one of the personalities of this century of European experimental science, linking the importance of this study with the sailing. His name was linked to science for his works which showed a trend for modern scientific spirit.
- Jacinto Freire de Andrade Vida de D. João de Castro, Lisbon, 1651 (English translation by Sir Peter Wyche in 1664).
- Diogo de Couto, Décadas da Ásia, VI.
- The Roteiros, or logbooks of Castro's voyages in the East (Lisbon, 1833, 1843 and 1872) are of great interest.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.