João de Castro


João de Castro was a Portuguese nobleman and fourth viceroy of Portuguese India. He was called Castro Forte by the poet Luís de Camões. Castro was the second son of Álvaro de Castro, the civil governor of Lisbon. His wife was Leonor de Coutinho.

Early life

As the younger son of Álvaro de Castro, João was destined for the church. He studied mathematics under Pedro Nunes, along with Louis, Duke of Beja, son of King Manuel I of Portugal, with whom he formed a lifelong friendship. At eighteen, he went to Tangier, where he was made a knight by Dom Duarte de Menezes, the governor. He remained for several years.

Voyages to India and the expedition to Egypt

In 1535 he accompanied Dom Louis to the siege of Tunis, where he refused knighthood and reward from Emperor Charles V. Returning to Lisbon, the king awarded him the small commandership of São Paulo de Salvaterra in 1538.
Soon after this, he left for India with his uncle Garcia de Noronha, and on his arrival at Goa participated in the relief of Diu. In 1540 he served on an expedition to Suez under Estêvão da Gama, who knighted his son, Álvaro de Castro, a child of thirteen, in recognition of D. João.
After D. Garcia's death, D. Estêvão da Gama succeeded him, and D. João de Castro joined D. Estêvão on an expedition to the Red Sea. D. Estêvão da Gama departed on December 31, 1540, with 12 large galleons and carracks, and 60 galleys.
De Castro kept a detailed journal of the voyage with maps, calculations, pictures, and detailed notes of the coasts of the Arabian Peninsula as well as of those of the countries which today are known as Somalia, Eritrea, Ethiopia, Sudan, and Egypt. He traveled as far as Suez and to several ports in the shores of the Sinai Peninsula, all included in the Roteiro do Mar Roxo.
Unlike other viceroys, João de Castro had much interest in Indian culture and religion and was involved, in collaboration with the humanist André de Resende, in writing a book on Indian art. His estate of Penha Verde, in Sintra, also contains the two famous black stones of Cambay, brought by João de Castro and his son, Álvaro de Castro.

Later life

Returning to Portugal, João de Castro was named commander of a fleet in 1543 to clear Atlantic Europe, seas of pirates. In 1545 he was sent to India with six ships to assist Martim Afonso de Sousa, who had been dismissed as the viceroy. Seconded by his sons and by João Mascarenhas, João de Castro overthrew Mahmud, King of Gujarat, and defeated the army of the Adil Khan. He also captured Bharuch, subjugated Malacca, and traveled by the passage of António Moniz into Ceylon; and, in 1547, was appointed as viceroy by king John III of Portugal.
After the victory of his Armada in the relief of Diu, he asked the King not to prolong his term of office beyond the ordinary three years, and to allow him to return to the Sintra Mountains in Portugal. After his victory over Mahmud and the Adil Khan, D. João de Castro set about rebuilding Diu, and to obtain money sent an appeal to the citizens of Goa, who responded. He did not live long to fulfill this goal, however, dying in the arms of his friend, Saint Francis Xavier, on 6 June 1548.
He was buried at Goa, but his remains were afterward exhumed and conveyed to Portugal, to be reinterred 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, vigor 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, which 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 longitude, because 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 carried out a series of experiments, perhaps inspired by Pedro Nunes, that succeeded in detecting the phenomena, in particular, related to magnetism and the magnetic needle on board. When D. João de Castro attempted to determine the latitude of Mozambique on 5 August 1538, he noted the deviation of the needle 128 years before Guillaume Dennis of Nieppe, who is normally credited with this discovery. He observed on 22 December 1538 near Baçaim a magnetic phenomenon, for which there were variations of the needle because of the proximity of certain rocks, confirmed four centuries later. D. João de Castro refuted the theory that the variation of magnetic declination is not formed by geographic meridians.
João de Castro's recorded values of magnetic declination in the Atlantic and Indian oceans in the sixteenth century were useful for the study of terrestrial magnetism. He made 43 determinations of magnetic declination through 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. He discovered spatial variations of declination in the Bay of Bombay, which he attributed to the disturbing effects of underwater rock masses.
In the 1890s, G. Hellman, quoted by Chapman and Bartels, 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.