João de Castro
D. João de Castro was born on the 27th of February 1500 to Álvaro de Castro, the civil governor of Lisbon. As the second son in his family, he was originally destined for a life within the church rather than military service. This path changed when he began studying mathematics under Pedro Nunes alongside Luis, Duke of Beja. The Duke was the son of King Manuel I of Portugal and became a lifelong friend to the young scholar. At just eighteen years old, de Castro traveled to Tangier where he spent several years away from home. During this time, Dom Duarte de Menezes knighted him while serving as the local governor.
In 1535 de Castro accompanied Dom Luís to the siege of Tunis but refused knighthood or rewards from Emperor Charles V. He returned to Lisbon only to receive the commendation of São Paulo de Salvaterra in the Order of Christ in 1538. Soon after this honor, he left for India with his uncle Garcia de Noronha to participate in the relief of Diu upon arrival at Goa. In 1540 he served on an expedition to Suez under Estêvão da Gama who was then viceroy of India. Da Gama departed on the 31st of December 1540 with twelve large galleons and sixty galleys. One of those galleons was captained by de Castro himself during the journey.
During the voyage to India de Castro carried out experiments that detected magnetic needle deviation long before others recognized it. When he attempted to determine the latitude of Mozambique on the 5th of August 1538 he noted a specific deviation of the needle. This observation occurred one hundred twenty-eight years before Guillaume Dennis of Nieppe received credit for the discovery in 1666. He observed another magnetic phenomenon on the 22nd of December 1538 near Bassein which was confirmed four centuries later. De Castro made forty-three observations of magnetic declination through measurements over the entire route around Africa. The instrument he used was called the Bussola de Variacão which had been developed by Felipe Guillén a decade earlier in Seville.
In 1545 de Castro was sent to India with six ships to replace governor Martim Afonso de Sousa. Seconded by his sons including Fernão who was killed in action, and by João Mascarenhas, de Castro overthrew Mahmud King of Gujarat. He defeated the army of Adil Khan and captured Bharuch while subjugating Malacca. In 1547 he was appointed viceroy by King João III of Portugal due to his victory at the second siege of Diu. After this victory he asked the king not to prolong his term beyond three ordinary years so he could return to the Sintra Mountains. He rebuilt Diu using money received from citizens of Goa before his death.
Unlike other viceroys de Castro showed deep interest in Indian culture and religion during his time abroad. He collaborated with humanist André de Resende to write a book on Indian art that documented local traditions. His estate of Penha Verde located in Sintra contains two famous black stones of Cambay retrieved by him and his son. These artifacts remain part of his legacy today alongside his written records. De Castro kept a detailed journal of his voyage which included maps calculations pictures and notes about coasts of the Arabian Peninsula. The Roteiro do Mar Roxo covered regions known as Somalia Eritrea Ethiopia Sudan and Egypt today.
De Castro died in the arms of his friend Saint Francis Xavier on the 6th of June 1548 while still serving as Viceroy. He was buried at Goa before his remains were exhumed and transported back to Portugal for reinterment in the convent of Benfica. Historians later recognized him as a key figure in maritime science due to his extensive work on magnetism. In the 1890s G. Hellman quoted by Chapman and Bartels considered de Castro the most important representative of scientific maritime investigations of the time. His method was universally introduced on ships and used until the end of the sixteenth century. Luís de Camões called him Strong Castro in poetry celebrating his life and achievements.
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Common questions
When was João de Castro born and who were his parents?
D. João de Castro was born on the 27th of February 1500 to Álvaro de Castro, the civil governor of Lisbon.
What scientific discovery did João de Castro make regarding magnetic needles?
During the voyage to India de Castro carried out experiments that detected magnetic needle deviation long before others recognized it. He made forty-three observations of magnetic declination through measurements over the entire route around Africa using an instrument called the Bussola de Variacão which had been developed by Felipe Guillén a decade earlier in Seville.
Why was João de Castro appointed viceroy of India in 1547?
In 1547 he was appointed viceroy by King João III of Portugal due to his victory at the second siege of Diu where he defeated the army of Adil Khan and captured Bharuch while subjugating Malacca.
How did João de Castro die and when did this event occur?
De Castro died in the arms of his friend Saint Francis Xavier on the 6th of June 1548 while still serving as Viceroy. He was buried at Goa before his remains were exhumed and transported back to Portugal for reinterment in the convent of Benfica.
Which historical figures praised or studied the work of João de Castro after his death?
Historians later recognized him as a key figure in maritime science due to his extensive work on magnetism and Luís de Camões called him Strong Castro in poetry celebrating his life and achievements. In the 1890s G. Hellman quoted by Chapman and Bartels considered de Castro the most important representative of scientific maritime investigations of the time.