Short answers, pulled from the story.
João de Castro (the 27th of February 1500 - the 6th of June 1548) was a Portuguese nobleman, scientist, writer, and colonial administrator who served as the fourth Portuguese Viceroy of India from 1545 to 1548. He is significant both for his military campaigns in India and for his pioneering observations of terrestrial magnetism, which preceded credited discoveries by more than a century.
On the 5th of August 1538, de Castro recorded a deviation of the magnetic needle near Mozambique, a phenomenon not formally credited to another discoverer until Guillaume Dennis of Nieppe documented it in 1666 - 128 years later. He made 43 observations of magnetic declination across the route around Africa and refuted the theory that magnetic declination aligns with geographic meridians.
The Roteiro do Mar Roxo was de Castro's detailed navigational journal from the 1540-41 Red Sea expedition led by Estêvão da Gama. It contained maps, calculations, drawings, and observations of the coastlines along the Arabian Peninsula and regions now known as Somalia, Eritrea, Ethiopia, Sudan, and Egypt, including the ports of Suez and the shores of the Sinai Peninsula.
De Castro used the Bussola de Variacão, an instrument developed by Felipe Guillén approximately a decade earlier in Seville. With it, he made 43 observations of geomagnetic declination across the Atlantic and Indian Oceans. The method he tested was universally adopted on ships and remained in standard use through the end of the sixteenth century.
João de Castro died on the 6th of June 1548 in the arms of Saint Francis Xavier. He was buried at Goa before his remains were exhumed and transported to Portugal for reinterment in the convent of Benfica.
De Castro overthrew Mahmud, King of Gujarat, and defeated the army of the Adil Khan. He also captured Bharuch, subjugated Malacca, and traveled through António Moniz's passage into Ceylon. His victory at the second siege of Diu in 1547 led King João III of Portugal to appoint him viceroy.