China
Africa
Explore chapters and articles related to this topic
Portuguese Claims to the Exclusive Domination of the Indian Ocean Regions
Published in K.S. Mathew, Shipbuilding, Navigation and the Portuguese in Pre-modern India, 2017
During the second half of the seventeenth century, other altitude instruments began to appear in Portuguese navigation, but the greater part of those known were imported from England without any contribution from the Portuguese instrument-makers. The double-arc quadrant or Davis quadrant developed by John Davis at the end of the sixteenth century is one among them. Semi-quadrant and octant were used in navigation. Another instrument known as Nonius was also in use for navigation.35
Monteiro da Rocha and the international debate in the 1760s on astronomical methods to find the longitude at sea: his proposals and criticisms to Lacaille’s lunar-distance method
Published in Annals of Science, 2022
Fernando B. Figueiredo, Guy Boistel
In the Portuguese navy of the time, the ‘balestilha’ (cross-staff) was the most common instrument to measure angles, preferring it to the Davis quadrant.58 The Davis quadrant was susceptible to twist due to its construction in separate and later joined parts, which caused some maintenance problems and reading errors. On the contrary, the cross-staff was easy to build and adjust and little prone to distortions. In fact, if used as a backstaff (the Sun does not drop in the eye), it was a very accurate instrument.59 However, Monteiro da Rocha recommends using modern octants, which had «a small telescope instead of the sight hole.» Through the telescope, the image of the celestial body is magnified and be seen with more distinction, which, as Monteiro points out, is of «significant advantage for adjusting the image of the body with the horizon, reaching a precision of four or five times more than if we had made it by sight.» (MS BNP 511, fl.45v).60