The achievements of Columbus and Da Gama are immeasurably enhanced when we consider the inadequate means at their disposal, their small and ill-formed ships, and their defective knowledge of navigation. The mariners compass had been in use for nearly two centuries, and it was Columbus himself who first observed the phenomena of variation. But the compass and rough sea-card were the only appliances, until the learned Nuremberger, Martin Behaim, invented the application of the astrolabe to purposes of navigation which enables mariners to ascertain their latitude. This was in the year 1480. The astrolabe was used by Vasco da Gama on his first voyage round the Cape of Good Hope; but the movement of a ship rendered accuracy impossible, and the liability to error was increased the necessity for three observers. One held the astrolabe by a ring passed over the thumb, the second measured the altitude, and the third read off. The astrolabe was a metal circle graduated round the edge, with a limb called the Alhidada fixed to a pin in the centre, and working round the graduated circle. The instrument had two sights fitted upon it, one at each end, and was suspended by a ring so as to hang vertically on one hard, while the alhidada was worked up and down until the sun could be seen through both the sights. It then gave the zenith distance. The Ordenanzas of the Spanish council of the Indies record the course instruction prescribed for pilots, which included the De Sphoera Mundi of Sacobosco, the spherical triangles of Regiomontanus, the Almagest of Ptolemy, the use of the astrolabe and its mechanism, the adjustments of instruments, cartography, and the methods of observing the movements of heavenly bodies. The only observations employed by the ancients for finding the longitude were those of the eclipses of the moon, and it was not until 1610 that Galileo discovered another method by observation of Jupiters satellites.
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