The discovery of a late 15th century astrolabe on a Portuguese shipwreck off Oman provides a fascinating insight into an exciting period of the Age of Discovery.
Found within the wreckage of the Esmeralda, a carrack that sailed in Vasco da Gama‘s 1503 fleet to India, it is one of the most unique of more than 3,000 bronze artefacts so far found on the vessel since its discovery in 2015. Indeed it is reckoned to be the oldest such instrument found to date.
Better known as an astronomer’s tool, the astrolabe was miniaturised and adapted for navigational purposes in the 15th century, at a time when mariners and explorers began plying their trades further and further from home. No longer safe to rely on their navigational experience and dead-reckoning, these pioneers began to turn to ‘scientific’ instrumentation to supplement their knowledge of the treacherous seas.
Early attempts at measuring the position of a ship away from the coast relied on the Pole Star, ‘the most easily observable heavenly body…The altitude of the Pole Star – its angle above the horizon – grew less as a ship sailed further south, and so gave an indication of how far south she had sailed’. (Parry, 1963, p.107)
Initially calculated by rough-eye estimates, these ‘measurements’ were enhanced during the 15th century by the popularisation of the mariner’s quadrant.
The quadrant was soon superseded, amongst the Portuguese at least, by the astrolabe, which:
Consisted of a brass disk engraved with a stereographic projection of the heavens and a rotatable grill, by means of which the movements of the more conspicuous heavenly bodies could be followed. It was principally intended as a calculating device for the use of astronomers; but on its reverse side it was graduated in degrees round the perimeter and fitted with a rotating sight bar or alidade for observing altitudes…Only the reverse side of the instrument was useful – or indeed comprehensible – to seamen. (Parry, 1963, p.108)
That said, da Gama is recorded to have used a larger astrolable on his breakthrough voyage to India than the one found on board the Esmeralda. This was primarily for use on shore to determine his latitude. How much benefit he derived from the device is not wholly clear, its limitations in uncharted lands and inclement weather unlikely to have filled him with confidence.
It is not even clear how skilled a navigator da Gama was, for little is known of his life prior to setting off on that historic voyage around the Cape of Good Hope in 1497. There are snippets of information suggesting that he studied mathematics and navigation at an inland school in the 1480s, which may have enabled some familiarisation with instruments such as the quadrant and astrolable.
Yet one of the most enduring stories of da Gama’s voyage ironically centres on him dispensing with all of his navigational tools, when a mutiny broke out amongst his fearful crew. It is worth repeating the account in some detail, if only in the words of da Gama’s chronicler:
Crew: We have had enough. This is indeed a terrible and evil place as we were told.
Da Gama: No! We go on! We go to India.
As the crew threatened open rebellion, da Gama slipped away to his cabin, returning with a bundle of his charts and his navigational instruments.
Da Gama: What are these?
Crew: Your charts and instruments, Captain.
The Captain stepped forward and threw his possessions into the sea.
Da Gama: Now there is no returning! We go on with da Gama and with God!
That was the last of the dissension in the ranks.
Ordering his men to trust in God – not to mention in his own divine leadership – da Gama freed himself of the burden of science, his successful landfall near Calicut in 1498 testament to the holy honour bestowed upon him by his patron Dom Manuel I.
At least that’s how the chronicle portrays it, for such a foolhardy act would surely have been beyond even a man of da Gama’s fiery temperament.
In addition to being able to read and compile maps and charts as a way of plotting their course, the late medieval mariners needed to adapt to the new tools of their trade and this required some schooling.
For many it was a case of learning on the job, the seaman’s apprenticeship an invaluable if brutal introduction into life on the open ocean. The Portuguese developed navigational schools in the 15th century, with Prince Henry the Navigator’s semi-mythical institution at Sagres both a practical training centre and a somewhat primitive think tank at the same time.
Vasco da Gama’s voyages to India encompassed all of this 15th century learning, not to mention the personal characteristics that mattered; an explorer possessed of a character befitting of his mission, a master whose ambition for the House of Aviz was unbounded, and the navigational tools that guided the way.
The discovery of the astrolabe points to a time when maritime culture was undergoing profound changes, enforcing a level of exactitude and professionalism previously unattainable. This in turn opened up the globe to the processes of mercantilism and imperialism, with the benefits and detriments these brought to so many far-flung peoples.
Beautiful and beguiling, the astrolabe has stood the test of time, a marker in the ocean upon which Portugal, that impoverished cousin of Spain, created the first global maritime empire.
Within a few years of its inception the Carreira da India was in full flow, exotic spices and magnificent beasts flowing back to Lisbon along with troves of gold and treasure. A template had been created for Western Europe to grow rich, to upend its backward and unenlightened reputation and propel its states towards a glory hitherto unseen.
The world was truly never the same again.
Parry, J. H. The Age of Reconnaissance: Discovery, Exploration and Settlement, 1450-1650 (1963)