A Disc on the Seabed of History: Vasco da Gama’s Astrolabe and the Beginning of Empire

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.

Laser scanners have revealed the etches on the astrolabe

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.

A 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The quadrant was soon superseded, amongst the Portuguese at least, by the astrolabe, which: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.

Dom Manuel I of the House of Aviz

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.

A fleet of the Carreira da India departs Lisbon in a 1593 engraving by Theodor de Bry

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)


The Christian Mission: from ‘Soldiers of God’ to Humanitarian Educators

Roman Catholic missionaries were amongst the most important explorers during the European Age of Discovery, venturing where no others would dare in the hope of inculcating ‘primitive heathens’ into their all-powerful church. Today such missionaries receive little attention yet they remain active nonetheless, seeking not only to spread Christian doctrine across the globe but also to translate bibles into indigenous languages in an attempt to create a more uniform Christian teaching within ethnically-diverse states.

A Jesuit missionary instructs schoolboys in the Congo - 1930s
A Jesuit missionary instructs schoolboys in the Congo – 1930s

A good case in point is the Democratic Republic of the Congo. Accurate statistics are hard to come by in this vast and war-torn nation, although it is possible that up to 80% of the population identifies itself as Catholic. Either way, this branch of the Church certainly makes up the overwhelming majority.

The Kingdom of Kongo (as the western part of the DRC was then known) was one of the first states in the African interior exposed to the proselytizing missionaries of Europe. Diogo Cao – the famed Portuguese navigator – first reached the Congo River in 1482 and on a subsequent voyage in 1485 he is recorded to have penetrated deep inland and met with the Manisono, the chief adviser to the King (the Manicongo). Several African natives accompanied the return voyage to Lisbon, on which Cao died.

In 1490-91 another voyage set sail for the Kongo with the stated purpose of converting the natives to Christianity. Much historical debate remains over which Catholic order undertook the mission. Contemporary chroniclers suggest it was the Franciscans, although subsequent claims have been made for both the Dominicans and the Order of St John the Evangelist taking the lead. Either way, the mission was remarkably successful. The Manisono was converted, along with several provincial chieftains, and it was not long before the Manicongo himself had taken the cross, being christened Joao in honour of the Portuguese king.

Nzinga a Nkuwu - otherwise known as Joao I
Nzinga a Nkuwu – otherwise known as Joao I

There is a paucity of first-hand sources for this period, as one might expect. However, the ease with which the Manicongo and his people were converted has been attributed by some to the belief that several friars remained in Kongo after Cao’s second voyage. Either that, or the natives that were returned to Portugal were instructed in the European tongue and were therefore well-placed to translate the necessary sacraments – with their supposed merits – to their kinsmen.

It was certainly one of the more successful of the European missionary ventures and Catholicism has retained a presence in the Congo region ever since, with a variety of orders picking up the mantle of whoever first converted the natives.

Of course despite this, animist and other traditional forms of religion have persisted, sometimes cloaked within the guise of Christianity. This explains why the Congo mission has never ended. During Belgian colonial administration great efforts were made to translate the bible into the plethora of native languages that existed across the country, making a simultaneous push to improve literacy rates and thus help the proliferation of Christian doctrine.

The  Christian faith is practiced with local variants in the DR Congo
The Christian faith is practiced with local variants in the DR Congo

One persistent problem for the Christians – in their purest form – is that their religion is historically associated with conquest and colonisation. Therefore, traditional practices – such as the worshiping of ‘false idols, shamanism, spiritual healing and even human sacrifice – have persisted in certain provinces.

As such the role of the missionaries – at least as far as the Church is concerned – will never end. Their duties have taken on a more educational and humanitarian aspect over the past century, rather than simply existing to swell the numbers in the Christian ranks and to eradicate ‘pagan’ beliefs.

It is perhaps a more respectable role that these ‘soldiers of God’ now play, though no doubt examples remain of over-interfering, self-serving proponents of the faith. It also demonstrates the sheer absurdity of the early Catholic missions. They had a goal – and a staunch belief – that they would convert all in their path, smash the heathens and find long-lost Christian tribes dwelling deep within the interior of undiscovered states.

Presumably such sentiments have long been forgotten…and we move into a new age of the Christian mission.

Whatever your views on Christianity, these people have taken a central role in history. Let us hope that they continue to do so.


Boehrer, G.C.A, ‘The Franciscans and Portuguese Colonization in Africa and the Atlantic Islands, 1415-1499, Academy of American Franciscan History (Jan, 1955)