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URANOS FIGURES CÉLESTES, LÉGENDES & CIVILISATIONS grand public – éducateurs & pédagogues – chercheurs & passionnés |
Mise à jour le 14/05/2014
The southern Sky
from the Arabs to the Portuguese near 1500
This text reprend the intervention of Roland Laffitte at the Planetaria Symposium held in Lucerne, on 1rst‒5 May, 2014.
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Vasco da Gama left Lisbon on July 8th, 1497 as captain-major of a squadron of four naves. which, after 309 days of journey, reached Calicut on May 20th, 1498 (see Fig. 2) .
Fig. 1 : Portrait of Vasco da Gama dated of 1520. |
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By crossing the Indian Ocean with him, the Europeans benefited from the astronomical knowledge of Arabic sailors. The subject is to see what they learnt with them of the southern sky.
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Fig. 2 : Vasco da Gama’s first journey . |
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By crossing the Indian Ocean with him, the Europeans benefited from the astronomical knowledge of Arabic sailors. The subject is to see what they learnt with them of the southern sky.
The discovery of the Southern Sky by the Europeans
We can see below the southern sky, which appeared at him when Vasco da Gama turned the Cape of Good Hope, at night from 22 until 23 November 1497:
Fig. 3 : The southern sky in Cape of Good Hope on November 23th, 1497, near 1 hour in the morning.
The Europeans who crossed the equator all made a similar experience.
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All were able to admire, according to the terms of a letter there written in 1516 by the traveller Andrea Corsali to Julien de Medici, quite unknown celestial objects of them, quite particularly : “due nugolete di ragionevole grandeza”, i.e. “two small clouds of reasonable size”, as well as “una Croce meravigliosa”, which it does not to say that it is “a marvellous Cross”.
Fig. 4 : Drawing of the “due nugolete” by Andrea Corsali |
Very soon, the Portuguese squadron met by accosting in the south of present Mozambique, populations in relation with Arab traders, because for more than five centuries the latter have been active in this region. As Vasco da Gama had embarked Arabs or at least people knowing the Arabic language, he got very fast informs on the Arab settlements.
The contact of Vasco da Gama with Arab navigation
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In fact, the first one he approached was Mozambique Island, where he found his first Arab pilot. So he could join easily Mombasa, where the reception did not encourage him to linger. And he finally reached Malindi In fact, the first one he approached was Mombasa where the reception did not encourage him to linger, and so he reached Malindi where he was welcome and where the sultan gave him a pilot who could make him cross the Indian Ocean to attain India. By having navigated for many centuries from the northern extremities of the Red Sea and the Persian Gulf down to the south of Madagascar in latitude, and from the oriental coast of Africa to China in longitude, the Arab sailors have recorded a long experience in the form of route guide. These were giving all the useful indications to go from such o such port in such other one and or treaties of navigation (see Fig. 7)). Fig. 5 : Vasco da Gama’s route in the Indian Ocean. |
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(1421‒1500) احمد ابن ماجد |
At the end of the XVth century, one of masters of this discipline was Ahmad Ibn Mâjid to whom the Europeans wanted to see the pilot who had guided Vasco da Gama from Mabindi to Calicut. That is not the case. The man was already too old to navigate and he was much more than a pilot. Nevertheless, his works, which overtake fifty, gives us, in verses as in prose, a good idea of the knowledge of time.
Fig. 6 : Imaginary portrait imaginaire of Ahmad Ibn Mâjid. |
Incidentally, the famous navigator gives, in one of his nautical poems, sailors yet renowned for their boldness who want take sea at Sofala for the north (Kilwa and Zanzibar) this advice: you should not absolutely leave before May 2nd because the
Fig. 7 : Places mentioned by the navigator Ibn Mâjid.
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winter monsoon, because opposite currents as well as storms and cyclones which damage the Mozambique Channel, make before this date the sea very dangerous. That allows measuring the luck of Vasco da Gama and his companions, who exactly crossed this zone at the most precarious time and went out of it yet safe and sound. On the other hand, the route from Malindi to Calicut sometimes benefits according to years from April of very propitious conditions thanks to the summer monsoon, which blows of the south-west whereas Somali’s current, streaming in the same way, still accelerates the progression (see Fig. 8). |
Fig. 8 : The sommer monsoon. |
The Portuguese sailors were very surprised to notice that the Arab pilots did not use either charts or even the magnetic compass. They knew it, certainly, but it was not of current practice. They supplied to it by the use of route-guides, often oral, giving, when leaving every port, the direction to keep on the sidereal compass, a precise knowledge of stars allowing to establish the latitude and the hours in the night, indications which were adding to precious land-marks, presence of marine animals and birds, etc.
Navigation from Malindi to Calicut with an Arab pilot
If we take the indications given by the Arab route guides, the journey from Malindi to Calicut might be done in two phases :
PHASE I. When leaving the port, the ship is heading for Northeast.
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It makes it by the sidereal compass the Arabs call dâ’irat al-akhnân, litterally “the circle of branches”, or al-dâ’ira al-afaqiyya, “circle of horizons”. That is a wood dial which divides the horizon into 32 sectors indicating the directions thanks to risings and settings of specified 15 stars which could be qualified as azimuthal stars, and to which it is necessary to add al-Jâh, “the Very-High”, i.e. soit α Umi = Polaris.
Fig. 9 : Direction Northeast. |
As for the South Pole, it may be materialized by a star, but may also found by the passage in the meridian of some brilliant southern stars or asterisms, first of all Suhayl, i.e. α Car= Canopus (see Fig. 9 & 11), what make call this position Qutb Suhayl, i.e. “Suhayl’s Pole”. We have so 32 positions. The Arab sidereal compass looks like in a striking way of rose of winds which we can see on Fig. 11.
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By leaving the African port, Vasco da Gama’s pilot headed for the rising of al-ᶜAyyûq, i.e. α Aur = Capella, which points to Northeast on the sidereal dial (see Fig. 12). In fact, this star is not visible on the evening on the eastern horizon at this period of the year. However, he knows that its location is marked by the appearances of al-Qâ’id, which we call up of the same name as the Arabs, i.e. Elkaid, et 1/3 of the distance which separates this star of al-Simâk al-Râmih, which the Latin clerks named Azimech Alrameh (i.e. α Boo, today Arcturus). |
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Fig. 10 : Sidereal compass in a book of Ibn Mâjid. |
Fig. 11 : Portuguese rose of winds. |
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al-ᶜAyyûq = α Aur (Capella)
Fig. 12 : al-ᶜAyyûq, i.e. α Aur on the sidereal compass |
The ship keeps this direction until it reached the latitude of Calicut.
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The question is to know how to calculate this latitude. It can be measured in a simplest way holding with the tense arm, the palm of the hand parallel to the horizon line, and by counting the number of fingers – in arabic : asâbiᶜ –, with which a star is above the horizon. The replacement of the hand by a small board provided with a graduated string and named kamâl (see Fig. 13 et 14) allows to measure altitudes more precisely by giving 1/3 and 1/4 finger, knowing that the finger – isbac – is estimated, in the Arab nautical treatises, to 1° 36’, that is to say 1,607 degrees (they are, according to Ibn Mâjid, 224° on the meridian). |
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Fig. 13 & 14 : exemples de kamâls |
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According to Ibn Mâjid’s treaties, the pilots have at their disposal, even it is not formalized, a true table indicating the range of latitudes in which the stars are used, that is to say : for northern Indian ocean, al-Juday, i.e. α UMi = Polaris; for the equatorial latitudes, al-Farqadayn, i.e. βγ UMi = « the Guards »; and for the most austral latitudes, al-Naᶜsh, i.e. εζ UMa = Alioth and Mizar (see Fig. 15).
Fig. 15 : Ranges of utilisation of stars for measuring latitudes by Ibn Mâjid
PHASE II. The pilot heads for East by maintaining in the kamâl the height of al-Juday, i.e. α UMi = Polaris at the altitude of Calicut, that is to say 6 fingers (see Fig. 16).
Fig. 16 : Measurement of the heigh of al-Juday, i.e.α UMi = Polaris.
When the ship is approaching the bank, he just has to locate a special land-mark placed in the northeast of the city, namely the mountain which the Arabs call mount Maqdar and which is today the peak Vellari Mala, considered for looking like a camel’s hump.
Fig. 17 : Keeping the heigh of al-Juday 4 fingers.
Remark :
The Arabs used the zâm which is worth 1/8 of finger. Converted in times of navigation by normal wind, it makes 3 hours, what corresponds to the meridian 224 x 8 = 1792 zâms, and so a speed of 4 knots.
Fig. 18 : Estimation of the duration of the journey from Mabindi to Calicut.
If, according to the Arab treaties, the estimated duration for going from Malindi to Calicut is of 27 days, we realize that it took in fact 35 days, what, considering the numerous maritime hazards, is not a serious gap. The journey thus went off wellé.
The southern by the Arabs
Vasco da Gama embarked expert clerks of languages, history of the peoples, geography and astronomy. There is a chance that one of them read the Almagest in the Latin version which Gerard of Cremona had made it the Arabic translation. Let us imagine that during the crossing, this scholar asks the pilot, helped by a « Moorish » translator of the captain-major, what stars he uses to recognize himself in the southern sky. This one describes then to our eager to know traveller the sky similar to which he had already seen when Vasco da Gama’s squadron had doubled the Cape of Good-Hope (see Fig. 3).
The indications of the pilot and the explanations of the translator allow our scholar to locate some stars present in Almagest but invisible in the latitude Lisbon (see Fig. 19).
The bright star of Argo ship, today α Car, the Greeks called Kanôpos and the Latins Canopus, is Suhayl by the Arabs.
Other stars did not be named by the Greek have got a appellation by the Arabs.
* so this one of the end of Eridan (α Eri), which the pilot call al-Salbâr, visible at the beginning of the night ;
* the same for the figure of al-Qaws wa-l-Sahm, literally “the Bow and the Arrow”, visible on the early hours, two objects drawn by groups of stars considered in the Almagest as outside the figure of Piscis austrinus (to day respectively β Tuc + βγδμλγ Gru for the bow and t αβ Gru + α Phe for the arrow).
Fig. 19 : The southern sky at Malindi at the beginning of the night of April 24th, 1497.
Then comes a beautiful surprise: the famous “wonderful cross” the sailors can admire when they have crossed equator, and which the Almagest does not differentiate as a special celestial figure situated in the constellation of Centaurus, is also considered as a cross by the Arabs (αβγδ Cen). The pilot shows him precisely two crosses. One is Salîb al-Qutb, “the Cross of the Pole”. It is the one about which we have just spoken. He qualifies it as janûbiyya, “austral”, in order to distinguish it from another, shmâliyya, “boreal » that one, which he calls al-ᶜAwâ-idh and which he places in the Head of the Dragon (βγεν Dra). Then he strongly recommends not confusing it with another cross which ornaments the southern sky and which he names, for this reason, al-Salîb al-Kâdhib, “the False Cross”.
Fig. 20 : The southern sky at Malindi at the end of the bight from April 24th to 25th, 1497.
As for the two small clouds whose Almagest does not speak at all (LMC & SMC), the pilot described as the Sahâbatân Baydhâ’, “Two White Clouds”, the Large one ‒ al-Kabîr ‒ also named Qaddamâ Suhayl, “the Feet of Suhayl”, the other is, of course, the Small one ‒ al-Saghîr.
It is clear that the name of the Magellanic Clouds, given to these celestial objects very late in the nineteenth century, helps to erase the contribution of Arab nautical arts and astronomy in the patrimony of the European Renaissance.
For more details on this point, see the following articles (in french) on this site:
Les Nuages de Magellan : les choses et les mots
MASSIGNON, Louis, « Les Nuages de Magellan et leur découverte par les Arabes »
ainsi que :
La découverte de la Croix du Sud
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