Fatimid supremacy on the seas was possible due to their knowledge of astronomy and geography. The Fatimid Caliphs were great patrons of science and learning, thus encouraging research in astronomy and geography. There were big observatories in Cairo where scholars could follow the movement of the stars. Ibn Yunus, who worked in these observatories, is considered one of the greatest astronomers of the Islamic World. An astrolabe (instrument for measuring altitude of the sun and stars at sea) developed in the time of Imam Moiz is still kept in the British Museum in London. Geography formed an important part of the curriculum at the Al-Azhar. The Caliph Moiz was greatly interested in geography and commended a piece of silk to be made at Tustar in Persia, representing, in gold and colours, a map of the world, which cost him 22,000 Dinars
Sherali Alladina, “Ismaili Navigation”, Ismaili Mirror, Dec ’74, p.11
Ismailimail 
The Astrolabe in its final form was perfected by Muslim scientists although a cruder version was invented in pre-Islamic times.
This instrument was instrumental in fostering the great voyages of discovery by the Portuguese during the 15th century. The great Portuguese navigator Vasco Da Gama had a Muslim navigator on board who was an expert at using the Astrolabe. As a result this explorer was able to discover a sea route around the southern part of Africa to India. The South African province of Natal and present-day Maputo in Mozambique were discovered by Vasco Da Gama. At school we used to joke that
Vasco Da Gama
ate a banana
and went to bed without his pyjama.
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Excerpt from an article published on the Institute of Ismaili Studies website, entitled ‘Muslim Philosophy and the Sciences’:
http://www.iis.ac.uk/view_article.asp?ContentID=106391
‘Astronomical measurements required innovation in measuring instruments. Here, too, Muslim stronomers surpassed their predecessors by designing new instruments, revising older ones, and sometimes building extremely large instruments to increase accuracy. The astrolabe is an example of an astronomical instrument that was derived from the Greeks but was improved by Islamic astronomers. Primarily used for determining the position of celestial bodies, it was combined with a number of movable plates and arcs to graphically solve complex trigonometrical functions and thereby determine direction or time of prayer.’
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