The astrolabe was one of the most important aids to the early explorer. Invented by the Greeks, then developed by the Muslims, it helped travellers by land or sea determine their latitude on earth
Latitude is the distance north or south of the equator. Once a traveller can determine latitude, they can then know how far north or south they are. Together with a rough estimate of the distance travelled, this can then be used to work out how far east or west they have journeyed. Early travellers had no such knowledge of latitude, and navigated by known landmarks, using the hills and rivers to plot their course. However, this system was obviously of no use when in unfamiliar territory, or when crossing the open sea, where there were no natural landmarks. What travellers needed was some sort of mechanical instrument to help calculate latitude.
The early astrolabes combined a planisphere (a star chart that used a rotating, adjustable disc to work out which stars were visible that day) and a dioptra, or sighting tool, to measure angles. By such primitive means, the angle of the Sun, Moon, and stars, and their position in the heavens relative to the ground, day or night, could be worked out. Armed with this knowledge, travellers could then plot their latitude to find out where they were on Earth. It is not completely clear who invented the first mechanical astrolabe. Credit is usually given to Hypatia of Alexandria, due to a letter written to her by one of her students, Synesius of Cyrene. However, this seems unlikely, given the fact that Hypatia’s father, Theon of Alexandria, had already written a treatise – sadly now lost – on the working of the astrolabe. It is also claimed that Ptolemy had used an astrolabe in his calculations for Tetrabiblos, his book on astrology, some two centuries earlier. Apollonius of Perga, a Greek astronomer who lived in what is now southern Turkey, is thought to have invented an astrolabe around 350 years before Ptolemy. All the individuals attributed to the invention of the astrolabe were Greeks living in the Roman Empire, and the Greeks continued to use and develop astrolabes during the succeeding Byzantine Empire. John Philoponus, a Byzantine philosopher from Alexandria, wrote the earliest extant treatise in Greek on the instrument in around 530. By the mid-600s, Severus Sebokht, a scholar and bishop who lived in Mesopotamia, wrote a major treatise on the astrolabe, and described it as being made of brass, indicating that by then it was an instrument of some complexity.
In the 640s, Muslim armies from Arabia occupied most of the Byzantine Empire, and knowledge of the astrolabe passed to the Arabs. They further developed it, adding new dials and discs to make it more complex and accurate. In its most advanced form, the astrolabe consisted of a mater, or base plate, marked with a degree scale around its outer rim. On top of this, two rotating plates and a ring were aligned to work out the angles of the known stars, and thus determine latitude. If the latitude was already known, the astrolabe could be used in reverse to calculate the time. While the Arabs used the astrolabe as a navigational tool, they also discovered a more specific purpose for the instrument. When praying, Muslims have to face the holy city of Mecca, the birthplace of Muhammad. By using the astrolabe, Muslims could calculate qibla – the direction of Mecca. Used both as a secular and a religious instrument, the astrolabe soon became indispensable for travellers and worshippers alike.
“It uses as its servants geometry and arithmetic, which it would not be improper to call a fixed...truth” SYNESIUS OF CYRENE ( C. 373- C. 414 CE), WRITING TO HIS FRIEND PAEONIUS
Right: Moorish Astrolabe. Its rotating rings and plates could be used to establish latitude or time Far right: The Ptolemaic universe. This illustration from 1708 shows Ptolemy’s geocentric view of the universe, in which Earth is orbited by the Sun,...
Left, Mihrab. A semi-circular niche in the wall of a mosque, a mihrab shows the direction of the holy Kaaba or ‘House of Allah’ in Mecca. This mihrab is in the Bou Inania Madrasa, Fes, Morocco Below: Ibrahim Ibn Said al-Sahli created the celestial...