The as­tro­labe

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

Timeless Travels Magazine - - TRAVEL INVENTION -

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.

Early in­ven­tors

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.

Mus­lim ad­vances

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...

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