Jabir bin Eflah

Câbir bin Eflah, full name Ebû Muhammed Câbir bin Eflah el-İşbîlî, was a Muslim astronomer and mathematician who lived in Andalusia in the first half of the 12th century. As its name suggests, it is from the city of İşbiliye, that is, today's Sevilla. In medieval Europe, it was called Geber by its Latinized name; Because of this name, he is sometimes confused with the alchemist Câbir bin Hayyan, but they are different people.

There is very little information about his life. It is accepted that he lived between approximately 1100 and 1150. Musa bin Maimonides (Maimonides) wrote that he met Cabir's son; This note is one of the bases used to determine his period.

Review of Almagest

The work that made Câbir known is his book called İslâhu'l-Mecistî (Correction of Almagest). This work written on Ptolemy's Almagest is considered the first comprehensive criticism of this work in the West of the Islamic world. Câbir replaced the Menelaus theorem that Ptolemy used in calculations with methods based on spherical trigonometry and thus simplified the calculations.

A theorem in spherical trigonometry is named after him. He also differed from Ptolemy in the order of the planets: he placed Venus and Mercury above the Sun rather than below it.

It is also suggested that he may be the inventor of the torquetum, a tool used to convert between global coordinate systems.

The work was translated into Latin by Gerard of Cremona; It was translated into Hebrew by Moshe ibn Tibbon and then by Jacob ben Makhir in the 13th century. Thanks to these translations, he reached a wide readership in Europe. In Andalusia, thinkers such as Ibn Rushd and al-Bitrûcî benefited from his criticisms. In the 15th century, Regiomontanus used Câbir's material without citing the source in his writings on spherical trigonometry; this was expressed by Cardano in the following century.

Câbir bin Eflah takes his place in the history of science as one of the leading figures of the Andalusian tradition, in which Ptolemaic astronomy was questioned.