Abū Ishāq Ibrāhīm al-Zarqālī

(1028–1087), Latinized as Arzachel, was a leading Arab mathematician and the foremost astronomer of his time. He flourished in Toledo in Castile, Al-Andalus (now Spain).

The Arzachel crater on the Moon is named after him.

Astronomy

Instruments

Combining theoretical knowledge with technical skill, he excelled at the construction of precision instruments for astronomical use. He constructed a flat astrolabe that was 'universal,' for it could be used at any latitude. This instrument came to be known as the Saphaea in Latin Europe. Al-Zarqali also built a water clock capable of determining the hours of the day and night and indicating the days of the lunar months.

Al-Zarqālī also wrote a treatise on the construction of an instrument (an equatorium) for computing the position of the planets using diagrams of the Ptolemaic model. This work was translated into Spanish in the 13th century by order of King Alfonso X in a section of the Libros del Saber de Astronomia entitled the "Libros de los laminas de los vii planetas."

Theory

Al-Zarqālī corrected Ptolemy's geographical data, specifically the length of the Mediterranean Sea. He was the first to prove conclusively the motion of the aphelion relative to the fixed background of the stars. He measured its rate of motion as 12.04 seconds per year, which is remarkably close to the modern calculation of 11.8 seconds. He also contributed to the famous Tables of Toledo, a compilation of astronomical data of unprecedented accuracy. Al-Zarqālī was famous as well for his own Book of Tables. Many "books of tables" had been compiled, but his almanac (Spanish-Arabic al manakh; "calendar) contained tables which allowed one to find the days on which the Coptic, Roman, lunar, and Persian months begin, other tables which give the position of planets at any given time, and still others facilitating the prediction of solar and lunar eclipses. He also compiled valuable tables of latitude and longitude. The was the first almanac in the modern sense, in that it was the first to provide entries that directly give "the positions of the celestial bodies and need no further computation". The work provided the true daily positions of the sun, moon and planets for four years from 1088 to 1092, as well as many other related tables.

His work was translated into Latin by Gerard of Cremona in the 12th century, and contributed to the rebirth of a mathematically-based astronomy in Christian Europe. It was later adapted as the Tables of Toledo in the 12th century and the Alfonsine tables in the 13th century. Four centuries later, Copernicus mentioned his indebtedness to Al-Zarqālī and quoted him, in the book that gave new meanings to the term 'revolution,' De Revolutionibus Orbium Coelestium.

In dealing with Ptolemy's complex model for the planet Mercury, in which the center of the deferent moves on a secondary epicycle, al-Zarqālī noted that the path of the center of the primary epicycle is not a circle, as it is for the other planets. Instead it is approximately oval and similar to the shape of a pignon. Some writers have misinterpreted al-Zarqālī's description of an earth-centered oval path for the center of the planet's epicycle as an anticipation of Kepler's sun-centered elliptical paths for the planets.

See also

• Arzachel (crater)
• Arzachel (band)
• Zij
• Islamic astronomy
• Islamic scholars
• List of Arab scientists and scholars

Notes

Further reading

• E. S. Kennedy. A Survey of Islamic Astronomical Tables, (Transactions of the American Philosophical Society, New Series, 46, 2.) Philadelphia, 1956.

External links

• A Zahoor's capsule biography of Al-Zarqali
• 'Transmission of Muslim astronomy to Europe'