Al-Battani

By Kaveh Hosseini

He measured the year more accurately than anyone before him — and his tables guided astronomers for six hundred years. Abu Abdallah al-Battani, known to the Latin West as Albategnius, worked from his observatory at Raqqa, on the Euphrates in what is now Syria, during the late Abbasid age. He took Ptolemy's Almagest as his starting point and then did what Ptolemy's successors were supposed to do: he checked it against the sky, and corrected it.

Al-Battani's program was relentless observation. Over decades at Raqqa (and later near Damascus), he recorded solar and lunar eclipses, equinoxes, and planetary positions with instruments of his own refinement. His great work, the Zij al-Sabi — the Sabian Tables — compiled the results: a solar year of 365 days, 5 hours, 46 minutes, 24 seconds, against the modern 365 days, 5 hours, 48 minutes, 45 seconds — an error of barely two minutes. He measured the obliquity of the ecliptic, the precession of the equinoxes, and the eccentricity of the Sun's apparent orbit more precisely than Ptolemy, and he was the first to recognize that the Sun's distance varies — that its apparent size changes through the year, implying an elliptical reality behind the circular model.

His mathematical legacy may be even greater than his tables. Al-Battani was among the first astronomers to abandon the clumsy Greek chords and work directly with sines — and he introduced, or at least systematized, the use of the tangent and cotangent, giving them rules and tables. This was the decisive step in creating spherical trigonometry as an independent discipline: the mathematics of triangles drawn on the surface of a sphere, without which neither astronomy nor navigation nor geodesy can function. Copernicus, three centuries later, quoted al-Battani's observations in De Revolutionibus and used his data; Regiomontanus built on his trigonometry.

Al-Battani came from a family of Sabian star-worshippers of Harran — pagans in an Islamic empire, tolerated for their astronomy — and rose to a governorship under the caliphs. He died in 929 near Samarra, returning from Baghdad where he had gone to protest an unjust tax levied on his people of Raqqa. The tax was lifted; the astronomer did not survive the journey. His tables outlived the caliphate that had sheltered him by half a millennium.

Impact on civilization

Al-Battani turned Ptolemaic astronomy from a received system into an empirical science, proving that even the Almagest must answer to observation. His solar year stood as the most accurate for centuries; his sine-based spherical trigonometry became the mathematical engine of astronomy, navigation, and cartography. Copernicus built directly on his data, and his tables served astronomers from Cordoba to Samarkand. He remains the model of the working scientist — observe, compute, correct, repeat — the patient craft by which astronomy actually advances, century after century.