Al-Battani

Al-Battani was an astronomer and mathematician of the Islamic Golden Age who carried out more than four decades of careful astronomical observations that significantly corrected and refined the earlier work of the Greek astronomer Ptolemy. Using instruments he built or improved himself, Al-Battani measured the length of the solar year with remarkable precision for his era, calculating it as 365 days, 5 hours, 46 minutes and 24 seconds, a figure only a few minutes off the modern value, and he refined measurements of the inclination of Earth's axis and of the precession of the equinoxes, the slow wobble in Earth's rotational axis. Al-Battani made major contributions to trigonometry, being among the first astronomers to replace the older Greek geometrical method of chords with the sine function in astronomical calculations, and he introduced the use of tangent and cotangent functions along with detailed trigonometric tables, tools that became fundamental to later astronomy and mathematics in both the Islamic world and, through Latin translation, in medieval Europe. His major astronomical work, known in its Latin translation as De Motu Stellarum, compiled a star catalog together with detailed solar, lunar and planetary tables and was studied by European astronomers, including Copernicus, for centuries after his death.