An analysis of Ibn al-Shāṭir’s star table

Abstract

Ibn al-Shāṭir’s (1306–1375/1376 AD) star table in his Jadīd zīj, comprising of the equatorial coordinates and magnitudes of 89 stars, is edited and analyzed in this paper on the basis of the extant manuscripts going back to the late 14th and early 15th centuries. It established a new tradition of arranging the celestial coordinates in the star tables in Egypt and Syria after him. The right ascensions (mean absolute error MAE = 20.0′, mean error μ = –2.8′, standard deviation σ = 29.1′) and the declinations ( MAE = 21.1′, μ = –3.2′, σ = 29.3′) are nearly of the same degree of precision. The stars in the region RA ~ 120°–180° generally have the least errors in both declination and right ascension. The declinations of the southern stars were measured more precisely than those of the northern ones. The values for the declinations of the stars in the region δ ~ –30°–0° (the middle of the sky towards the south of the horizon of Damascus) are significantly the most accurate. A systematic northward shift can be seen in the declinations of the southern stars. The declinations of 15 of 18 stars spreading out in RA ~ 67°–121° show a southerly, downward shift. More than 65% of the stars have the errors in both the declination and right ascension less than 32′. No outlier in the tabular coordinates exceeds ±98′. Also, Ibn al-Shāṭir measured the magnitudes of, at least, a few stars: he assigned a correct magnitude of +4 to λ Ori, a component of the star cluster in the Orion that was considered a nebulous object in the Almagest star catalogue, and presented more precise values for the magnitudes of α Sco, α Oph, β Cas, κ Ori, γ Gem, and β CMi than Ptolemy and al-Ṣūfi.