 The serious errors which Ulugh Beg found in previous Zij star catalogues (many of the earlier ones were simply updates on Ptolemy's work, adding the effect of precession to the longitudes) induced him to redetermine the positions of 992 fixed stars, to which he added 27 stars from al-Sufi's "Book of Fixed Stars" (964), which were too far south for observation from Samarkand.This catalogue, one of the most original of the Middle Ages, was edited by Thomas Hyde at Oxford in 1665 under the title "Tabulae longitudinis et latitudinis stellarum fixarum ex observatione Ulugbeighi", by G. Sharpe in 1767, and in 1843 by Francis Baily in vol.xiii.