Rabbi Sujurmenza Zag was a Jewish convert of 13th-century Spain who helped King Alfonso X of Castile with his scientific works.
Zag de Sujurmenza was commissioned by the king to write Astrolabio redondo (spherical astrolabe), Astrolabio llano (flat astrolabe), Constelaciones (constellations) and Lámina Universal (an instrument that improved on the astrolabe); he also translated the book Armellas de Ptolemy, and wrote about the Piedra de la sombra (stone of the shadow, or sundial), Relox de agua (clepsydra, or water clock) Argente vivo o azogue (quicksilver or mercury) and Candela (candle clock). Of his works, the most important are those of the "round astrolabe" and the "flat astrolabe". In the first, the author rises to profound scientific considerations that reveal the vast knowledge he possessed in sciences. It has been said that with this book, Zag Sujurmenza reformed the character of the science of astronomy and contributed to its advancement, without losing sight of the studies of Arab scholars, following in their footsteps and correcting their errors. Other works of Zag de Sujurmenza, if less extensive, are not without merit.