Robert of Chester (Latin: Robertus Castrensis) was an English arabist of the 12th century. He translated several historically important books from Arabic to Latin, by authors such as Abu Musa Jabir Ibn Hayyan and Al-Khwarizmi including:Liber algebrae et almucabola Al-Khwārizmī's book about algebra translated in 1145 Liber de compositione alchimiae a book about alchemy translated in 1144 A Testament of Alchemy, a compendium of alchemy from the Byzantine world, translated in 1144In the 1140s Robert worked in Spain, where the division of the country between Muslim and Christian rulers resulted in opportunities for interchange between the different cultures. However, by the end of the decade he had returned to England. Some sources identify him with Robert of Ketton (Latin: Robertus Ketenensis) who was also active as an Arabic-Latin translator in the 1140s.However, Ketton and Chester, while both places in England, are a long way apart. Also, when in Spain, Robert of Ketton was based in the Kingdom of Navarre, whereas Robert of Chester is known to have worked in Segovia.While translating Al-Khwārizmī's book about algebra, Robert of Chester made an interesting error that lives on today. Arabic script, like Hebrew script, consists of consonants with vowels punctuated underneath and often omitted. The sine originated in India, and was adopted by the Arabs who spelled the consonants as "jb". When Robert translated the word, not understanding the Hindu origin, he supplied Arabic vowels yielding the word for bay or inlet. In Latin, a bay or inlet is sinus. And thus, the origin of the trigonometric function named sine.