His book Maqalat fi al-Jabr wa al-Muqabila is a masterpiece on algebra and has great importance in the development of algebra. He was a great thinker and poet, living most of his life in Samarkand, in Central Asia. The Sultan Malikshah Jalal al-Din called him to the new observatory at Ray around 1074 and assigned him the task of determining a correct solar calendar. Khayyam's calendar was remarkably accurate. It had an error of one day in 3770 years.