A 60-km diameter lunar crater located on the southern hemisphere of the moon is named after him as “Nasireddin”.

Khajeh Nasir al-Din Tusi (1201–1274) was a Persian polymath and prolific writer.

Tusi, one of the greatest Persian scholars, was also known as a philosopher, mathematician, astronomer, theologian, physician, and prolific writer.

Born in Toos, northeastern Khorasan region, he started his studies at an early age.

His areas of study included Arabic, the Qur’an, Hadith, Shi’a Jurisprudence, logic, philosophy, mathematics, medicine, and astronomy.

He is also the founder of Maragheh observatory, an astronomical observatory established in 1259, with a library, which is said at one time to have held over 400,000 volumes of literature related to astronomy and astrology.

As the armies of Genghis Khan swept his homeland, he fled to join the Ismailis and made his most important contributions to science during this time, while he was moving from one stronghold to another.

He finally joined Hulagu’s (Genghis Khan’s son) ranks, after the invasion of the Alamut castle of the Hashshashin (Assassins) by Mongol forces, according to the portal of Khajeh Nasir ad-Din Toosi University.

The Mongol ruler, Hulagu Khan, built an observatory in Maragheh for Tusi.

He is considered the first of the Maragheh School of astronomers, who made significant contributions to the development of astronomy. Notable members of that school were Mu’ayyad al-Din al-’Urdi and Qutb al-Din al-Shirazi.

Tusi made very accurate tables of planetary movements as depicted in his book Zij-i ilkhani (the Ilkhanic Tables). This book contains astronomical tables for calculating the positions of the planets and the names of the stars.

His planetary system was the most advanced of his period and was used extensively until the development of the heliocentric model in the time of Copernicus. Between Ptolemy and Copernicus, he is considered the most eminent astronomer of his time.

He was perhaps the first to treat trigonometry as a separate mathematical discipline and in his Treatise, on the Quadrilateral, he was the first to list the six distinct cases of a right-angled triangle in spherical trigonometry.

He invented the geometrical technique called a Tusi-couple for his planetary models, which generates linear motion from the sum of two circular motions.

He also calculated the value of 51″ for the annual precession of the equinoxes and contributed to the construction and usage of some astronomical instruments including the astrolabe.

He gave the first extant exposition of the whole system of a plane and spherical trigonometry.

One of his significant activities was the establishment of a great library in the city of Maragheh in addition to the Observatory.

Upon the command of Hulagu, many valuable and scientific books that had been plundered from Baghdad, Damascus, Mosul, and Khurasan were brought to the library.

Accompanied by his students, he traveled to Baghdad to collect the remaining plundered books, but he could not complete this mission and passed away on June 25, 1274, in Baghdad, and according to his will, he was buried in the Shrine of Kazimayn, Kadhimiya, Iraq.

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