By: Mohammed Sirajuddin
Trans. By: Atya Tabassum

Ibn Yunus is one of the greatest astronomers of the Islamic world in the Middle Ages. His full name is Abul Hasan Ali ibn ‘Abd al-Rahman ibn Ahmad ibn Yunus al-Sadafi al-Masri. But he is well known by the short name of Ibn Yunus. He belonged to a respectable family. His ancestor was a companion of Imam Shafi’i (RA), a leading scholar of Islamic jurisprudence, and his father was a renowned historian and a muhaddis – scholar of hadith. Ibn Yunus was not only a famous astronomer and astrologer but also a famous poet. Ibn Yunus was a courtier of the Caliph al-Aziz (975-996 AD) and had the opportunity to work at the Dar al-Hikma Observatory in Cairo. While studying and observing astronomical phenomena, he received an order from the Fatimid ruler Khalifa al-Aziz to compile astronomical tables.

He started compiling these tables in 990 AD during the lifetime of the Caliph, but this work was completed after the death of the Caliph during the reign of his son, the Caliph Al-Hakim (966 – 1020 AD). That is why he named this Zij after him and named it “Al-Zij Al-Hakimi Al-Kabeer”. In the middle ages, before Ibn Yunus, Muslim astronomers had compiled more than two hundred astronomical tables. Of these, Ibn Yunus’ tables are considered the best.

David A. King writes in his article that the existing chapters of the Zij al-Hakimi are found in two unpublished manuscripts, found in the Leiden and Oxford Libraries. A third one is also found in Paris. The importance of Ibn Yunus came to light in the Western world when the Leiden manuscript was studied in the eighteenth century. The attention to his book made him known for his services to astronomy. After the loss of the translation of Leiden and Paris manuscript made by Sedillot, Delambre published a summary in 1891. The German scholar Carl Schoy has since published several translations and analysis of the Zij, and articles about Sundial Theory and spherical astronomy.

Ibn Yunus has recorded his observations in his book Al-Zij Al-Hakimi Al-Kabir. His observations relate to the planetary conjunctions (conjunction of two planets), conjunction of planets in Qalb al-Assad (the brightest star in the constellation Assad) and the eclipses of the sun and moon. He describes about forty planetary conjunctions in his book. Based on his observations, he has recorded some distances which are in accordance with today’s calculations. An excerpt from this book is given here for the interest of the readers.

“A conjunction of Venus and Mercury in Gemini, observed in the western sky: The two planets were in conjunction after sunset on the night [of Sunday 19 May 1000 corresponding to 13 Jumadi al-Sani 390 AH]. The time was approximately eight equinoctial hours after midday on Sunday … . Mercury was north of Venus and their latitude difference was a third of a degree.”

Modern knowledge of the positions of the planets confirms that his description and his calculation of the distance being one-third of a degree is exactly correct.(2)

Dr. Qadir Lone writes in his book, ‘The Scientific Achievement of Muslims’ that the amount of motion of the stars discovered by Bin Yunus is more accurate than that of any other Muslim scientist. The astronomers of the West who have benefitted from Ibn Yunus include the name of a great scientist like Laplace (1749-1827). And he refers William Draper.

“The treatise of Albategnius on the science of the stars is spoken of by Laplace with respect. He also draws attention to an important fragment Ibn-Yunis, the astronomer of Hakeem, the Khalif of Egypt 1000 A.D.”(2)

Ibn Yunus measured the position of Qalb al-Assad in 1003 AH and the degree of the motion of planets he discovered was more accurate than the degree given by any other Muslim scientist. It is written in the ‘Scientific Achievements of the Muslims’ that Ptolemy stated this movement as one degree in one hundred years and Al-Battani stated as one degree in every sixty-six years. Ibn Yunus discovered this movement as one degree in 70.25 years, which is very close to the actual amount of “one degree in seventy two years”. This is considered to be an important discovery of Ibn Yunus.

Ibn Yunus was a famous astrologer and he devoted much of his time to predict through astrology. He authored a book titled Kitab bulugh al-umniyya (“On the Attainment of Desire”), a work concerning the heliacal risings of Sirius, and on predictions concerning what day of the week the Coptic year will start on. This book consists of twelve chapters.

Ibn Yunus’s second major and most important work was to compile astronomical tables of time, which were used in Cairo until the nineteenth century to watch time. The collection of these time tables is called Kitab Ghayyat al-Ashfa (very useful tables). This collection contains 200 pages of tables with most of them containing up to 180 entries.

Ibn Yunus has given details about the fixed times of worship of Muslims in his book. They are determined by the motion of the sun and the planets, and they change over time throughout the year. These time tables have been in use in Cairo until the late nineteenth century.

Ibn Yunus’s biographer Al-Masji calls Ibn Yunus eccentric or a strange man. He states that Ibn Yunus was a careless and forgetful man dressed in torn old clothes and looked like a lazy man. One day when he was in good health and in good spirits, he predicted his own death which would take place in seven days. After dealing with his personal affairs, he moved into his house and erased ink marks on his manuscripts. After that he continued to recite the Holy Qur’an until he died, the same day that he prophesied his death, that is, he died in 1008 AD.

Dr. Qadir Lone, quoting William Draper, writes that Ibn Yunus was the first to use a pendulum to measure time. William Draper writes, “Their (Muslim) great physician was Ibn Yunus, who did the most valuable work on the measurement of time.” He then writes:

“He first applied pendulum to the measure of time.” (3)

References:
1- Dictionary of Scientific Biography, Vol. 18, P. 574.
2- Scientific Achievements of Medieval Muslims, Dr. Ghulam Qadir Lone, P. 179.
3- Ibid, P. 180.