Born in Birjand, Iran, ‘Abul ‘Ali Ibn Mohammad Ibn Hussain Birjandi was a prominent 16th-century mathematician, physicist and astronomer.

Al Birjandi was a pupil of Mansour Ibn Mu‘in Al Deen Al Kashi at the famed Samarqand observatory, as well as of the equally prominent Saif Al Deen Taftazani, then the world’s leading authority on astronomical commentaries.

With such an academic pedigree, it was not long before Al Birjandi produced work of his own. He wrote commentaries, most notably on the works of Nasser Al Deen Al Tusi, whose exceptional contributions on Ptolemy’s “Almagest” revolutionised the scientific community by rigorously testing the Greek astronomer’s findings using astrolabes.

Although difficult to ascertain, historians believe that Al Birjandi’s evaluations of various Arab astronomical discoveries made it to Europe, which benefited scientists such as Copernicus.

Though little is known of Al Birjandi’s personal life, his sharp analyses and ability to digest complex ideas into understandable prose transformed him into a leading “transmitter” and someone who made a difference. He died in Birjand in 1528.

Contributions to astronomy

After what must have been an early education, Al Birjandi ended up at the Ulugh Beg observatory in Samarkand, in today’s Uzbekistan. Built in the 1420s by the astronomer Ulugh Beg, a grandson of Tamerlane, the observatory was considered by scholars to have been one of the finest in the Muslim world.

Several famous Muslim astronomers worked here, including Al Kashi and Ali Qushji, before the facility was destroyed in 1449 after the civil strife that arose from various lords challenging the Chinese empire (Ulugh Beg was beheaded). In 1908, the observatory was rediscovered and rebuilt, but it never regained its previous standing.

Still, it was at Samarkand that Al Birjandi continued a debate initiated by Ali Al Qushji over the rotation of the Earth. As he analysed various questions, Al Birjandi wondered what might occur if the Earth were moving in the cosmos, and developed a hypothesis similar to the notion advanced by Galileo Galilei (1564–1642) that it is engaged in a circular inertia, though fortunately for the Persian, he was spared house arrest and various ecclesiastical punishments that befell the Italian polymath.

What Al Birjandi described was based on his numerous observations, best articulated in a response to one of Qutb Al Deen Al Shirazi’s arguments: “The small or large rock will fall to the Earth along the path of a line that is perpendicular to the plane (sath) of the horizon; this is witnessed by experience (tajribah). And this perpendicular is away from the tangent point of the Earth’s sphere and the plane of the perceived (hissi) horizon. This point moves with the motion of the Earth and thus there will be no difference in place of fall of the two rocks.”

While this may sound trivial to a 21st-century century reader, it was a significant step forward in astronomy then, without the modern telescopes and computers to verify stellar movements throughout the cosmos. 

Over a short period, Al Birjandi composed several works that provided tables of the positions of stars, which he calculated for various years, and wrote commentaries on Al Kashi’s attempts to correct Al Tusi’s opus.

In addition, he also wrote a few independent works, whose subjects included cosmology, ephemerides (the study that focuses on celestial navigation by providing the positions of naturally occurring astronomical objects in the sky at any given time), astrolabes, as well as a treatise on the distances and sizes of the planets, and another work on the creation of almanacs. Most famously, Al Birjandi completed his “Sharh Al Tadhkirah” (Commentary on the Tadhkirah) in 1508 to test Al Tusi’s lunar theory. 

Al Birjandi and the “Tusi-couple”

Simply stated, Al Tusi’s contribution to the post-13th century intellectual history of Islam must be one of the most significant by any scientist, as he became the standard torchbearer in so many disciplines. In astronomy, Khawajah (Master or Teacher) Tusi, as he was known during his lifetime (after his death, Tusi was often referred to as Ustaz Al-Bashariyyah — teacher of mankind — as well as Al Mu‘allim Al Thalith, the third teacher after Aristotle and Al Farabi), built an observatory at Maragha in northwestern Iran, where renowned scientists, including astronomers from China, participated in research and scientific observations to help the Mongol ruler Hulegu in his conquests.

His “Memoir on the Science of Astronomy” (Al Tadhkirah fi-‘ilm Al hay’a), which gives a somewhat coherent and unified account of astronomy, led to Al Birjandi focusing on the relation between theory and observation. Al Birjandi’s “Sharh” of Tusi’s epochal work, which was completed in 1508CE, showed how the master reevaluated Ptolemy’s “Almagest” and Euclid’s “Elements”.

Such scientific assessments and reassessments allowed various scholars to advance knowledge and establish the foundations of later work that propelled us into the modern era.

Historians believe that Al Birjandi’s “Sharh” of the “Tusi-couple”, a term coined by Edward Kennedy in 1966, may well have inspired Copernicus as he formulated the rotation of stellar objects. In fact, the mathematical device in which a small circle rotates inside a larger circle twice the diameter of the smaller one demonstrated the simultaneous rotations of both, to cause a point on the circumference of the smaller circle to oscillate back and forth in a linear motion along a diameter of the larger circle.

Remarkably, Copernicus used a similar model to advance his theory of trepidation — which combined the oscillation of the equinoxes (a spurious notion) with a change in the obliquity of the ecliptic that is now acknowledged as an authentic motion of the Earth’s axis — using Mercury as a case study.

While there is no concrete evidence to make the link, historians now suspect that either Copernicus or another European author had access to an Arabic astronomical text, most probably Al Birjandi’s “Sharh”.

According to some sources, the 16th-century scientist Guillaume Postel, a French universalist who was adept at Arabic, Syriac and other Semitic languages and who travelled extensively in the Ottoman Empire and throughout the Muslim world, may well have brought back the text to Europe and made it available to Copernicus.

In any case, the “Tusi-couple” was used by Copernicus in his reformulation of mathematical astronomy. This is not to belittle the Polish scientist’s immense contributions but to simply highlight how this mathematical model — for converting circular motions to reciprocating linear motion — that was first advanced by Euclid and later enhanced by Proclus in France in the middle of the 14th Century, was likely refined by Al Tusi and transmitted by Al Birjandi. 

A transmitter of knowledge

Al Birjandi’s commentary on the “Tadhkirah” is a good illustration of the methods of transmission that characterised scientific advances in the Muslim world. In analysing Al Tusi’s work, Al Birjandi highlighted its relevance — how such scientific work added value to core knowledge, provided various explanations of what major discoveries actually meant and, above all else, engaged the scientific community in philosophical discussions.

To be sure, Al Birjandi was not only influenced by Al Tusi but also by many of his predecessors, including Ptolemy, Ibn Al Haytham, Euclid, Qutb Al Deen Al Shirazi and Theodosius. His genius was to tie loose ends, follow certain opinions, test others and make more inquiries, all to better understand man’s ultimate role in life.

While Al Birjandi objected to the application of the Tusi-couple to the celestial spheres regarding the necessity of rest between two motions, his curiosity was boundless, as he wished to test whether or not the curvilinear or spherical versions of the Tusi-couple produced a slight longitudinal inclination.

Legacy to Arabs and Muslims

Because a Muslim starts his day before sunrise to verify the break of dawn to perform his first prayers, concludes his day waiting for the time of dusk to start his evening entreaties, verifies the setting of new moon during Ramadan to start and break his fast, and is obliged to know the direction of the Ka‘bah to offer his daily appeals to the Lord, it was natural that astronomy preoccupied Arab and Muslim scientists.

Hence so many of them added value to the discipline. In fact, as a Muslim recites the Quran, several verses command him to contemplate the outer space and the creation of the Heaven and the Earth, including: “And that He (Allah) is the Lord of Sirius” (the star which the pagan Arabs used to worship, Quran 53: 49), and “Allah is the one who created seven Heavens and from Earth like them [of corresponding type]; [Allah’s] command descends among them so that you may know that Allah is capable of anything and that Allah knows everything” (Quran 65: 12).

If the first Muslim caliph to pay attention to astronomy was the Abbasid Abu Ja‘far Al Mansour, it was leading astronomers who first translated key books by Euclid, Archimedes and Ptolemy as well as other Greek scientists into Arabic, before embarking on their own original research.

All were preoccupied by the dissemination of knowledge, which was why they encouraged the study of astronomy and mathematics. Al Birjandi was one of those rare characters who was both innovative as well as articulate, as he explained complex notions in easily understood prose and, in doing so, bridged the gulfs that separated civilisations engaged in the quest for truth. 

Dr Joseph A. Kéchichian is an author, most recently of, “Legal and Political Reforms in Sa‘udi Arabia”, London: Routledge, 2013. 

This article is the twenty-fourth of a series on Muslim thinkers who greatly influenced Arab societies across the centuries.