Maragha: The Observatory the Conqueror Paid For
The Islamic Golden AgeThe Year After Baghdad
In February 1258 Hulegu, a grandson of Genghis Khan, took Baghdad and killed al-Musta'sim, the last Abbasid caliph to reign in the city. Within roughly a year he was paying for an observatory on a hill above Maragha, in the mountains of East Azerbaijan Province in what is now northwestern Iran, a region whose people today are largely Azerbaijani Turks. The man he put in charge, Nasir al-Din al-Tusi, was a mathematician who had spent years in Ismaili service at the fortress of Alamut until the Mongols took it in 1256. Hostile later writers accused him of engineering both surrenders. That charge has been argued over for centuries and rests on partisan sources rather than on anything al-Tusi wrote, so it is best left open. What is certain is stranger: he walked out of a conquered castle as the conqueror's scientific adviser, and asked for a building.
Money, Stone, and a Number Nobody Can Check
Al-Tusi did not ask for a grant. He asked for waqf, the Islamic religious endowment that locks property permanently to a purpose, with agents in each city collecting a tenth of the revenues. Maragha is often described as the first observatory funded that way, and the arrangement did what endowments do. Hulegu died in 1265; the observatory ran for roughly half a century. An Iranian team under Parviz Varjavand dug the hill in the 1970s and found a central tower 22 metres across, a corridor 3.1 metres wide cut along the meridian, and in it some 5.5 metres of the great mural quadrant. Fourteenth-century biographers put the library at 400,000 volumes; historians read that as inflated, and a figure of 40,000 also circulates. Nobody can check.
The Inheritance, and the Flaw in It
Twelve years of observation went into the Zij-i Ilkhani, tables finished in 1272 under Hulegu's son Abaqa. They were not a purely Islamic product: a Chinese astronomer whom Persian sources call Fao Munji worked there, and the tables include a chapter on the Chinese-Uighur calendar. The frame it all sat in was foreign too: Ptolemy's Almagest, out of second-century Alexandria, inherited through centuries of Arabic translation and read beside Persian and Indian tables. Ptolemy predicted well and contained something the Maragha astronomers rejected: the equant, a point about which a planet swept equal angles in equal times though it was not the centre of its own circle. Uniform circular motion, the physical premise of the system, quietly broke.
Two Circles Inside Each Other
Al-Tusi's answer appears as early as 1247, in his commentary on the Almagest, where he used it for the latitude motions of the inner planets; he developed it further in his later Memoir on Astronomy. Roll a small circle inside one of twice its diameter, and a point on the small circle traces a straight line, back and forth. Circular motion, all of it uniform, producing an oscillation, which is enough to do the equant's work without the equant. Al-Urdi, the engineer from Damascus who built the instruments, supplied a second geometrical result now called Urdi's lemma. A century later Ibn al-Shatir, timekeeper at the Umayyad Mosque in Damascus, used both to rebuild the models of Sun, Moon and planets with no equant anywhere in them.
The Same Geometry, and No Paper Trail
In 1957 Victor Roberts showed that Ibn al-Shatir's lunar model is mathematically identical to the one Nicolaus Copernicus published in 1543. Identical, but not the same idea: Ibn al-Shatir's universe was firmly Earth-centred, and he never proposed anything like heliocentrism. The Tusi couple turns up in De revolutionibus, and so does Urdi's lemma, unattributed. Around 1300 the Byzantine scholar Gregory Chioniades carried Maragha material west and put it into Greek, and Greek manuscripts containing the device survive in Italy. There the trail stops. Many specialists think Copernicus picked the geometry up somehow; others hold that independent rediscovery is at least as plausible. No document linking them has been found.
Key Dates to Remember
- 1247 — Al-Tusi sets out the two-circle device later called the Tusi couple, in his commentary on Ptolemy's Almagest, to handle the latitude motions of the inner planets.
- 1259 — A year after sacking Baghdad, Hulegu orders an observatory built on the hill above Maragha, with al-Tusi in charge and religious-endowment revenues to pay for it.
- 1272 — The Zij-i Ilkhani is completed under Hulegu's son Abaqa, after twelve years of observation; its chapters include the Chinese-Uighur calendar.
- 1371/72 — Ibn al-Shatir, the Damascus mosque timekeeper who had already rebuilt the planetary models without the equant, builds his sundial for the Umayyad Mosque minaret.
- 1957 — Victor Roberts shows that Ibn al-Shatir's lunar model is mathematically identical to the one Copernicus published in 1543 — reopening a transmission question still unsettled.