Islamic Astronomy and Geography

David A. King

• Éditeur: Routledge
• Année: 2022
• ISBN:
  9781000585155
• Édition: 1
• Sujet: History
• Sous-sujet: World History

This volume of 12 studies, mainly published during the past 15 years, begins with an overview of the Islamic astronomy covering not only sophisticated mathematical astronomy and instrumentation but also simple folk astronomy, and the ways in which astronomy was used in the service of religion. It continues with discussions of the importance of Islamic instruments and scientific manuscript illustrations. Three studies deal with the regional schools that developed in Islamic astronomy, in this case, Egypt and the Maghrib. Another focuses on a curious astrological table for calculating the length of life of any individual. The notion of the world centred on the sacred Kaaba in Mecca inspired both astronomers and proponents of folk astronomy to propose methods for finding the qibla, or sacred direction towards the Kaaba; their activities are surveyed here. The interaction between the mathematical and folk traditions in astronomy is then illustrated by an 11th-century text on the qibla in Transoxania. The last three studies deal with an account of the geodetic measurements sponsored by the Caliph al-Ma'mûn in the 9th century; a world-map in the tradition of the 11th-century polymath al-Bîrûnî, alas corrupted by careless copying; and a table of geographical coordinates from 15th-century Egypt.

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Too Many Cooks … A New Account of the Earliest Muslim Geodetic Measurements

Introduction

The measurement of the length of one degree on the meridian by astronomers commissioned by the Abbasid Caliph al-Ma’mūn in Baghdad ca. 830 is in one sense well known, and in another, clouded in obscurity. Different versions of the observations are recorded by Ibn Yūnus (fl. Cairo ca. 990) and al-Bīrūnī (fl. Central Asia ca. 1025), as well as by various later writers. Particularly those versions in which two groups of astronomers laden with instruments are reported heading off in opposite directions along a meridian in the middle of the desert simply boggle the mind. There is a substantial secondary literature on these reports, including important contributions by C. A. Nallino, S. H. Barani, A. Sayılı, E. S. Kennedy, R. P. Mercier, and F. J. Ragep.¹ In this paper these well-known accounts will be discussed, but only briefly, for my main purpose is to present a new contemporaneous account of the measurements.

The earliest Muslim latitude and longitude measurements in Mecca and Baghdad, also commissioned by al-Ma’mūn, are less well known, and they too are clouded in obscurity. Until recently they were known only from passing remarks by al-Bīrūnī,² and it was Aydin Sayılı who first drew attention to that scholar’s brief mention of the simultaneous lunar eclipse observations in the two cities.³ It was clear that the main purpose of the measurements was to derive the qibla at Baghdad, but details were lacking.⁴

In 1985 Dr. Y. Tzvi Langermann of the Hebrew University in Jerusalem published a newly-discovered treatise on both sets of observations by the contemporary astronomer Ḥabash,⁵ and I have come across a different account by Yaḥyā ibn Aktham,⁶ the judge appointed by al-Ma’mūn to oversee the observations. In this paper I shall compare both of these reports in the light of the later accounts of Ibn Yūnus and al-Bīrūnī. Mercier’s recent overview of the evidence available to him, which included the treatise of Ḥabash, draws attention to the obvious inconsistencies in the sources. From the outset it must be borne in mind that this newly-discovered report of Yaḥyā ibn Aktham raises yet more questions and casts but little light on the confusion.

In the sequel I use the following notation freely:

q	qibla (local direction of Mecca, measured from the local meridian)
L	terrestrial longitude
ΔL	longitude difference from Mecca
ΔLB	longitude difference between Baghdad and Mecca
Δϕ	latitude difference from Mecca
ΔϕB	latitude difference between Baghdad and Mecca
ϕ	latitude
ϕB	latitude of Baghdad
ϕM	latitude of Mecca.

In fact, q is a trigonometric function of ΔL, ϕ and ϕ_M, but most of the earliest mathematical procedures, including the one used by al-Ma’mūn’s astronomers, were approximate, and one popular method used for several centuries involved only ΔL and Δϕ.⁷

In the translations the distinction between numbers written in words and those expressed in Arabic numerals has been preserved. Numbers expressed in alphanumerical (abjad) notation are underlined. Insertions in the text to smooth the flow of the translation, as well as occasional citations of the Arabic original, are in parentheses; restorations are in square brackets; and my comments in curly brackets.

1. Ibn Yūnus’ account

In Chapter 2 of his monumental work, a zīj or astronomical handbook with tables,⁸ dedicated to the Fatimid Caliph al-Ḥākim and called the Ḥākimī Zīj, Ibn Yūnus⁹ refers in general terms to the measurement of longitude differences by means of lunar eclipse observations,¹⁰ and then briefly refers to the geodetic measurements:¹¹

Discussion of the (distances in) cubits between places: Sanad ibn ʿAlī mentioned in a statement of his which I found that al-Ma’mūn ordered Khālid ibn ʿAbd al-Malik al-Marwarrūdhī and him to measure the amount of one degree of a great circle on the surface of the earth. He said: “We both set off together for this (purpose). (al-Ma’mūn) ordered ʿAlī ibn ʿĪsā al-Asṭurlābī and ʿAlī ibn al-Buḥturī to do the same and they went off in a different direction.” Sanad ibn ʿAlī said: “Khālid ibn ʿAbd al-Malik and I travelled to (a place) between W’mxh¹² and Tadmur ( = Palmyra) and there we measured the amount of a degree of a great circle on the surface of the earth. It was fifty-seven miles (mīl). ʿAlī ibn ʿIsā and ʿAlī ibn al-Buḥturī also made measurements and the two of them found the same as this. The two reports from the two directions (telling of) the two measurements with the same result arrived at the same time.”

Aḥmad ibn ʿAbdallāh, known as Ḥabash, stated in his book in which he mentioned the observations (of the compilers) of the Mumtaḥan (Zīj) in Damascus, that al-Ma’mūn ordered that one degree of a great circle on the surface of the earth should be measured. He said: “They set off for this (purpose) in the desert of Sinjār, (travelling) until two measurements of the meridian altitude on the same day differed by one degree, and then they measured the distance between the two places, and it was 56 and one-quarter miles, where each mile is four thousand cubits, these being the “black cubits” that were adopted by al-Ma’mūn.”

Ibn Yūnus continues with a description of the way in which the distance along the meridian can be measured. One takes two ropes, each 50 cubits long. Lay out the first one in the meridian. Then lay out the second one along the first one, starting at the mid-point of the first. And so on … (for 56-odd miles!).¹³ He concludes with a discussion of measuring rods, describing instruments of square cross-section and 24 times as long as they are wide, adding that these would cost 1000 dirhams if made of silver, or 1000 dinars in gold. Since, according to a 14th-century Egyptian source, Ibn Yūnus was paid...