Eclipses in the Middle East from the Late Medieval Islamic Period to the Early Modern Period: Part 1: The observation of six lunar eclipses from the Late Medieval Islamic Period
By S. Mohammad Mozaffari
Journal of Astronomical History and Heritage, Vol.16:3 (2013)
Abstract: This paper deals with the analysis of data obtained from observations of two sets of three lunar eclipses in the Late Medieval Islamic Period. The first trio consists of the lunar eclipses of 7 March 1262, 7 April 1270 and 24 January 1274, observed by Muḥyī al-Dīn al-Maghribī from the Maragha Observatory (in north-western Iran), and the second includes those of 2 June and 26 November 1406, and 22 May 1407, observed by Jamshīd Ghiyāth al-Dīn al-Kāshī from Kāshān (in central Iran). The results are that al-Maghribī’s values for the magnitudes of these eclipses agree excellently with modern data, and his values for the times when the maximum phases occurred agree to within five minutes with modern values. Al-Kāshī’s values for the times of the maximum phases show a rather larger divergence from modern data, varying from about ten minutes to about one hour. The errors in all six values both astronomers computed from their own solar parameters for the longitude of the Sun at the instant of the opposition of the Moon to the Sun in these eclipses remain below ten minutes of arc. The motivation for doing these observations was to measure the lunar epicycle radius r in the Ptolemaic model. Al-Maghribī achieved r = 5;12 and al-Kāshī r ≈ 5;17,1 in terms of the radius of an orbit of R = 60 arbitrary units. It is argued that comparing with modern theory, neither of these two medieval values can be considered an improvement on Ptolemy’s value of r = 5;15.
Introduction: Over the last few decades around fifty observational reports of solar and lunar eclipses dating to the Early Medieval Islamic Period (ca. AD 801-1000) have been investigated in depth. Alongside reports preserved by other cultures (particularly the Chinese), they have been used to obtain estimates for the rate of the deceleration of the Earth‟s rotation and the cumulative amount of the change in the length of the day, or ΔT, that is, the difference between Terrestrial Time and Universal Time .
This was perhaps a main characteristic of the rise of astronomy in communities where eclipses were seen as remarkable and frequent celestial events, and were observed so that the data obtained (regardless of how accurately they might be determined) could be compared with those computed on the basis of contemporary tables and theories. This was perhaps the reason why a great deal of energy and effort was employed to make more precise observations of eclipses. For example, Ibn Yūnus (d. 1007) from Cairo gathered reports from some local astronomers and others who had witnessed eclipses, and data they supplied helped him to determine a better estimate for the magnitude of each eclipse and solar-lunar altitudes at specific phases. And the earliest known attempts to reconcile theory with observations in Medieval Islamic astronomy might have been produced in this way. For instance, Ibn Yūnus reported that the Baghdad astronomer Ibn Amājūr (ca. the late ninth to the early tenth century) found that the true longitude of the Moon was 16′ behind that computed from the Mumtaḥan zīj composed by Yaḥyā b. Abī Manṣūr at Baghdad in about AD 830.