Determination of Obliquity of the Ecliptic by Observing Solstices in Medieval Islam

Document Type : Research/Original/Reqular Article

Author

Assistant Prof., University of Tehran

Abstract

The Obliquity of the Ecliptic (in the terms of Islamic astronomy: Meyl aʿzam / Kolli, literally: the greatest / total declination [of the sun]) is one of the important astronomical parameters for which numerous observations have been made in Medieval Islam to determine it. The most reliable method for this task, according to ancient scholars,  was to find the maximum and minimum annual meridian solar altitudes (by observing meridian solar altitudes on a few days close to the summer and winter solstices) and to calculate half the difference between the two. Most of these observations were made with a large brick quadrant known as a libna, in some cases with metal graduated rings of various sizes, and in at least two cases with a very large brick sextant, known as the Fakhri Sextant. Some Islamic astronomers, such as Khujandi, considered the obliquity to have a decreasing trend, and some others (mostly in Andalusia) considered its changes to be oscillating, following the incorrect assumption known as Trepidation / “The Accession (Iqbal) and Recession (Idbar).” But some, such as Biruni and Ibn al-Shatir, attributed the difference in the results of the observations to the error of the observational instruments and / or the observer  (especially about Khujandi and Abu al-Fazhl Hirawi). Hence, many astronomers have pointed out different values ​​obtained for the Obliquity, and Biruni has provided a more or less chronological list of the Obliquity observations up to his time. The present survey presents, for the first time, an analytical and chronological list of these observations with numerous considerations on some details and points related to these observations

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References

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Journal for the History of Science
Volume 23, Issue 1 - Serial Number 38
September 2025
Pages 173-202

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