Disparities in Global Hijri Calendars: A Comparative Study of Muhammadiyah’s KHGT and Diyanet Türkiye during the Period 1447–1457 AH

Abstract:
This study compares the Muhammadiyah Unified Global Hijri Calendar (KHGT) and Diyanet Türkiye, two global calendar systems that both refer to the 2016 International Hijri Calendar Unity Congress in Istanbul but result in different date determinations in several cases. This study employed a qualitative approach using a comparative-documentary method to analyze official documents from both institutions, crescent (hilal) visibility maps, and institutional correspondence between Muhammadiyah and Diyanet. Of the 131 first-of-the-month dates that could be compared during the period 1447–1457 AH, 129 cases (98.5%) yielded identical dates, while differences occurred in 2 cases (1.5%), namely Ramadan 1447 AH and Rabi’ al-Awwal 1455 AH. The analysis shows that these differences do not arise from different astronomical threshold criteria, but rather from differences in the type of crescent altitude parameter used: KHGT uses a geocentric parameter consistent with the objective-universal hisab tradition, whereas the operational application of Diyanet’s visibility criteria in this study is interpreted as a topocentric approach that reflects the actual visibility of the crescent from the Earth’s surface. This difference shifts the position of the “first visibility” boundary curve on Diyanet’s map westward, such that the first point at which the parameters are met is located at a different location from that of KHGT, significantly affecting date determination in cases involving astronomical boundaries. These findings indicate that the differences between the two calendars ultimately lead to a question that has not been explicitly addressed in the KHGT documents: whether the 5°/8° parameters are intended as objective geocentric astronomical conditions or as an analogy to imkanurukyat, which astronomically requires a topocentric approach.

Keywords: KHGT, Turkish Diyanet, Global Hijri Calendar, Geocentric, Topocentric