Determining the start of the Hijri month is a critical issue in astronomy because it relates to the observance of Islamic rituals. Although MABIMS member states have agreed on the “imkanur rukyat” criteria as a common guideline to harmonize the Hijri calendar, discrepancies in determining the start of the month still occur, including between Malaysia and Brunei Darussalam, which share a common geographical location. These differences are influenced by the implementation of the criteria, institutional mechanisms, the rukyat verification process, and geographical conditions. This article presents a field study using a normative-empirical approach. Primary data were obtained through interviews with staff from the Astronomy Division of the Sarawak Mufti’s Office in Malaysia, as well as staff from the Sharia Court and the Survey Department of Brunei Darussalam. Secondary data were sourced from regulations, crescent moon observation reports, astronomical literature, and official MABIMS documents. The data were analyzed descriptively, analytically, and comparatively using the burhani epistemological theory to assess the rationality, empirical observations, and scientific arguments in the application of the imkanur rukyat criteria. The research findings indicate that both Malaysia and Brunei Darussalam apply the 2021-revised MABIMS imkanur rukyat criteria through the integration of the hisab and rukyat methods, noting that Brunei places greater emphasis on the visibility of the crescent moon during observations on the 29th of each Hijri month. The differences lie in institutional structure, mechanisms for verifying rukyat results, and the management of observational data. An epistemological analysis based on burhani reasoning indicates that the application of these criteria is grounded in empirical evidence and rational astronomical considerations. Geographical factors, such as horizon conditions, atmospheric characteristics, and observation locations, also influence the likelihood of the crescent moon’s visibility and the decision-making process. Despite technical and administrative differences, both countries remain committed to applying the imkanur rukyat criteria in a scientific and sharia-compliant manner to support the harmonization of the Hijri calendar in the MABIMS region.

[Penentuan awal bulan Hijriah merupakan isu penting dalam ilmu falak karena berkaitan dengan pelaksanaan ibadah umat Islam. Meskipun negara-negara anggota MABIMS telah menyepakati kriteria imkanur rukyat sebagai pedoman bersama untuk mewujudkan harmonisasi kalender Hijriah. Namun, perbedaan penetapan awal bulan masih ditemukan, termasuk antara Malaysia dan Brunei Darussalam yang memiliki lokasi geografis berdampingan. Perbedaan tersebut dipengaruhi oleh implementasi kriteria, mekanisme kelembagaan, proses verifikasi rukyat, dan kondisi geografis. Artikel ini merupakan penelitian lapangan dengan pendekatan normatif empiris. Data primer diperoleh melalui wawancara dengan pegawai Bagian Falak Jabatan Mufti Sarawak, Malaysia, serta pegawai Mahkamah Syariah dan Jabatan Ukur Brunei Darussalam. Data sekunder berasal dari peraturan, laporan observasi hilal, literatur ilmu falak, dan dokumen resmi MABIMS. Data dianalisis secara deskriptif-analitis dan komparatif menggunakan teori epistemologi burhani untuk menilai rasionalitas, observasi empiris, dan argumentasi ilmiah dalam penerapan kriteria imkanur rukyat. Hasil penelitian menunjukkan bahwa Malaysia dan Brunei Darussalam sama-sama menerapkan kriteria imkanur rukyat MABIMS revisi 2021 melalui integrasi metode hisab dan rukyat, dengan catatan bahwa Brunei lebih mempertimbangkan keterlihatan hilal saat observasi pada tanggal 29 setiap bulan hijriah. Perbedaannya terletak pada struktur kelembagaan, mekanisme verifikasi hasil rukyat, dan pengelolaan data observasi. Analisis epistemologi burhani menunjukkan bahwa penerapan kriteria tersebut didasarkan pada pembuktian empiris dan pertimbangan astronomi yang rasional. Faktor geografis, seperti kondisi ufuk, karakter atmosfer, dan lokasi observasi, turut memengaruhi peluang keterlihatan hilal serta proses pengambilan keputusan. Meskipun terdapat perbedaan teknis dan administratif, kedua negara tetap berkomitmen menerapkan kriteria imkanur rukyat secara ilmiah dan syar'i untuk mendukung harmonisasi kalender Hijriah di kawasan MABIMS.]