Astronomy Professor Criticizes Global Hijri Calendar, Muhammadiyah Responds

TEMPO.CO, Jakarta - Indonesian professor of astronomy and astrophysics from the National Research and Innovation Agency (BRIN), Thomas Djamaluddin, criticized the Global Hijri Calendar a la Turkiye or the Single Global Hijri Calendar. According to him, the use of this calendar has the potential to create more discrepancies in Indonesia.

"The Global Hijri Calendar only accommodates calculation-based followers and ignores visual sighting-based followers, hence differences will still occur at local, national, and global levels," he said to Tempo on Thursday, June 26, 2025.

The Global Hijri Calendar a la Turkiye or the Single Global Hijri Calendar (KHGT) was launched to be implemented starting from 1447 Hijriah. The new Islamic calendar year begins on 1 Muharam. "KHGT begins with the difference in the determination of 1 Muharam 1447 Hijriah, then will be tested with the scientific accuracy of the determination of the beginning of Ramadan later," said Thomas through his written explanation.

The government and most Islamic community organizations are using the new criteria agreed upon by the religious affairs ministers from Indonesia, Malaysia, Brunei Darussalam, and Singapore, abbreviated as Mabims. The new Islamic year 1447 Hijriah according to the KHGT version, according to Thomas, falls on June 26, 2025, while the Indonesian government has designated it as a national holiday on June 27, 2025.

According to Thomas, the difference in dates is due to the use of different criteria. At sunset on June 25, 2025, in the Southeast Asian region, the position of the new crescent moon did not meet the new Mabims criteria, so 1 Muharam in Indonesia falls on June 27, 2025. Meanwhile, according to the Turkish criteria or KHGT, the moon's position has met the criteria, and the conjunction occurred before dawn in New Zealand, thus marking the beginning of the new Islamic year or Hijriah on June 26, 2025.

The concept of a day in KHGT refers to the opinion of Jamaluddin Abdur Razzaq from Morocco who proposed that the start of the day should be at 00:00 globally, which means midnight in the areas around the international date line. Indications of differences, according to Thomas, could also occur in determining the beginning of fasting or the upcoming 1 Ramadan.

Muhammadiyah, an Islamic community organization using the Global Hijri Calendar, responded to this criticism through its official website. One of the criticisms is that the Global Hijri Calendar is considered to force certain regions into the new month even if the crescent moon has not been sighted, or to delay other regions that have already sighted the moon for the sake of global uniformity, especially when the moon is still below the horizon in the eastern regions.

Based on the explanation from Muhammadiyah's astronomy expert, Arwin Juli Rakhmadi Butar-butar, scientific arguments to respond to this criticism can be summarized into several points. Moon phases, including the crescent, are global phenomena, while the sighting of the crescent is local. This means that even if the crescent is not visible in a certain region due to its position below the horizon, astronomically the crescent still exists after the conjunction (ijtima').

The conjunction, as the end point of the lunar cycle, marks the beginning of the existence of the crescent, even if it is not yet seen with the naked eye. Thus, the crescent that is below the horizon is still considered as the definitive crescent in the global context.

The change in moon phases correlates with elongation, which is the angular distance between the Sun and the Moon, influenced by the difference in the angular speed of the two celestial objects. After the conjunction, the elongation continues to increase over time, so the crescent that was initially below the horizon in a certain region will continue to grow and become visible in other regions. In the global context, if the possibility of sighting is fulfilled in one place, the crescent is considered to exist globally even if in other regions the moon is still below the horizon.

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