MUHAMMADIYAH.OR.ID, YOGYAKARTA—A global Islamic calendar have several criteria for its validity, according to Oman Fathurrahman, a member of the Muhammadiyah Council for Tarjih and Tajdid. They are unification, globality, and prohibition of new moon sighting.
First, the global Islamic calendar should be applicable to all Muslims in the world without divided the earth into a number of zones.
Second, the applied time system is in line with the International Date Line (IDL). It is an internationally accepted demarcation of the surface of earth, running between the south and north poles and serving as the boundary between one calendar day and the next. It passes through the Pacific Ocean, roughly following the 180.0° line of longitude and deviating to pass around some territories and island groups. Crossing the date line eastbound decreases the date by one day, while crossing the date line westbound increases the date.
Third, it is not allowed to postpone the beginning of the month when hilal (a crescent moon) is visible in an area. According to Muhammad Odeh, there are five levels of moon visibility: easily visible, visible but with difficulty, visible only with optical aids, not possible to be seen, and impossible to be seen.
“For example, on Friday, January 4, 2030, when the sun sets, the crescent moon is already visible in the United States of America, whereas in Wellington, New Zealand, the moon has already set, 5 minutes before sunset. In such a scenario, New Zealand must still commence the first Ramadan 1451 on Saturday, January 5, 2030. It is not permissible to further postpone it because the crescent moon has already been sighted in the western part of America,” said Oman at the Seminar on the Science Integration in Hisab (Islamic Astronomy), Rukyat (Moon Sighting), and the Unifying Global Calendar conducted at Universitas Muhammadiyah Yogyakarta on Friday (2/6).
Moreover, Oman mentioned criteria for determining the new moon in the Global Islamic Calendar. The new month begins, anywhere in the world, before 12:00 midnight Universal Time Coordinated (UTC) when two criteria are met: 1) the angular distance (elongation) between the sun and the moon at sunset reaches 08° or more, and 2) the height of the moon above the horizon at sunset reaches 05° or more.