Introduction

For Muslims, the Hijri (Islamic) calendar is vital because it determines the timing of major religious observances such as the Ramadan fast, Eid al-Fitr, Eid al-Adha, and the Hajj pilgrimage. Accuracy in determining the start of a Hijri month is not just a matter of timekeeping; it directly affects the validity of worship and the unity of religious practices.

However, to this day, Muslims in different parts of the world often begin Ramadan or celebrate Eid on different days. This discrepancy stems from two primary methods: rukyat (physical sighting of the moon) and hisab (astronomical calculation). The debate between the two is not merely technical, but also touches on how religious teachings are understood alongside modern scientific advancements.

In today’s globalized era, this issue has grown more complex. Muslims live not only in Muslim-majority nations but also as minorities in Europe, the Americas, and East Asia. In these settings, date certainty is crucial to align with workplaces and social systems that require advance planning. On the other hand, advancements in astronomy have made it possible to calculate the moon’s position with extreme precision, opening the door for hisab to become a more systematic and predictable solution.

This article explores the hisab method as a scientific approach to the Hijri calendar, including its religious foundations, its differences from rukyat, and how it is applied globally.

Rukyat as a Traditional Method

The rukyat method relies on the direct sighting of the hilal (the first thin crescent moon) after the conjunction phase. In practice, rukyat is performed on the 29th day of the Hijri month at sunset. If the crescent is sighted, the next day is declared the start of the new month. If it is not seen, the current month is completed to 30 days.

The basis for rukyat comes from a hadith of Prophet Muhammad, which explicitly instructs Muslims to begin and end fasting based on sighting the crescent:

“Fast when you see it (the crescent) and break your fast when you see it. If it is obscured from you, then complete the number of Sha’ban to thirty.” (Narrated by Bukhari No. 1909, Muslim No. 1081)

The strength of this method lies in its adherence to the Prophet’s traditions. However, in practice, rukyat is highly dependent on natural conditions like weather and geographical location. Differences in observation sites can lead to varying results between regions, causing countries to start their months on different days.

Hisab as a Scientific Approach

Unlike rukyat, the hisab method determines the start of a Hijri month based on astronomical calculations of the sun and moon’s positions. In practice, hisab does not rely on physical sightings but uses precisely calculated mathematical and astronomical data. This method evolved alongside the science of falak (Islamic astronomy), providing certainty regarding the exact time of ijtima’ (conjunction) and the crescent’s position at sunset. In this context, hisab is viewed as a scientific approach that allows Muslims to create a systematic and planned Hijri calendar years in advance.

The basis for hisab can be found in Quranic verses affirming that the orbits of the sun and moon are orderly and can be calculated:

“The sun and the moon [move] by precise calculation.” (QS. Ar-Rahman: 5)

“And He determined for it phases—that you may know the number of years and account [of time].” (QS. Yunus: 5)

The hisab system is generally divided into several levels:

1. Hisab ‘Urfi (Conventional Calculation)

Hisab ‘urfi is a simple calculation method not based on the actual astronomical position of the moon. This system assigns a fixed number of days to each month (alternating between 29 and 30 days) and uses standard cycles over a year or several years. Because it is conventional, hisab ‘urfi has historically been used for administrative or civil purposes in Islamic history, rather than for religious worship which requires precise lunar positions. Therefore, it does not account for actual visibility or real-time astronomical data.

2. Hisab Hakiki (True Astronomical Calculation)

Unlike hisab ‘urfi, hisab hakiki is based on actual astronomical calculations of the sun and moon. This method calculates the time of conjunction, the crescent’s height, elongation, and other parameters related to visibility. Consequently, it is considered more accurate and scientific. Over time, hisab hakiki has become the backbone of modern Islamic calendar systems, though its implementation varies depending on the criteria used.

• Hisab Hakiki Taqribi (Approximate): An early form of calculation using simple approaches with limited accuracy. It relies on classical astronomical tables and basic formulas, yielding approximate results. However, it played a vital role in history as a bridge between traditional and modern astronomy.
• Hisab Hakiki Tahqiqi (Verified): An advanced version of the approximate method with higher accuracy. It uses detailed astronomical data and accounts for variables affecting the moon’s position. It is close to modern scientific standards and is widely used by astronomers for precise calculations.
• Hisab Hakiki Kontemporer (Contemporary): The most advanced form of hisab, utilizing modern technology and the latest astronomical data. Calculations are done using computers, specialized software, and highly accurate ephemeris data. This method allows for pinpoint predictions of the moon’s position and visibility. It is frequently used by official institutions worldwide.

As a point of scientific reinforcement, contemporary hisab hakiki currently utilizes standard celestial positioning algorithms employed by international scientific institutions (such as NASA). These modern computer calculations offer an extremely high degree of positional accuracy, reaching the order of a fraction of an arcsecond. It is this mathematical precision that provides near-absolute temporal certainty in the digital era.

Global Practices of Hisab

The use of hisab varies widely across the globe, heavily influenced by social and geographical conditions.

1. Western Europe

In Western Europe, geographical and demographic constraints push Muslim communities toward calculation-based approaches. In the UK, organizations like Wifaqul Ulama refer to hisab criteria developed by Her Majesty’s Nautical Almanac Office (HMNAO). In France, although the Grande Mosquée de Paris traditionally follows Morocco’s rukyat-based announcements, a growing number of Muslim organizations are shifting to astronomical calculations for consistency and practicality. In Germany, the Muslim community, whose majority comes from the Turkish diaspora, follows the decisions of Diyanet, the official religious body of Turkey, which uses a combination of hisab and rukyat with a strong tendency toward calculation-based certainty.

The factor driving the adoption of hisab in Europe is not purely theological. The skies of Europe, which are often covered by clouds, make rukyat technically unreliable. Moreover, the demands of professional life in Western European countries make date certainty an uncompromisable need.

2. North America

The most significant development in North America occurred in 2007, when the Fiqh Council of North America (FCNA) officially adopted the hisab method for determining the start of the Hijri month. This historic decision was based on three main considerations. First, the often-cloudy geographical condition of North America makes rukyat unreliable as a single method. Second, the status of Muslims as a minority in this region creates an urgent need for date certainty for work, school, and community coordination. Third, in terms of fiqh, accurate hisab is considered more capable of fulfilling the Sharia goal of certainty and preventing division.

FCNA uses the criterion of conjunction before midnight GMT as the main requirement, with the additional requirement of the moon setting after the sun in the Americas. This decision is followed by many mosques in the United States and Canada, although some communities, especially those from South Asia, still prefer to refer to Saudi Arabian announcements.

3. Turkey

Turkey is one of the most consistent examples in institutionalizing hisab at the state level. Since the founding of the modern Turkish Republic, the Diyanet İşleri Başkanlığı has used astronomical calculations as the primary basis for the official Islamic calendar. Turkey even publishes an annual Hijri calendar that lists all important dates, including the start of Ramadan and Eid al-Fitr based on hisab calculations. Turkey’s influence in this regard goes beyond its own territorial borders, as the widespread Turkish Muslim diaspora in Germany, the Netherlands, Belgium, and Austria brings the hisab-based calendar tradition to their communities in Europe.

4. Malaysia and Singapore

In Southeast Asia, Malaysia and Singapore developed a model that is now widely referred to as the most balanced approach between the two traditions. Malaysia through JAKIM (Jabatan Kemajuan Islam Malaysia) and Singapore through MUIS (Majlis Ugama Islam Singapura) use hisab as a planning instrument as well as a selection filter for rukyat reports. The applied criteria establish that the hilal must be at a minimum height of 3 degrees above the horizon and an elongation of at least 6.4 degrees at sunset. If these hisab criteria are not met, no rukyat reports can be accepted, and the month is automatically completed to 30 days.

This approach is technically known as Imkan Rukyat (the possibility of seeing the hilal), which combines the power of hisab certainty with the traditional authority of rukyat. It does not reject rukyat, but places hisab as a prerequisite that must be met before rukyat can be accepted as proof. The coordination between Malaysia, Singapore, and Brunei Darussalam within the framework of MABIMS makes these three countries almost always determine the start of the month on the same day.

5. Indonesia

Indonesia represents the complexity as well as the richest dynamics in the global landscape of Hijri calendar determination. As the country with the largest Muslim population in the world, Indonesia is a meeting arena for two major traditions that simultaneously represent the two largest Islamic organizations in the world.

Nahdlatul Ulama (NU) traditionally uses rukyat as the main method and places the physical observation of the hilal as an irreplaceable condition. On the other hand, Muhammadiyah uses a method they call Hisab Hakiki Wujudul Hilal, where the start of the month is determined if at sunset the moon is already above the horizon, regardless of whether it can be physically observed or not. The Indonesian government through the Ministry of Religious Affairs takes a middle position by adopting the Imkan Rukyat approach based on MABIMS criteria, which was updated in 2021 to a minimum hilal height of 3 degrees and an elongation of 6.4 degrees.

6. Asian Countries with Minority Muslim Communities

In East Asian and mainland Southeast Asian countries with minority Muslim communities such as Japan, South Korea, China, Vietnam, Thailand, and the Philippines, practical considerations dominate methodological choices. Muslim communities in these countries generally refer to one of three sources of authority: Saudi Arabian decisions, Malaysian or Singaporean decisions based on Imkan Rukyat, or hisab calendars published by local Islamic institutions.

In Japan, most mosques, including Tokyo Camii managed by Turkey’s Diyanet, use the hisab calendar. This is not purely a theological choice, but a response to Japan’s rigid professional life structure in terms of time discipline and leave application procedures. In South Korea, the Muslim community centered around the Seoul Central Mosque generally follows announcements from countries using hisab, especially Turkey and Malaysia, with similar considerations.

The conditions faced by Muslim communities in these countries collectively illustrate why hisab is not just a theological preference, but a rational response to the real needs of Muslims living as minorities in a social system that does not accommodate date uncertainty for religious holidays.

Visual Summary of Global Criteria

To help general readers understand the variations in these standards, here is a visual summary of the astronomical criteria adopted in different regions:

Toward a Global Hijri Calendar

The methodological divide often leads to different holiday dates across borders. This has sparked the idea of establishing a unified Global Hijri Calendar.

One growing approach is setting universal astronomical criteria. In this model, if criteria are met anywhere on Earth, the whole world follows the same date. This bridges the predictability of hisab with the religious roots of rukyat. Unifying the calendar does not mean erasing one method but integrating both.

Conclusion

The debate between hisab and rukyat is a testament to the dynamic intellectual tradition in Islam. Both methods stand on strong foundations, both textually and rationally.

In our hyper-connected modern world, the need for time certainty is paramount. Hisab offers a systematic, planned solution, especially for Muslims living in environments that require schedule predictability.

Developing a global Hijri calendar that harmonizes both methods is a vital step toward reducing discrepancies and strengthening global Muslim unity. This approach is not only scientifically sound but also aligns with the core Sharia principles of ease and certainty.

– Written by Naflah –

References

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NU Online. (n.d.). Metode hisab: Perhitungan astronomis. https://nu.or.id/opini/metode-hisab-perhitungan-astronomis-fVBT5

Djamaluddin, T. (2016). Proposal Ringkas Penyatuan Kalender Islam Global. LAPAN, Indonesia.

Maliha, N. F., & Junaidi, A. (2025). Utilization of artificial intelligence for a global Hijri calendar: Harmonizing hisab and rukyat in the digital era. In Annual International Conference on Islamic Education (AICIED) Proceedings (Vol. 3). IAIN Ponorogo.

Arafat, M., & Budiwati, A. (2025). Syamsul Anwar’s vision: Establishing a unified global Islamic calendar. AL-AFAQ: Jurnal Ilmu Falak dan Astronomi Fakultas Syariah Universitas Islam Negeri (UIN) Mataram, 7(1), 1–17.