The Short Answer

The Umm al-Qura calendar is Saudi Arabia's official civil Hijri calendar, computed in advance for Mecca. Since 1423 AH (2002) its rule is short: a new month starts if, on the 29th, the conjunction falls before sunset at Mecca and the Moon sets after the Sun. It does not test whether the crescent can be seen, and by independent estimates the crescent is still invisible to the naked eye from Mecca in roughly 75 per cent of the months it starts. The religious start of Ramadan and the Eids is declared separately by the Saudi Supreme Court.

Every year, as Ramadan approaches, a familiar pattern plays out across the Muslim world: Saudi Arabia announces the start of the month, several other countries follow the next day, and observers are left wondering how astronomers and religious authorities in the same region can reach different conclusions on the same evening. At the centre of this recurring puzzle is the Umm al-Qura calendar, one of the most widely referenced, and most frequently misunderstood, Islamic calendar systems in existence.

What Is the Umm al-Qura Calendar?

The Umm al-Qura calendar is the official civil Hijri calendar of Saudi Arabia. It is computed in advance from astronomical calculation using the geographic reference of Mecca, and it takes its name from Mecca itself. "Umm al-Qura" is Arabic for "Mother of Towns" or "Mother of Villages", a Quranic title for Mecca appearing in Surah Al-An'am (6:92) and Surah Ash-Shura (42:7).

The calendar was introduced in 1346 AH (1927 CE), taking its name from the Umm al-Qura newspaper, the kingdom's official gazette, in which the calendar was first published. Today, dates are computed by the Institute of Astronomical and Geophysical Research at the King Abdulaziz City for Science and Technology (KACST) in Riyadh.

One point must be stated plainly, because confusion on this single issue causes more misunderstanding than anything else. The Umm al-Qura calendar is a civil administrative calendar computed in advance. The actual start of Ramadan, Eid al-Fitr, and Eid al-Adha for religious observance is declared separately by the Saudi Supreme Court based on testimony from crescent sighting witnesses. These are not the same system. The calendar tells government offices, schools, and businesses when to observe public holidays months or years in advance. The Supreme Court declaration, by contrast, is made on the evening of the 29th of each month and determines the religious commencement of a new month. They often agree, but they can and do differ.

The Exact Rule: What the Umm al-Qura Calendar Actually Computes

The current rule, in force since 1423 AH (approximately 2002 CE), starts the new Hijri month if, on the 29th day of the current month, both of the following conditions are met at sunset in Mecca.

First, the geocentric conjunction (the astronomical new moon) must have occurred before sunset at Mecca. Second, the Moon must set after the Sun at Mecca. That is the complete rule. Nothing more.

This is a purely geometric criterion. It does not ask whether the crescent is visible. It does not require the crescent to reach a minimum altitude above the horizon. It does not apply any elongation threshold. It only requires that the Moon be above the geometric horizon at the moment of sunset, after the conjunction has already taken place. This type of rule is sometimes called a "Wujud al-Hilal" rule, from the Arabic meaning "the existence of the crescent". The crescent merely needs to exist above the horizon geometrically, regardless of whether any human observer could ever detect it there.

How the Rule Changed Over Time

The Umm al-Qura calendar rule has been revised several times since its introduction. The original 1346 AH formulation was replaced in 1420 AH (1999 CE) with an intermediate rule, which was itself replaced in 1423 AH (2002 CE) with the current formulation. Historical scholarship by Robert van Gent of Utrecht University shows that the pre-2002 rules were even more permissive, meaning they triggered the start of a new month even earlier relative to actual crescent visibility than the current rule does.

Why the Umm al-Qura Rule Does Not Guarantee Crescent Visibility

The threshold required to trigger a new month under the Umm al-Qura rule is met well before the crescent can be seen with the naked eye. By independent astronomical estimates, the crescent is still invisible to the naked eye from Mecca in roughly 75 per cent of the cases where this rule declares a new month.

The Yallop q-value and the Odeh V-value are the two leading scientific criteria for crescent visibility. Both are calibrated against large databases of actual crescent sighting reports and incorporate the empirical Danjon limit as a hard floor. A crescent that satisfies the Umm al-Qura rule can still fall in what Yallop classified as Zone E or Zone F, meaning not visible even with optical aid such as binoculars or a telescope.

The Danjon limit itself is important here. The minimum angular separation between the Moon and the Sun at which a crescent has ever been reliably observed with the naked eye is approximately 7 degrees of elongation. The Umm al-Qura rule can trigger a new month at elongations well below this physical threshold. For a full explanation of what the Danjon limit is and why it matters, see The Danjon Limit: Why Some Crescents Are Physically Impossible to See.

A Worked Example: Ramadan 1447 AH

On the evening of 29 Shaban 1447 (17 February 2026), the astronomical conjunction had occurred the same day at 12:01 GMT. The Moon's elongation at Mecca sunset was very small. The New Crescent Society, independent astronomers, and multiple visibility modelling tools confirmed the crescent was geometrically impossible to see anywhere in Asia, Africa, or Europe that evening. The Yallop and Odeh maps showed no visibility zone anywhere on Earth.

Yet a sighting was announced and Ramadan was declared to begin on 18 February 2026 in Saudi Arabia, while most other countries, following their own sighting traditions or more conservative calculation rules, began Ramadan on 19 February 2026. This one day difference, small in absolute terms, affects the timing of the entire month and consequently the date of Eid al-Fitr. For more on why this happens across different communities, see why Muslims start Ramadan on different days.

The Recurring Controversy: Announced Sightings That Puzzled Astronomers

The Ramadan 1447 example is not an isolated incident. A study of 42 Saudi High Judiciary Council Ramadan new-moon announcements from 1962 to 2001 found that more than half were too early relative to what astronomers calculated as the earliest possible visibility date. The announced sightings described crescents that, by the best available models of crescent visibility physics, should not have been detectable.

Several factors contribute to this pattern. The Umm al-Qura rule sets a very low geometric bar, which means that whenever the calendar declares a new month is possible, witness testimony is being sought on evenings when the Moon is extremely young and the crescent extremely thin. Judging a faint crescent against a bright twilight sky is genuinely difficult, and false positives are well documented in the crescent sighting literature globally. The religious gravity of the declaration, particularly for Ramadan, creates strong social pressure toward positive reports from witnesses.

It is essential to be clear that this is documented astronomical evidence, not a religious verdict. Saudi Judiciary committees act on sworn testimony in good faith within an established legal tradition. The astronomical analysis simply shows that the geometric threshold which prompts the search for witnesses is set lower than the physical threshold at which witnessing is possible.

How the Saudi Religious Sighting Declaration Differs

Because the distinction matters, it is worth describing the Saudi Supreme Court declaration process in its own terms. The Supreme Court seeks witness testimony from trained observers stationed across the kingdom on the evening of the 29th of each lunar month. If credible testimony of a crescent sighting is received, the new month begins the following day. If no credible testimony is received, the current month is extended to 30 days and the new month begins the day after.

In principle, this process is closer to the classical sighting tradition of Islamic jurisprudence than a purely calculated calendar is. The difficulty is that the Umm al-Qura calendar's low geometric threshold means the Supreme Court is frequently seeking testimony on evenings when the crescent, by astronomical reckoning, cannot be seen. The two systems are intertwined in practice even though they are distinct in design.

The Umm al-Qura Calendar vs Other Islamic Calendars

The table below summarises how the Umm al-Qura rule compares to other major Islamic calendar approaches.

Calendar or Rule	Trigger	Threshold	Typical result
Umm al-Qura (Saudi civil)	Calculation	Conjunction before sunset and Moon sets after Sun at Mecca	Can precede naked-eye visibility
Saudi Supreme Court declaration	Witness sighting	Actual crescent reports from trained observers	Roughly aligns with Umm al-Qura most years
MABIMS Imkan al-Rukya	Calculation	Altitude 3 degrees or more, elongation 6.4 degrees or more	More conservative; based on empirical Danjon limit
Local naked-eye sighting (ikhtilaf al-matali)	Sighting	Real crescent visible in local region	Most conservative; never precedes actual visibility

For a side-by-side look at how calculation, sighting and hybrid engines can be combined in one system, see the triple-engine Hijri calendar. The MABIMS criteria (used by Malaysia, Brunei, Indonesia, and Singapore) represent a middle position: they use calculation, as the Umm al-Qura calendar does, but they apply a visibility threshold derived from observed crescent records. The result is that MABIMS dates generally avoid the most extreme cases, though a crescent at 3 degrees of altitude and 6.4 degrees of elongation can still need optical aid, whereas Umm al-Qura dates more often fall on evenings when no crescent can be seen.

What Hilal Vision Shows vs the Umm al-Qura Rule

Hilal Vision computes the Yallop q-value and the Odeh V-value for each candidate crescent evening. Both criteria are calibrated on large databases of actual crescent visibility records and include the empirical Danjon limit as a hard lower floor. The platform's interactive map shows the actual visibility zones for each evening: which regions of the world can see the crescent with the naked eye, which require optical aid, and which have no realistic chance of any sighting.

Users can compare these visibility maps directly against Umm al-Qura dates to assess whether a given calendar date corresponds to genuine observability. When the calendar date falls on an evening where Hilal Vision's map shows no visibility zone anywhere on Earth, the mismatch is immediately apparent.

This is an educational and analytical function. Hilal Vision does not issue religious rulings and does not adjudicate between different scholarly opinions on calendar methodology. The goal is to give observers, researchers, and interested members of the public access to the best available astronomical data.

Frequently Asked Questions

What does Umm al-Qura mean?

It is Arabic for "Mother of Towns" or "Mother of Villages", a Quranic honorific for Mecca. The title appears in Surah Al-An'am (6:92) and Surah Ash-Shura (42:7). The calendar took the name from the Umm al-Qura newspaper, the Saudi official gazette, in which it was first published in 1927 CE.

Is the Umm al-Qura calendar the same as the Islamic lunar calendar?

No. It is one specific national civil calendar based on astronomical calculation. To understand the underlying system it implements, see how the Islamic lunar calendar works. Many Muslim-majority countries use different calculation rules, different geographic reference points, or sighting-based declarations. There is no single universally adopted Islamic calendar, which is itself the source of much of the variation in Ramadan and Eid dates between countries.

Why does the Umm al-Qura calendar sometimes start Ramadan a day before other countries?

Because its threshold (conjunction before sunset and Moon sets after Sun at Mecca) is often met when the crescent cannot yet be seen, and sometimes when it cannot be seen from anywhere on Earth. In a scan of the 304 months from 1423 AH to September 2026, only 41 (about 13 per cent) had a crescent that was naked-eye visible from Mecca itself on the start evening. Countries that follow visibility-based systems, whether through actual sighting or through more conservative calculation criteria like MABIMS, will not begin the month until the crescent reaches a detectable threshold.

Can the Umm al-Qura calendar be used to predict Ramadan years in advance?

Yes. Since it is a purely calculated calendar, dates can be computed for future years with precision. KACST publishes the calendar in advance for this reason. However, the actual religious start of Ramadan in Saudi Arabia depends on the Supreme Court's sighting declaration, which is made on the evening of the 29th of Shaban and can occasionally differ from the calendar's pre-computed date by one day.

Does the Umm al-Qura calendar apply outside Saudi Arabia?

Not officially. It is Saudi Arabia's national civil calendar. Other countries have their own national Hijri calendars and their own religious declaration processes. Many Muslims worldwide consult the Umm al-Qura calendar as a reference point, but it carries no formal authority outside the kingdom.

References

van Gent, R. H. The Umm al-Qura Calendar of Saudi Arabia. Utrecht University. https://webspace.science.uu.nl/~gent0113/islam/ummalqura_rules.htm
Middle East Eye. Will the UAE break with Saudi Arabia on the start of Ramadan? https://www.middleeasteye.net/news/will-uae-break-saudi-arabia-ramadan-start-date-moon-sighting
Middle East Eye. Saudi announces Ramadan Wednesday, most others start day later. https://www.middleeasteye.net/news/saudi-announces-ramadan-wednesday-most-others-start-day-later
Al Jazeera. When is Ramadan 2026 and how is the moon sighted. https://www.aljazeera.com/news/2026/2/16/when-is-ramadan-2026-and-how-is-the-moon-sighted
Yallop, B. D. (1997). A Method for Predicting the First Sighting of the New Crescent Moon. NAO Technical Note No. 69.
Odeh, M. S. (2004). New Criterion for Lunar Crescent Visibility. Experimental Astronomy, 18, 39 to 64.

Clear skies and happy sighting.

The Umm al-Qura Calendar Explained: Saudi Arabia's Official Hijri Rule