Does the Umm al-Qura Calendar Follow the Crescent? 300 Months Checked Against the Sky

The Short Answer

Not in the sense of waiting for it. The Umm al-Qura calendar is computed years ahead from a rule, and the rule asks only that the Moon has passed conjunction before sunset and sets after the Sun at Mecca. It never claims that a crescent could be seen.

We took the 300 months that began between March 2002 and May 2026 (1423 to 1447 AH, every month since the current rule came into force) and computed the crescent at Mecca on the evening before each one began:

• In 215 months (72 per cent) the crescent was in Yallop's Zone E: below the visibility threshold for any method, even a telescope.
• In 41 months (14 per cent) it was in Zone A or B, visible to the naked eye under good conditions. Another 27 were Zone C and 12 Zone D, where binoculars or a telescope are needed.
• In 5 months the Moon was a fraction of a degree below the horizon at sunset (Zone F), borderline cases of the rule itself.
• At 162 of the 300 evenings (54 per cent) the Moon was younger than the youngest crescent ever confirmed with the naked eye, 15 hours 32 minutes.
• 20 of the 25 Ramadans began after a Zone E evening.

This is a statement about the sky, not a verdict on anyone's observance. What the numbers do and do not mean is set out below.

What the Rule Says

Since 1423 AH (2002) the Umm al-Qura calendar starts a new month if, on the 29th evening, conjunction has happened before sunset in Mecca and the Moon sets after the Sun there. There is no visibility threshold: the Umm al-Qura calendar explained covers the rule, its earlier versions and the difference between this civil calendar and the religious declarations that the Saudi Supreme Court makes after witnesses are heard.

A rule like that is easy to satisfy even when the Moon is far too young to see. The question this post asks is how often that happens.

What We Checked

For each Hijri month the Umm al-Qura table (the one the calendar page uses) gives the date of day 1. We took the evening before it, which is the evening on which the new month would be sought, and computed at sunset in Mecca:

• the Moon's age, the hours since conjunction;
• its altitude above the horizon;
• the Yallop q-value and zone, and the Odeh V-value and zone, from the arc of vision and the width of the crescent (see the science behind the crescent).

Yallop's zones run from A (easily visible) to E (below the threshold for any method) and F (Moon below the horizon).

The Result, Drawn

The upper bar sorts the 300 evenings by zone. The lower chart sorts them by the Moon's age, in two-hour steps, and marks the naked-eye record.

The age chart peaks between 10 and 16 hours. Half of all the evenings had a Moon younger than 14.6 hours, nine in ten younger than 24.7 hours, and the oldest was 34.0 hours. The youngest was 0.9 hours, on 14 September 2004, the evening before 1 Sha'ban 1425.

Ramadan and the Two Eids

Some months matter more than others. Here is the same count for the three months that begin a major observance, 25 evenings each:

So in 20 of the 25 years the evening before 1 Ramadan had a crescent that no method could be expected to show. The median age of the Moon on those 25 Ramadan evenings was 13.1 hours.

Does a Different Criterion Change the Picture?

Yes, the percentages move, and the conclusion does not. Odeh's criterion reads the same evenings more favourably:

Under Odeh 89 evenings (30 per cent) were A or B, and 121 (40 per cent) were E. Yallop's stricter edge puts 72 per cent in E. Either way a large share of the months began after an evening when the crescent could not be seen.

What This Does and Does Not Mean

It does not say that any month was wrong. The Umm al-Qura calendar is a civil calendar, fixed by a published rule so that a country can plan its year. It is not a sighting calendar and does not claim to be one. The dates of Ramadan and the Eids for religious observance are declared separately, after the Supreme Court hears witnesses, and the two roughly agree in most years, as that post explains.

It does say why dates differ between countries. On a Zone E evening nobody could be expected to see the crescent, so a country that begins its month only on a sighting would normally have completed 30 days and begun a day later than the table, all else being equal. This is one reason the start of Ramadan can differ by a day between countries that follow a calculated calendar and those that follow local sighting.

It is Mecca only. The zones describe the crescent at Mecca. A crescent can be visible further west on the same evening, and some traditions accept that. The post on local and global sighting covers it.

It has limits. The altitude is measured without refraction, and the five Zone F months are within about 0.2 degrees of the horizon, so the rule and our horizon disagree by a few minutes there. The zones are statistical thresholds from two published criteria, not predictions for a given sky: weather, observer skill and the height of the observing site all matter, as the post on the youngest crescents ever seen shows.

Frequently Asked Questions

Why only 300 months?

The current rule has been in force since 1423 AH, and 1447 AH ended in June 2026. Months that have not begun yet are left out, so every evening counted has already happened.

Could the crescent be seen on a Zone E evening?

Not by any method in the two criteria, including a telescope. On a Zone C or D evening binoculars or a telescope may find it, and on a Zone A or B evening it can be seen with the naked eye in good conditions.

How is the Moon's age measured?

In hours from the nearest new moon (conjunction) to sunset at Mecca. It is the same age that the youngest-crescent records use.

Where can I check a given month?

On the calendar page you can switch between calendar systems and see the crescent forecast for any evening on the visibility map.

See Also

• The Umm al-Qura calendar explained
• The science behind the crescent: Yallop and Odeh
• The youngest crescent moons ever seen
• Global versus local moon sighting
• Why 30 years? The arithmetic of the tabular calendar

References

• 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.
• van Gent, R. H. The Umm al-Qura Calendar of Saudi Arabia. Utrecht University. https://webspace.science.uu.nl/~gent0113/islam/ummalqura_rules.htm
• The counts and the chart are reproduced by docs/research/umm-al-qura-evenings.mjs and packages/astronomy/src/ummAlQuraEvenings.ts in the Hilal Vision source.