The Crescent Wave: Watch Global Moon Sightings Roll In Live

For 28 days of the lunar month, the sky holds few surprises. But on the 29th night, millions of Muslims worldwide look to the western horizon immediately after sunset. This synchronised, global effort to spot the hilal, the first visible crescent, is one of the oldest living traditions in human history.

At Hilal Vision, we realised that while the act of sighting the crescent is local, the phenomenon itself is a planet-wide wave. To capture this, we built the Crescent Wave: a live, interactive map that turns the 29th night into a thrilling, globally shared event. To understand why the map looks the way it does, you first have to understand the geometry that creates the wave.

What the "New Moon" Actually Is

Before going further, it is worth being precise with terms, because most confusion about crescent sighting comes from sloppy language. The conjunction, also called the new moon, is the instant the Moon passes between the Earth and the Sun. At that moment the Moon's illuminated face points entirely away from us, and it is invisible from everywhere on Earth. It is not a thing you can see.

The hilal, by contrast, is the new crescent: the first thin sliver of reflected sunlight that becomes perceptible some 15 to 40 or more hours after conjunction. The hilal is what observers actually hunt for. Throughout this article, "new moon" and "conjunction" mean the invisible instant, and "hilal" or "crescent" means the visible sliver. If you want the full grounding, our explainer on what the hilal is and how it differs from the new moon covers the distinction in depth.

The Geometry of the Wave

Here is the mechanism that produces the wave, stated correctly. The Sun sets at a given longitude and then the sunset terminator, the moving line dividing day from night, sweeps across the globe from east to west. Tokyo and Jakarta reach sunset hours before Dubai, which reaches it hours before London, which reaches it hours before Los Angeles.

While that terminator travels westward, the Moon is quietly doing its own work. It orbits the Earth eastward and gains roughly 12 to 13 degrees of elongation per day, where elongation is the angular separation between the Moon and the Sun in the sky. Crucially, the gap in local sunset times between the far east and the far west of the inhabited world is large. Sunset in Jakarta and sunset in Los Angeles on the same calendar date can be separated by something close to 15 hours of real time.

In those 15 hours the Moon continues moving away from the Sun. At a rate of about 12 to 13 degrees per day, 15 hours of extra separation buys the western observer roughly 7 to 8 additional degrees of elongation compared with the eastern observer who looked earlier the same evening. That is an enormous difference when the entire crescent-visibility problem turns on the first 7 to 12 degrees of elongation.

So the wave is not driven by the Moon "ageing" in any mystical sense. It is driven by the simple fact that later sunsets, further west, observe a Moon that has had more time to pull away from the Sun. The western crescent is higher above the horizon, thicker, brighter, and accompanied by a longer lag time between sunset and moonset. A crescent that is hopeless in Tokyo can be a telescope target in Dubai and a comfortable naked-eye object in New York, all on the very same evening.

This is also why a crescent can be visible in some countries and not others on the same night, a subject we unpack in why the crescent is visible in some countries but not others.

The Vocabulary of Visibility

The Crescent Wave map and the science behind it rest on a small set of geometric quantities. Defining them precisely makes everything else clearer.

• ARCV (arc of vision): the difference in altitude between the Moon and the Sun at sunset. It measures how high the crescent sits above the dying glow once the Sun has gone.
• ARCL (arc of light): the Moon-to-Sun elongation. This is the angle that drives the crescent's width and underpins the Danjon limit. It is the single most important number in the whole problem.
• DAZ (difference in azimuth): the horizontal angle between the Sun and the Moon along the horizon. When DAZ is large, the Moon sets well to the side of the sunset point rather than directly above it.
• Lag time: the interval between sunset and moonset. A longer lag means the crescent lingers in a darkening sky, giving the observer more time and a darker background.
• Crescent width (W): the topocentric width of the lit crescent in arcminutes. It grows as elongation grows.
• Topocentric parallax: a correction for where you stand on the planet. Because the Moon is relatively close, its position seen from the Earth's surface differs from its position seen from the Earth's centre. Near the horizon this lowers the Moon's apparent altitude by up to about 0.95 degree compared with the geocentric value, which can be the difference between a crescent being above or below the visibility threshold.

A vital warning: moon age, the number of hours since conjunction, is a poor predictor of visibility, and it is not a parameter in either of the serious scientific criteria. Two crescents of identical age can have very different elongations depending on where the Moon sits in its slightly elliptical orbit. What actually decides visibility is the combination of ARCV, ARCL, W and DAZ. The "age myth" persists in popular writing, but the models that power the Crescent Wave ignore age entirely.

The Yallop q-value Zones

The references to "Zone A" and "Zone C" later in this article, and the colours you see on the map, come from the Yallop criterion. Bernard Yallop of HM Nautical Almanac Office reduced the whole visibility question to a single number, the q-value, computed at his "best time" (roughly four ninths of the lag time after sunset):

q = (ARCV - (11.8371 - 6.3226·W + 0.7319·W² - 0.1018·W³)) / 10

where W is the topocentric crescent width in arcminutes. The resulting q-value falls into one of six mutually exclusive bands. These are non-overlapping ranges, not a cumulative ladder:

Note that Zone F means the crescent is below the Danjon limit and cannot be seen by any means. It does not mean the Moon has set.

The complementary Odeh criterion (Odeh, 2004), derived from 737 observation records, expresses the same physics through a V-value with four regions:

Odeh's empirical optical-aid limit sits at roughly 6.4 degrees of elongation. The app computes both Yallop and Odeh side by side for each location; you can explore the comparison on the science behind the crescent page or directly in the methodology section.

The Danjon Limit: The Hard Edge of the Wave

The Crescent Wave has a leading edge, a sharp boundary west of which sightings become possible and east of which they are impossible. That edge is anchored by the Danjon limit, the minimum elongation below which a crescent simply cannot be perceived. André Danjon first reported the effect in 1932 and quantified it in 1936, placing it at roughly 7 degrees, with later work by Fatoohi, Stephenson and Al-Dargazelli (1998) deriving a naked-eye figure near 7.5 degrees and an optical-aid limit near 6.4 degrees.

Why the crescent vanishes below this elongation is still debated and should be treated as a live hypothesis rather than settled fact. Danjon attributed it to lunar topography: at grazing sunlight the cusps of the crescent are foreshortened and broken up by mountains and crater rims. Schaefer instead emphasised a photometric brightness fall-off toward the cusps, while others point to atmospheric seeing and the contrast threshold of the eye. Our deep dive on the physics of the Danjon limit weighs these competing explanations. Whatever its cause, the limit gives the wave a definite leading edge: until a longitude's elongation has climbed past roughly 7 degrees, no green pin can legitimately appear there.

A Worked West-versus-East Example

Picture a contested 29th night. Conjunction occurs in the morning, Universal Time. By the time the Sun sets over Jakarta that evening, the elongation has reached only about 6 degrees. That is below the Danjon limit, so the crescent there falls into Zone F: invisible to the naked eye and to telescopes alike. Honest observers in Jakarta report a negative sighting, and that is exactly what the science predicts.

Now follow the terminator westward. Roughly 15 hours later the Sun sets over Los Angeles. In that interval the Moon has gained close to 7 to 8 further degrees of elongation, putting the arc of light near 13 to 14 degrees. The crescent is now well clear of the Danjon limit, the width W has grown, the ARCV is comfortably positive, and the q-value lands in Zone B or even Zone A. A clear-eyed observer in Los Angeles sees the hilal without difficulty.

Same date, same Moon, opposite outcomes, with the disagreement explained entirely by geometry rather than by piety or eyesight. This is precisely why two countries can declare the start of a month on different days, and why a global view matters. You can explore any night yourself on the global visibility map, inspect the underlying geometry on the moon dashboard, and check how a given date threads through the Hijri calendar.

The "Election Night" Experience

Before Hilal Vision, finding out whether the crescent had been sighted meant waiting for official announcements or scrolling through scattered social media posts. The Crescent Wave changes that. On the 29th night, the global map goes live. As users submit sighting reports through the app, results plot onto the map in real time:

• Green pins indicate a positive sighting.
• Red pins indicate the crescent was not seen.

Watching it is like watching election results roll in. You can see the visibility boundary forming as the sunset terminator sweeps westward across the continents, and you can test whether the empirical data, what people actually saw, matches the theoretical predictions of the Yallop and Odeh models. When reports cluster on the "wrong" side of a predicted boundary, that is a genuinely interesting data point worth investigating, often a matter of atmospheric refraction and local weather or favourable terrain and altitude.

Your Sightings Count

Every report you submit is a data point that helps calibrate the visibility models. Hilal Vision awards difficulty-weighted points based on your Yallop zone, so a hard Zone D catch earns far more than an easy Zone A glance; the full scoring system, ranks, and badges are described in becoming a master observer. If you are just starting out, the beginner's guide to spotting the crescent will get you to your first confirmed sighting.

A Contribution to Science

When you tap "I saw it" on Hilal Vision, you are not just logging points. You are adding to a growing dataset of crowdsourced astronomy, the same kind of observational record that ICOP, the Islamic Crescents' Observation Project founded by Mohammad Odeh in 1998, has been compiling for decades and that you can browse in the historical archive. Every report helps calibrate the visibility models, and negative reports matter just as much as positive ones for finding where the wave's true edge lies.

This 29th night, do not just wait for the news. Compute the prediction for your own location on moonsighting.live, step outside, look west, and join the wave. Observers who want cloud overlays and the extended ICOP archive can unlock them on the Pro tier.

References and Further Reading

• Yallop, B.D. (1997). A Method for Predicting the First Sighting of the New Crescent Moon. HM Nautical Almanac Office, NAO Technical Note No. 69.
• Odeh, M.Sh. (2004). "New Criterion for Lunar Crescent Visibility." Experimental Astronomy.
• Danjon, A. (1932, 1936). L'Astronomie. (The Danjon limit.)
• Fatoohi, L.J., Stephenson, F.R. and Al-Dargazelli, S.S. (1998). "The Danjon limit of first visibility of the lunar crescent." The Observatory.
• The Islamic Crescents' Observation Project (ICOP), International Astronomical Center: https://www.astronomycenter.net

Clear skies and happy sighting.