Crescent Moon Observation Log: How to Become a Master Hilal Observer

The Short Answer

Every valid report earns 10 points, whether you saw the crescent or not. A sighting from a location in Yallop Zone A or B adds 5, a report on the 29th day of the Hijri month adds 15 and an active community challenge adds 5, so one report earns between 10 and 35 points. Your total carries you through five ranks, from Newcomer at 0 points to Master Observer at 500.

Spotting the new crescent, the hilal, is one of the oldest skills in observational astronomy. It is also one of the hardest. A razor-thin arc of light, sometimes less than a third of an arcminute wide, has to be picked out of a bright twilight sky low over the western horizon, often within a window of just a few minutes before it sets. Doing this well takes preparation, patience and a genuine feel for the geometry of the sky.

This article is the reference for how Hilal Vision turns that skill into a measurable, rewarding progression. If you want the story of the live global sighting event itself, the "election night" map where reports roll in across the terminator, read Tracking the Crescent Wave. Here we go under the bonnet of the scoring: how the physics places each report in a Yallop zone, exactly how points are awarded, a worked example, the one automatic check on submissions, and the ranks and badges. By the end you should understand precisely how a logged sighting turns into points and how those points carry you towards Master Observer.

Your Crescent Log: a life-list of every report

Birdwatchers keep a "life-list", a permanent record of every species they have ever seen. Your Crescent Log is the astronomical equivalent. Every observation you submit, positive or negative, becomes part of a permanent, timestamped, location-stamped history of your relationship with the night sky.

The crucial design decision is that every report counts, positive or negative. A report earns points for the effort of going outside and looking, and a seen report from an easy evening (Yallop Zone A or B) earns a small bonus. The score is deliberately simple and does not try to rank how hard a sighting was. Spotting a fat, brilliant two-day-old crescent from a clear tropical horizon is a lovely experience; catching a sub-twenty-hour crescent through binoculars from a hazy northern European city is a feat of skill. The difficulty is preserved in the record instead: the zone, q-value and Odeh V-value computed for your point are stored with every report, so the scientific dataset shows which sightings were hard.

What actually makes a sighting difficult

Before we get to points, you need to understand what the engine measures. The single most important thing to unlearn is the "age myth". The Moon's age (the number of hours since conjunction, the invisible instant the new moon passes between Earth and Sun) is a poor predictor of visibility and appears in neither of the two scientific criteria we use. Two crescents of identical age can have wildly different visibility depending on geometry. What actually decides difficulty are these parameters:

• ARCV (arc of vision): the altitude difference between the Moon and the Sun at sunset. The higher the Moon sits above the sinking Sun, the longer it lingers in a darkening sky and the easier it is to see.
• ARCL (arc of light): the Moon-Sun elongation. This drives the crescent's intrinsic width and brightness. Below roughly 7 degrees of elongation, the crescent effectively cannot exist as a visible object at all.
• W (topocentric crescent width): the width of the lit arc in arcminutes, as seen from your spot on Earth. A thicker crescent is brighter and more forgiving.
• DAZ (difference in azimuth): the horizontal separation in azimuth between Sun and Moon. This affects the tilt and apparent length of the crescent.
• Lag time: the interval between sunset and moonset. A short lag means the crescent sets almost as soon as the sky begins to darken, giving you only seconds to a couple of minutes of usable window.
• Atmospheric extinction: low on the horizon, light passes through many air masses, dimming and reddening the crescent. Refraction, humidity and dust all eat into an already faint signal.
• The Danjon limit: the minimum elongation below which a crescent simply cannot be perceived, approximately 7 degrees of arc of light. André Danjon first reported the effect in 1932 and quantified it in 1936; later work by Fatoohi, Stephenson and Al-Dargazelli (1998) put it nearer 7.5 degrees, while the optical-aid limit sits around 6.4 degrees. For the deeper physics of why the cusps vanish first, see The Danjon Limit, and for the role of weather and the atmosphere see Atmospheric Refraction, Weather and Moon Visibility.

When you submit an observation, the engine computes all of these for your exact location and sunset time. It does not ask you to. The full mathematical treatment of how these combine into a prediction lives in The Science Behind the Crescent; here we use the output of that machinery to place each report in a zone.

From physics to a single figure: the Yallop zones

Rather than judge each report against six separate parameters, the engine collapses the geometry into a single number using the Yallop criterion (1997). Bernard Yallop, of HM Nautical Almanac Office, defined a visibility value, the q-value, evaluated at the "best time", about 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 q-value sorts every location into one of six mutually exclusive zones. These are non-overlapping bands, not a cumulative "greater-than" ladder; a location in Zone C is firmly in Zone C, not also in B and A.

A note on Zone F that trips up many newcomers: it means the crescent is below the Danjon limit and cannot be seen by any means. It does not mean the Moon is below the horizon. A claimed sighting from a place where the Moon is below the horizon at your time is, by definition, physically impossible, and the app rejects such a report (see the section on the automatic check below).

Hilal Vision also computes the independent Odeh V-value (Odeh 2004), derived from 737 observation records, roughly half drawn from ICOP. Both values are stored with each report. Where Yallop and Odeh disagree at the margins of a band, those reports are exactly the data points that help refine the models.

How points are calculated

Scoring has four components, each a flat amount that is added together.

1. Base points. Every valid, submitted report earns 10 points, whether the result is positive ("I saw it") or negative ("I looked and could not see it"). Negatives matter: a confirmed non-sighting from a clear site is genuinely useful scientific data, and the log credits you for the effort and honesty.

2. The Zone A or B bonus. A report in which you saw the crescent (with the naked eye or optical aid) earns 5 more points when the computed Yallop zone for your location and time is A or B. Reports of "not seen" never earn this bonus, and Zones C, D and E earn none.

3. The 29th night bonus. A report made on the 29th day of the Hijri month earns 15 more points, because that is the evening when the crescent search matters for the calendar. The day is taken from the Umm al-Qura Hijri calendar.

4. The community challenge bonus. If there is an active community challenge for the current month, your report counts towards it and earns 5 more points.

A single report can therefore earn between 10 and 35 points. Your total points set your rank, while the monthly leaderboard resets each month.

A worked example: London Zone D versus Bogota Zone A

Numbers make this concrete. Imagine the same lunar evening, which is also the 29th day of the Hijri month, seen from two cities.

Bogota, Colombia. Late sunset, the Moon riding high above the western horizon, a generous lag and a thick crescent. The engine computes a q-value of about +0.28. That is comfortably above +0.216, so Bogota sits in Zone A. An observer there glances west and sees the crescent immediately with the naked eye.

• Base points: 10
• Seen in Zone A: +5
• 29th night: +15
• Score: 10 + 5 + 15 = 30 points

London, United Kingdom. Hours earlier on the same date, far to the north, the same crescent is younger in geometric terms: lower ARCV, a shorter lag, a thinner W and more atmosphere to punch through near the horizon. The engine computes a q-value of about -0.19. That falls in the band -0.232 < q ≤ -0.160, which is Zone D: visible only with optical aid. Our London observer plans carefully, sets up binoculars on a clear western horizon, sweeps the right patch of sky at the best time, and catches the crescent.

• Base points: 10
• Seen in Zone D: no zone bonus
• 29th night: +15
• Score: 10 + 15 = 25 points

The comparison shows what the score does and does not measure: effort and regularity, not difficulty. The London sighting was far harder, yet it earns 5 points fewer than the easy glance in Bogota; a "not seen" report from either city would also have earned 25 points (10 plus the 29th night bonus). The difficulty is not lost, though. The zone and q-value are stored with each report, so anyone studying the data can see which sightings were hard. The app does not second-guess a naked-eye claim from a Zone D location; only a claim made where the Moon is below the horizon is rejected.

The reason a crescent can be a triumph in London and a casual glance in Bogota on the very same night is the same reason explored in Why a Crescent Is Visible in Some Countries and Not Others, and the local terrain and altitude factors that nudge a marginal site over the line are covered in Why Altitude and Terrain Matter.

What happens when you submit

The automatic check is deliberately light. When you submit a report, the app computes the crescent geometry for your coordinates and time. If you claim a sighting (naked eye or optical aid) but that geometry shows the Moon below the horizon at your location and time, the report is rejected, because no crescent can be seen from there. Reports of "not seen" are never rejected on this basis. Everything else is credited straight away; there is no manual review queue and no corroboration weighting. The sequence is:

1. Submission. You log the result (seen with the naked eye, seen with optical aid, or not seen), the time, and optionally a photograph or short note. Your coordinates are displaced by about a kilometre before storage, for privacy.
2. Geometry. The app computes the Yallop zone, q-value and Odeh V-value for your point and stores them with the report, together with temperature, pressure and cloud cover from live weather data where available.
3. Points. The base award and any bonuses described above are added to your totals, and your rank is recalculated.
4. Ledgering. The report is added to your Crescent Log and to the community dataset that helps calibrate the models for everyone.

You can always inspect the geometry behind any verdict yourself on the global visibility map and the moon dashboard, and cross-check your local prediction against the Hijri calendar. Comparing your past logs against the historical ICOP archive is one of the best ways to develop intuition for which marginal nights are worth the effort.

Ranks, badges and the road to Master Observer

Your total points carry you up five ranks: Newcomer (0 points), Observer (50), Astronomer (150), Scholar (300) and Master Observer (500). Because the points are flat amounts, you reach the top by taking part consistently: every report counts, and the 29th night is worth the most.

Badges mark milestones along the way. There are seven:

• First Light: submitted your first sighting report.
• Eagle Eye: 3 or more naked-eye sightings.
• Night Owl: reports on 3 nights in a row.
• Globetrotter: reported sightings from 3 or more different regions of the world (the app groups the world into regions such as the Arabian Peninsula, South Asia and Europe).
• Ramadan Ready: checked the crescent on 29 Sha'ban.
• Astronomer: explored all 6 Hilal Vision tools.
• Patron: supported Hilal Vision.

The seasonal community challenges and the social side of the monthly sighting event are described in Tracking the Crescent Wave, so we will not repeat them here.

How to climb deliberately

If your goal is genuinely to become a Master Observer rather than just to enjoy the occasional sighting, a few habits help:

• Report on the 29th night. A report on the 29th day of the Hijri month earns 15 bonus points, more than the base award. Use the prediction tools ahead of the 29th to see where your location stands, since a seen report from Zone A or B adds 5 more.
• Master your horizon. A clean, unobstructed western horizon at altitude can be the difference between a missed crescent and a confirmed catch. Terrain and elevation are levers you control.
• Log honestly, including negatives. A truthful non-sighting earns the same 10 base points as a sighting and improves the shared dataset.
• Learn the geometry, not the age. Internalise ARCV, ARCL, W, DAZ and lag. If you find yourself thinking in "hours since new moon", retrain on What Is the Hilal and the beginner's guide to spotting the crescent.

Every confirmed observation you add is more than a personal score. It is a contribution to a global, crowdsourced calibration of the visibility models, in the lineage of the Islamic Crescents' Observation Project, founded in 1998 by Mohammad Odeh under the International Astronomical Center.

Ready to start your log? Open moonsighting.live, compute the prediction for your own coordinates on the next 29th night, go outside and look. Observers on the Pro tier gain cloud-cover overlays and the extended ICOP archive, which together make choosing a promising evening and site far easier.

See Also

• Can You See the Crescent Moon With the Naked Eye? Naked Eye vs Binoculars vs Telescope
• How to Photograph the Crescent Moon: A Complete Astrophotography Guide

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. & Al-Dargazelli, S.S. (1998). "The Danjon limit of first visibility of the lunar crescent." The Observatory.
• The Islamic Crescents' Observation Project (ICOP) and the International Astronomical Center: astronomycenter.net.