How to Read a Crescent Moon Forecast: Age, Lag and Altitude

A crescent moon forecast gives, for one evening and one place, the numbers that decide whether the new crescent (the hilal) can be seen after sunset. The main ones are moon age (hours since the new moon), lag time (minutes between sunset and moonset), altitude (how high the Moon is at sunset) and elongation (how far it is from the Sun). No single number settles the question. Forecasters combine them into a visibility category, which is what the coloured zones on a world map show.

Key Takeaways

• Moon age is the time since the new moon. It is the most quoted number and the weakest guide on its own.
• Lag time and altitude tell you whether the Moon is above the horizon long enough, and high enough, to be picked out of the twilight.
• Elongation (arc of light) of about 10 degrees is near the lower limit of early reliable sightings, according to the US Naval Observatory.
• HMNAO and ICOP maps turn these numbers into zones: easily visible, visible in perfect conditions, optical aid needed, or not visible.
• A forecast is a likelihood, not a sighting and not a ruling. Confirm the start of a month with the authority you follow.

Why crescent forecasts exist

A forecast exists because the month, in the hadith, starts with a sighting. In Sahih al-Bukhari 1909, the Prophet ﷺ said to start fasting on seeing the crescent, stop on seeing it, and complete thirty days of Sha'ban if the sky is overcast. Muslims differ on how this is applied today, through local sighting, sighting elsewhere, or calculation. Those approaches are compared in moon sighting vs calculation.

Whatever method a community follows, the astronomy is the same. The new moon itself can be computed exactly. Whether the thin crescent will be seen afterwards cannot. The US Naval Observatory (USNO) crescent visibility page says it "cannot be predicted with certainty," because it depends on sky conditions and the observer's location, experience and preparation. A forecast narrows that uncertainty down to a likely answer for each place.

The numbers in a forecast, one by one

B.D. Yallop's NAO Technical Note No. 69, the method behind HM Nautical Almanac Office (HMNAO) maps, notes that the earliest methods relied on the Moon's age and the time from sunset to moonset. Twentieth-century methods moved to angles. Most forecasts print both kinds.

Moon age

Moon age is the time since the new moon, the instant the Sun and Moon line up. It is easy to compute and easy to quote. USNO gives the youngest reliable sightings as about 15.5 hours after the new moon by naked eye and 12.1 hours with a telescope, and says those are exceptional.

Age is a poor guide on its own because the same age gives a very different sky in different places and seasons. The example tables below show a 30-hour-old Moon that needs optical aid while a 27-hour-old one is easily visible.

Lag time

Lag time is the gap between sunset and moonset. If the Moon sets before the Sun, the lag is negative and there is no crescent in the evening sky at all. A short positive lag (10 or 20 minutes) usually means the Moon sinks into the haze near the horizon before the sky is dark enough.

Lag has a catch. At high latitudes the Moon sets along a shallow slant, so it can stay above the horizon longer without being any higher. In the February 2027 figures below, London's lag is about 66 minutes and Makkah's 45, yet the Moon is lower in London at sunset.

Altitude and arc of vision

Altitude is how high the Moon stands above the horizon at sunset. The arc of vision (ARCV) is the difference in altitude between the centres of the Sun and Moon. Yallop's note defines it as a geocentric value, measured as if from the centre of the Earth and ignoring refraction. Seen from the ground, the Moon near the horizon sits roughly a degree lower than its geocentric position, because of parallax. When two forecasts disagree by about a degree, check whether one is geocentric and the other topocentric (from the observer's location).

Elongation and relative azimuth

Elongation, which Yallop calls the arc of light (ARCL), is the angle between the centres of the Sun and Moon. USNO's page says early reliable sightings occur near an elongation of about 10 degrees.

The relative azimuth (DAZ) is the sideways part of that gap. Yallop shows that ARCL, ARCV and DAZ are linked, so only two of them are independent. A forecast that lists elongation and altitude has effectively told you the third.

Crescent width and best time

Crescent width (W) is the thickness of the lit sliver, in arc minutes. It grows with elongation. Yallop's test compares ARCV with a curve that depends on W, so a thicker crescent can be seen lower in the sky. ICOP uses a different test, the Odeh criterion. How those tests work is covered in crescent visibility criteria.

Forecasts usually evaluate the sky at a best time, not at sunset. Too early, the sky is too bright; too late, the Moon is low in the haze. Yallop's rule, summarised on R.H. van Gent's page on Yallop-based visibility maps, is sunset plus four-ninths of the lag. With a 45-minute lag, that is 20 minutes after sunset.

What a visibility map shows

A visibility map applies a criterion to every point on Earth for one evening and shades the result. HMNAO and the International Astronomical Center's Islamic Crescents' Observation Project (ICOP) both publish maps for each month, and their colours mean different things.

HMNAO maps (Yallop zones A to F)

HMNAO's maps use Yallop's six visibility codes. The note defines them as:

The Danjon limit is an elongation of about 7 degrees, as given on van Gent's summary of the Yallop zones. A Moon closer to the Sun than that falls in zone F and is classed as not visible.

The maps also shade where the Moon sets before the Sun, hatch areas where the new moon has not yet happened, and draw contour lines of the Moon's age in hours at the best time. A small marker shows the predicted place of first naked-eye or telescope visibility.

ICOP maps (Odeh criterion)

ICOP's monthly pages say the maps are made with the Accurate Times program "according to Odeh criterion." On the Ramadan 1448 page:

• Red: the crescent cannot be seen, because the Moon sets before the Sun or the topocentric new moon is after sunset.
• Blue: optical aid only.
• Magenta: optical aid, or naked eye for an experienced observer in superb conditions.
• Green: easily visible by naked eye.
• Uncoloured: the Moon is up after sunset, but too faint to see even with optical aid.

ICOP notes that its maps cover latitudes from 60 degrees north to 60 degrees south.

How to read either map

Find your location, then check three things: the colour, the date printed on the map, and the method. The visible zone generally lies to the west, because places further west see sunset later, when the Moon is older. A green or "A" zone that reaches Africa but not South-East Asia means the same evening is easy in one region and impossible in another. That is one reason communities following local sighting can start a month on different days, a point explored in global vs local moon sighting.

Worked example: a real HMNAO forecast (February 2026)

HMNAO's Crescent Moon Visibility for February 2026 covered the Ramadan 1447 crescent. Its summary PDF puts the new moon at 12:01 UT on 17 February 2026 and says a naked-eye sighting that evening was unlikely anywhere in the world.

For Wednesday 18 February, its table lists sunset (SS), moonset (MS) and a category for each city. The age, altitude and elongation columns below are not in the HMNAO table. They were computed for the same sunsets from USNO's Complete Sun and Moon Data for One Day and Celestial Navigation Data services. Altitudes are approximate topocentric values (the computed altitude minus the Moon's parallax, without refraction). Elongations are geocentric.

Cape Town shows why age misleads. Its Moon was older than Makkah's, with a wider elongation, yet HMNAO rated it the hardest of the four. The Moon sat low and well to one side of the sunset point (a relative azimuth of about 13 degrees), so most of that elongation was sideways rather than height. Jakarta's younger, lower Moon landed in between.

The calculated Umm al-Qura calendar, which this site uses, began Ramadan 1447 on 18 February 2026, starting at sunset on the 17th. Its rule, as summarised in van Gent's Umm al-Qura rules page, asks two things on the 29th at Makkah: whether the geocentric new moon happens before sunset, and whether the Moon sets after the Sun. It does not ask whether the crescent can be seen, so a calendar date and a visibility forecast can point to different evenings. New moon vs first crescent explains that rule.

Applying it: the Ramadan 1448 crescent (February 2027)

ICOP gives the new moon for Ramadan 1448 as 15:56 UT on Saturday 6 February 2027. The figures below are computed at local sunset with the same two USNO services and the same conventions: topocentric altitudes, geocentric elongations. They are not a visibility forecast; HMNAO and ICOP publish theirs closer to the date.

Saturday 6 February. In Makkah the Moon sets at 18:07, six minutes before the Sun, and the new moon itself comes after sunset, so neither Umm al-Qura condition is met. In New York the Moon is only about 6.4 hours old at sunset with a 10-minute lag; in Los Angeles about 9.5 hours with an 18-minute lag. Both ages are below USNO's 12.1-hour telescope record, so these are evenings to expect nothing.

Sunday 7 February:

Reading the table with the rules above:

1. The Americas have the oldest, highest Moon, with lags over an hour and elongations well above 10 degrees.
2. Makkah has a Moon about 23 hours old and about 9 degrees up at sunset, with an elongation just above 10 degrees. Its best time is around 18:34. The numbers are close to Jakarta's on 18 February 2026, which HMNAO rated "visible in perfect conditions," but that is a comparison, not a forecast.
3. Kuala Lumpur sits just under 10 degrees of elongation, with a Moon under 20 hours old and a 32-minute lag.
4. Cape Town again has an older Moon than Makkah but the lowest altitude, the same southern-hemisphere pattern as in 2026.

For the record, the calculated Umm al-Qura calendar begins Ramadan 1448 on Monday 8 February 2027, which starts at sunset on the 7th. The Ramadan 1448 calendar shows those calculated dates; local announcements may differ by a day.

Common questions

What lag time is needed to see the crescent?

There is no single cut-off. Lag was one of the oldest tests, and Yallop's note describes how later methods moved to angles instead. The same lag can mean a different altitude at different latitudes, as the London and Makkah figures show. As a rough rule of thumb from the tables above, a lag near or under 30 minutes (Cape Town and Kuala Lumpur in February 2027) is a warning sign, so check the map before planning to look.

Why do HMNAO and ICOP maps sometimes disagree?

They use different criteria (Yallop and Odeh) with different category boundaries. They can also differ in whether altitudes are geocentric or topocentric. Small differences at the edge of a zone are normal.

Does a green zone mean the month has started?

No. A forecast predicts what an observer could see. Whether a sighting is accepted, and for which region, is decided by the authority a community follows.

What to do next

Before a month starts, look up the new moon time on the moon phases page, then open the HMNAO or ICOP map for the evenings that follow and find your location. Note the lag, altitude and elongation for your city, and look west a little before halfway between sunset and moonset, the best time described above. For Ramadan, Eid and other key dates, confirm the start of the month with your local mosque or the authority you follow.