In the Name of Allah---the Most Beneficent, the Most Merciful.
Shafaq Ahmar: A Modern Understanding
Introduction
The disappearance of Shafaq Ahmar (the red twilight) marks the beginning of the time for the Isha prayer according to the overwhelming majority of Islamic jurists. For centuries, Muslims have relied upon direct observation of the western horizon to determine the disappearance of this red glow. However, atmospheric conditions, geographical location, seasonal changes, and modern light pollution often make such observations difficult or unreliable.
Modern atmospheric science now provides valuable insights into the physical nature of twilight. Rather than replacing the juristic definition, these scientific findings help us understand what Shafaq Ahmar actually is and how it behaves in Earth's atmosphere.
One of the most significant recent contributions is the 2024 study published in Scientific Reports (Nature Portfolio), which investigated the evolution of twilight brightness using Sky Quality Meter observations at urban, suburban, and pristine locations. Although the paper was not written to determine Islamic prayer times, its findings offer an excellent scientific framework for understanding the disappearance of the red twilight.
An important strength of the 2024 Nature study is that the overwhelming majority of its observation sites were located within approximately 1°–7° latitude in Malaysia, placing them very close to the Equator. Twilight geometry in this region remains remarkably consistent throughout the year because the Sun descends steeply below the horizon in all seasons. Consequently, the study provides an excellent representation of natural equatorial twilight. This is particularly relevant to the present study, which also uses equatorial observations (e.g., Jakarta) to investigate the disappearance of Shafaq Ahmar. The agreement between independent atmospheric measurements and equatorial observations enhances confidence that a solar depression near 14° represents a conservative threshold beyond which the visible red twilight has disappeared, leaving only faint achromatic residual twilight.
This article proposes a modern interpretation of Shafaq Ahmar based upon atmospheric optics while remaining faithful to the classical juristic definition.
The Classical Definition of Shafaq Ahmar
Classical scholars consistently defined Shafaq Ahmar as the visible red glow remaining on the western horizon after sunset.
The majority of jurists—including the Malikis, Shafi'is, and Hanbalis—considered the disappearance of this red glow to mark the beginning of Isha.
Imam Ibn Qudamah wrote:
"If he is in a place where the horizon is visible and the disappearance of the twilight can be observed, then when the redness disappears, the time of Isha begins."
He further explained that when the horizon is obstructed by mountains or buildings, one should wait until the white twilight disappears only because it serves as evidence that the red twilight has already vanished, not because the white twilight itself determines the beginning of Isha.
This distinction is extremely important.
The juristic criterion is the disappearance of the visible red glow, not the disappearance of every trace of scattered sunlight.
Understanding Twilight
Twilight is produced by sunlight that continues to scatter through Earth's atmosphere after the Sun has passed below the horizon.
However, twilight is not a single phenomenon.
It consists of several stages.
The Red Twilight
Immediately after sunset, sunlight travels through an extremely long atmospheric path.
Shorter blue wavelengths are scattered away.
The remaining direct and singly scattered light becomes rich in longer red wavelengths.
This produces the brilliant red and orange colours known as Shafaq Ahmar.
The White Twilight
As the Sun sinks farther below the horizon, the geometry changes.
The direct red illumination weakens rapidly.
The remaining twilight becomes dominated by multiple scattering occurring throughout the atmosphere.
Multiple scattering mixes wavelengths together.
Instead of producing vivid red colours, it generates a faint diffuse illumination that appears nearly white or grey to the human eye.
Thus, the physical atmosphere naturally transitions from:
Red twilight
White twilight
Complete darkness
These are three distinct physical stages rather than one continuous phenomenon.
Modern Measurements of Twilight
The 2024 Nature study measured twilight brightness using Sky Quality Meters under different observing conditions.
Instead of relying upon subjective visual observations, the researchers continuously recorded sky brightness as the Sun descended below the horizon.
Their measurements demonstrated several important facts.
Rapid Darkening
During the early stages of twilight, sky brightness decreases rapidly.
The dominant source of illumination—the strong scattered sunlight from sunset—is disappearing.
Gradual Transition
As the Sun approaches approximately 14° below the horizon, the brightness curve begins to flatten.
This means that the rate of darkening slows considerably.
The atmosphere has already lost most of its scattered sunlight.
Only weak residual twilight remains.
Natural Night
The researchers found that at pristine observing sites, the sky reaches its natural night background near 17.5° solar depression.
Importantly, this does not mean that the red twilight remains until 17.5°.
It simply marks the point where even the faint residual scattered light becomes almost indistinguishable from the natural night sky.
The Significance of 14° Solar Depression
The most informative feature of the Nature study is not the final transition near 17.5°.
Rather, it is the shape of the brightness curve.
The observations show that:
Most twilight brightness disappears before 14°.
Around 14°, the curve becomes noticeably flatter.
After 14°, the remaining brightness decreases only gradually.
This behaviour has an important physical interpretation.
Before 14°, the atmosphere is still losing the dominant twilight produced by sunset.
After 14°, the atmosphere contains only weak residual scattered light.
Consequently, the period between approximately 14° and 17.5° should not be understood as the persistence of the red twilight.
Instead, it represents the gradual disappearance of faint residual illumination.
Difference Between Urban, Rural, and Pristine Environments
This graph demonstrates that in urban environments the measured twilight brightness reaches an apparent plateau near 9.5° solar depression, not because natural twilight has ended, but because artificial skyglow dominates the measurements. Beyond this point, further fading of natural twilight cannot be distinguished from the constant background illumination produced by city lights. Consequently, the visible disappearance of Shafaq Aḥmar cannot be reliably determined in urban settings by naked-eye observation alone. This finding provides strong scientific support for the juristic principle of taqdīr (astronomical estimation) whenever the natural twilight signs are obscured by environmental factors such as light pollution.
Why the Remaining Twilight Appears White
Atmospheric optics explains this naturally.
The brilliant red sunset colours are produced primarily by single scattering.
Single scattering requires a direct geometric relationship between the Sun and the observer.
As the Sun sinks farther below the horizon, this geometry rapidly disappears.
What remains is multiple scattering.
Multiple scattering repeatedly redirects light through many atmospheric particles.
This process mixes wavelengths together.
Instead of producing vivid red colours, it produces weak diffuse illumination with little colour contrast.
To the naked eye, this remaining twilight appears nearly white or grey.
Although sensitive instruments may still detect minute spectral differences, the distinctive red glow has disappeared.
This distinction is essential.
Islamic jurisprudence concerns the visible red twilight, not microscopic spectral measurements.
Modern atmospheric studies also indicate that the visible red twilight frequently persists at solar depression angles shallower than 14°. The duration and intensity of the red glow are not governed solely by the Sun's geometric position below the horizon, but are strongly influenced by atmospheric conditions such as aerosol concentration, humidity, dust, cloud particles, and the efficiency of single scattering. Consequently, a fixed threshold such as 12° cannot guarantee the disappearance of the red glow under all conditions, even at the same location throughout the year. This variability explains why the red twilight may still be visible before 14° solar depression, whereas by approximately 14° the atmosphere has generally transitioned beyond the stage of dominant red scattering, leaving only a faint, visually achromatic residual twilight. Thus, 14° should be understood not as the earliest possible disappearance of Shafaq Ahmar, but as a conservative threshold that reasonably ensures its disappearance under ordinary natural conditions.
A Modern Understanding of Shafaq Ahmar
Combining classical juristic definitions with modern atmospheric science leads to an important synthesis.
The Nature study demonstrates that after approximately 14° solar depression, twilight brightness has already undergone most of its natural decline.
The remaining interval to approximately 17.5° consists only of weak residual scattered light.
Atmospheric optics explains that this remaining light is dominated by multiple scattering rather than the strong red single scattering responsible for Shafaq Ahmar.
Therefore, the most reasonable physical interpretation is:
At approximately 14° solar depression in equatorial regions, the residual twilight is visually dominated by faint white (achromatic) scattered light, with no discernible red glow (Shafaq Aḥmar) remaining to the naked eye.
This statement does not claim that every red photon has disappeared.
Rather, it recognizes the distinction between:
measurable spectral components detected by scientific instruments, and
the visible red twilight that forms the basis of the juristic definition.
Equatorial Observations
Near the equator, twilight geometry changes very little throughout the year.
Consequently, equatorial locations provide excellent natural laboratories for studying the disappearance of Shafaq Ahmar.
Repeated observations in equatorial regions indicate remarkable consistency in the disappearance of the visible red glow.
Such observations suggest that a solar depression of approximately 14° provides a conservative threshold, ensuring that the visible red twilight has ended.
This does not imply that astronomical twilight has ended.
Instead, it indicates that the physical phenomenon described by the jurists—the visible red glow—is no longer present.
Juristic Precaustions
إذَا ثَبَتَ هَذَا، فَإِنَّهُ إنْ كَانَ فِي مَكَان يَظْهَرُ لَهُ الْأُفُقُ، وَيَبِينُ لَهُ مَغِيبُ الشَّفَقِ، فَمَتَى ذَهَبَتْ الْحُمْرَةُ وَغَابَتْ، دَخَلَ وَقْتُ الْعِشَاءِ، وَإِنْ كَانَ فِي مَكَان يَسْتَتِرُ عَنْهُ الْأُفُقُ بِالْجُدْرَانِ وَالْجِبَالِ، اسْتَظْهَرَ حَتَّى يَغِيبَ الْبَيَاضُ، لِيَسْتَدِلَّ بِغَيْبَتِهِ عَلَى مَغِيبِ الْحُمْرَةِ، فَيَعْتَبِرُ غَيْبَةَ الْبَيَاضِ، لِدَلَالَتِهِ عَلَى مَغِيبِ الْحُمْرَةِ لَا لِنَفْسِهِ
Once this has been established, then if a person is in a place where the horizon is visible to him and the disappearance of the twilight (shafaq) can be clearly observed, the time of Isha begins as soon as the red twilight disappears and vanishes.
However, if he is in a place where the horizon is concealed from him by walls or mountains, he should wait until the white twilight disappears. He uses its disappearance as evidence that the red twilight has already disappeared. Thus, he considers the disappearance of the white twilight not because it is itself the criterion, but because it indicates that the red twilight has already vanished.
This statement from Ibn Qudamah is an important piece of evidence showing that, in the Hanbali school, the legal criterion for the beginning of Isha is the disappearance of Shafaq Aḥmar (the red twilight), while the disappearance of the white twilight is merely an evidentiary sign (qarīnah) when the red twilight cannot be observed directly.
The passage can be analyzed as follows:
"If a person is in a place where the horizon is visible to him and the disappearance of the twilight can be clearly observed, the time of Isha begins as soon as the red twilight disappears."
This establishes the primary rule:
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The actual cause ('illah) for the beginning of Isha is the disappearance of Shafaq Aḥmar.
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When the western horizon is clearly visible, there is no need to wait for anything else.
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Certainty is obtained through direct observation of the red glow.
He then says:
"If he is in a place where the horizon is concealed from him by walls or mountains, he should wait until the white twilight disappears."
Notice that he does not redefine the legal criterion. Rather, he addresses a practical problem:
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The observer cannot see the western horizon.
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Therefore he cannot be certain that the red twilight has disappeared.
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Since the white twilight disappears later, its disappearance serves as reliable evidence that the red twilight must already have vanished.
This is confirmed by the underlying reasoning:
"He uses its disappearance as evidence that the red twilight has already disappeared."
This distinction is extremely significant.
There are therefore two separate concepts:
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Legal criterion (ḥukm):
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Disappearance of Shafaq Aḥmar.
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Practical evidence (amārah or qarīnah):
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Disappearance of the white twilight when direct observation is impossible.
This reflects a broader legal principle in Islamic jurisprudence: when the original sign cannot be observed directly, a later and more certain indicator may be used to establish that the original condition has already occurred.
This passage also highlights the importance of certainty (yaqīn). Ibn Qudamah does not instruct people to estimate or guess when the red twilight has disappeared if they cannot see it. Instead, he instructs them to wait for an unmistakable sign—the disappearance of the white twilight—which removes doubt and confirms that the red glow has certainly vanished.
Consequently, this text supports the principle that Isha should not begin while any discernible Shafaq Aḥmar remains visible. If there is uncertainty due to obstructions or atmospheric conditions, one should rely on a later indicator that establishes with confidence that the red twilight has already disappeared. This emphasis on certainty aligns with the juristic maxim that acts of worship dependent on time should not commence until their prescribed time is established with confidence.
The objective of classical jurists such as Ibn Qudamah was to establish with certainty that Shafaq Aḥmar (the red twilight) had disappeared. When the western horizon was clearly visible, the disappearance of the red glow itself marked the beginning of Isha. However, when buildings, mountains, or other obstructions prevented direct observation, they instructed people to wait until Shafaq Abyaḍ (the white twilight) disappeared, not because it was itself the legal criterion, but because it served as reliable evidence that the red twilight had already vanished.
Today, systematic atmospheric observations and photometric studies provide another means of attaining that certainty. Multiple observational studies indicate that by approximately 14° solar depression, the residual twilight visible to the naked eye is predominantly faint white (achromatic) scattered light, while the red component (Shafaq Aḥmar) is no longer discernible under normal viewing conditions. Thus, a 14° threshold may be understood as a scientifically informed indicator that the legal criterion—the disappearance of Shafaq Aḥmar—has been fulfilled, eliminating the practical need to wait for the later disappearance of the white twilight.
The Hanbali understanding of Shafaq Aḥmar is closely connected with their broader conception of the duration of Maghrib time.
The distinction can be summarized as follows:
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Shafi'i School: Maghrib has essentially one immediate prescribed time. After sunset, the prayer should be performed without delay except for the time needed for purification, adhan, iqamah, and the prayer itself. The disappearance of the red twilight marks the beginning of Isha, but according to the dominant Shafi'i view, it does not represent a practical extension of Maghrib time because Maghrib is intended to be performed immediately after sunset.
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Hanbali and Hanafi Schools: Maghrib has a continuous valid time, just like the other prayers. Its time begins at sunset and remains valid until the disappearance of Shafaq Aḥmar, at which point Isha begins. Therefore, accurately identifying the disappearance of the red twilight is of much greater legal significance, since it determines both:
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the end of Maghrib, and
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the beginning of Isha.
This helps explain why Ibn Qudamah places so much emphasis on obtaining certainty that Shafaq Aḥmar has disappeared. His discussion is not merely about determining the start of Isha; it is equally about ensuring that the valid period of Maghrib has truly ended. If there is doubt because the horizon is obstructed, he advises waiting until the disappearance of the white twilight, since that provides certainty that the red twilight—and consequently the time of Maghrib—has already ended.
This perspective also strengthens your argument regarding a modern 14° observational threshold. If contemporary atmospheric studies reliably indicate that around 14° solar depression the red glow is no longer visible to the naked eye under normal conditions, then that threshold serves the same juristic function that Ibn Qudāmah sought: establishing certainty that Shafaq Aḥmar has disappeared. In this way, scientific observation becomes a modern means of confirming the classical legal sign, rather than replacing it with a new criterion.
Implementing Shafaq Ahmar Threshold in Islamic Sciences Online App
Islamic Sciences Online app implements this threshold as this formula:
1. The Equatorial Anchor (14°)
Our formula therefore begins exactly where the observational evidence is strongest.
In other words,
The formula is anchored to the latitude range where modern measurements of natural twilight are most stable and where the twilight brightness curve indicates that only weak residual scattered light remains beyond approximately 14° solar depression.
This is an excellent scientific justification.
2. Why increase the angle with latitude?
The Nature study also shows something fundamental:
Twilight does not behave identically at all latitudes.
Higher latitudes experience:
shallower solar descent,
longer atmospheric paths,
greater persistence of residual scattered light,
longer twilight.
Therefore,
the criterion corresponding to the disappearance of Shafaq Ahmar cannot reasonably remain fixed at 14° everywhere.
Our formula reflects this.
The term
genui{"trigonometry_learning_block":{"type_id":"TRIG_IDENTITY_PYTHAGOREAN"}}
naturally increases from
0 at the Equator,
0.25 at 30°,
0.50 at 45°,
0.75 at 60°,
1 at the poles.
Thus the additional angle varies smoothly from 0° to 4°.
3. What values does it produce?
| Latitude | Isha Angle |
|---|
| 0° | 14.0° |
| 15° | 14.27° |
| 30° | 15.0° |
| 45° | 16.0° |
| 60° | 17.0° |
| 66.5° | 17.37° |
| 90° | 18.0° |
This progression is remarkably reasonable.
Notice:
Equator → 14°
Mid-latitudes → 15–16°
High latitudes → 17–18°
These values closely span the range over which natural twilight transitions to astronomical darkness.
4. Physical interpretation
The Nature paper shows that after about 14°
As latitude increases,
that residual twilight persists longer because
sunlight travels through a much longer atmospheric path,
multiple scattering becomes more effective,
twilight duration increases.
Our latitude correction can therefore be interpreted as
a compensation for the increasing persistence of residual scattered twilight with latitude.
That is a very natural interpretation.
5. Why use (\sin^2(\phi))?
This is perhaps the most elegant aspect.
The square of the sine
is symmetric between north and south,
changes smoothly,
is nearly flat near the equator,
increases progressively toward the poles.
That matches twilight behaviour remarkably well.
It leaves the tropical world almost unchanged while gradually increasing the angle where twilight actually becomes more persistent.
Although the 2024 Nature study does not investigate polar or subpolar regions directly, it provides the underlying atmospheric physics needed to explain them:
Twilight persists throughout the night at high latitudes because the Sun never descends to the solar depression at which residual scattered light naturally fades. If the Sun does not reach approximately the threshold required for the disappearance of Shafaq Aḥmar, the red twilight cannot completely vanish, making juristic estimation (taqdīr) both physically justified and consistent with the classical Islamic approach.
Conclusion
The modern scientific study of twilight reveals that the atmosphere undergoes three distinct stages after sunset.
The first stage is the brilliant red twilight produced mainly by single scattering.
The second stage is a faint white twilight dominated by weak multiple scattering.
The third stage is complete astronomical darkness.
The 2024 Nature study demonstrates that the overwhelming majority of twilight brightness has already disappeared by approximately 14° solar depression, after which the brightness curve flattens considerably and only weak residual scattered light remains until the natural night background is reached near 17.5°.
These findings support a modern physical understanding of the classical juristic concept of Shafaq Ahmar.
The evidence suggests that the legal criterion is not the disappearance of every trace of scattered sunlight, but rather the disappearance of the visible red glow itself.
Accordingly, a scientifically coherent interpretation is:
At approximately 14° solar depression in equatorial regions, the residual twilight is visually dominated by faint white (achromatic) scattered light, with no discernible red glow (Shafaq Aḥmar) remaining to the naked eye.This interpretation preserves the classical definition of Shafaq Ahmar while benefiting from contemporary atmospheric science, providing a bridge between traditional Islamic jurisprudence and modern observational astronomy.
