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.
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.
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 having 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.
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, 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 Implications
This understanding agrees well with the principles established by classical scholars.
Imam Ibn Qudamah emphasized that when direct observation is not possible, one estimates the disappearance of the red twilight using reliable indicators.
Likewise, Shaykh al-Islam Ibn Taymiyyah explained, while discussing the extraordinarily long day of the Dajjal, that prayer times are determined by estimation (taqdīr) when the normal astronomical signs no longer provide practical guidance.
Modern atmospheric science provides such indicators.
Instead of replacing the Sunnah, it helps identify the physical phenomenon described by the Sunnah.
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, 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.
