Twilight & Depression Angles
Fajr and Isha are the two prayers directly tied to twilight — the period when the Sun is below the horizon but its light still illuminates the upper atmosphere. The angle matters.
Twilight phases
Twilight has three standard astronomical phases, each defined by how far the Sun's center is below the geometric horizon:
| Phase | Sun altitude | Sky condition |
|---|---|---|
| Civil twilight | 0° to −6° | Enough light to read outdoors without artificial light |
| Nautical twilight | −6° to −12° | Horizon just visible; stars bright enough for celestial navigation |
| Astronomical twilight | −12° to −18° | Sky dark enough for most astronomical observation |
| Night | below −18° | True astronomical darkness |
These phases are defined by the International Astronomical Union. The negative sign means the Sun is below the horizon.
Depression angles
A depression angle measures how far below the geometric horizon the Sun's center lies. If the Sun's altitude is −15°, the depression angle is 15°.
For Fajr and Isha, the Sun must reach a specific depression angle before the prayer time begins or ends:
- At Fajr, the depression angle marks the start of astronomical dawn — when the first light of the false dawn (al-Fajr al-Kadhib) gives way to true dawn (al-Fajr al-Sadiq), the spreading whiteness on the horizon.
- At Isha, the depression angle marks when redness has fully disappeared from the western horizon and true night has begun.
The exact depression angle for each event is what different Islamic organizations disagree on.
Why Islamic law uses twilight
Islamic jurisprudence defines Fajr as beginning when the "true dawn" spreads across the horizon — a visible astronomical phenomenon. The classical scholars (fuqaha) described this as the spreading white light that appears across the entire horizon, distinct from the earlier "false dawn" (a vertical streak).
This visible phenomenon corresponds precisely to a solar depression angle between roughly 15° and 20°, depending on:
- Atmospheric conditions (humidity, dust, aerosols)
- Observer's latitude and season
- Elevation above sea level
- Local horizon obstructions
Before modern astronomy, scholars relied on direct sky observation. The fixed-angle convention was an attempt to codify these observations into a single reproducible number.
The fixed-angle convention
In the 20th century, Islamic organizations adopted fixed depression angles for standardized prayer time calculation. The most common values:
| Organization | Fajr angle | Isha angle | Region |
|---|---|---|---|
| ISNA | 15° | 15° | North America |
| MWL | 18° | 17° | Global (Muslim World League) |
| Egypt | 19.5° | 17.5° | Egypt, many Arab countries |
| Umm al-Qura | 18° | 90 min after Maghrib | Saudi Arabia |
| Tehran | 17.7° | 14° | Iran |
| Karachi | 18° | 18° | Pakistan, parts of South Asia |
The variation reflects both different observational traditions and the conditions at the latitudes where each organization is based. Egypt's 19.5° was calibrated for the Nile Delta. ISNA's 15° was calibrated for North American mid-latitudes.
See Calculation Methods for the complete table.
Problems at extreme latitudes
At high latitudes (above ~48°N or S), fixed depression angles break down:
- During summer, the Sun may never reach −18° below the horizon — meaning Isha by the MWL method never begins
- During winter, twilight is brief and compression occurs — Fajr and Isha times are abnormally close together
Various fallback rules exist (1/7 of the night, proportional time, nearest city), but none are satisfying from a physical standpoint.
The Dynamic vs. Fixed Angles page explains how the DPC (Dynamic PrayCalc) algorithm addresses this using physics-grounded adaptive angles instead of fixed values.