Regarding why 15° (ISNA) was chosen in a North American context, many masājid adopted ISNA because it became a widely used standard for North America and because a number of local observation efforts in the US/Canada reported true dawn appearing closer to the mid-teens rather than a fixed 18° in many real-world settings. The Fiqh Council of North America (FCNA) notes that “Islamic dawn” (al-fajr al-ṣādiq) is defined by the appearance and spread of light on the horizon, and that this is not consistently predictable by a single fixed solar depression angle; it varies due to season, latitude, altitude, obstructions, clouds/light pollution, and even observer experience. Because of that variability, FCNA describes choosing an approximate regional standard as a matter of facilitation, and it specifically presents 15° as a reasonable “middle” approximation within a commonly observed range (often discussed as roughly 12°–18°), while acknowledging that different locations and conditions can shift what is actually seen. In other words, many communities choose 15° not because it is “the one fiqh angle,” but because it is an ijtihād-based approximation that has been supported by some observation projects in Western contexts and is easier to implement consistently in congregational life.

From a fiqh perspective, congregants should understand that the prayer-time “angles” are not themselves acts of worship; they are tools used to approximate the sharʿī sign. The sharʿī sign for fajr is al-fajr al-ṣādiq, and FCNA emphasizes that the sign is observational (the spreading horizontal light), while fixed-degree methods are approximations that can be off depending on local factors. This is why you will find reputable scholars and institutions on more than one side of the 15° vs 18° discussion, and why disagreements persist without implying that one side is careless with religion.

Practically, the difference between 15° and 18° is that 18° produces an earlier fajr time (often by 10–30 minutes depending on season and latitude, and more in some higher-latitude situations), while 15° is later and closer to sunrise. FCNA explicitly notes that what seems “safer” depends on whether you are talking about starting the prayer or starting the fast: praying fajr too early is invalid, so caution for prayer tends toward not praying until you are confident fajr has entered; for fasting, eating after fajr enters invalidates the fast, so caution for fasting tends toward stopping earlier. FCNA discusses this exact tension and why communities struggle when fixed calculations differ.

In terms of consistency for congregants, the strongest community principle is to follow the masjid’s established jamaʿah timetable for congregational prayer to preserve unity and avoid confusion, especially when the timetable is set by a recognized method and trusted leadership. If an individual remains personally concerned about fasting, a common practical approach (used by many careful people) is to stop eating at the earlier of the two fajr times while still praying with the masjid at its scheduled time, because the “safety direction” differs for fasting and prayer as noted above. If someone wants to go further, they can also listen for local scholarly guidance based on actual sightings or regional studies, because FCNA and others emphasize that local conditions can meaningfully affect visibility and, therefore, the best approximation.

If you want references that are written for community use, the FCNA article directly addresses the 15° vs 18° question, explains the observational definition of fajr, and explains why fixed angles vary and why 15° is sometimes adopted as a practical approximation. For a long-running compilation of observation-based discussion and methodology considerations (including the effect of latitude and season), moonsighting.com is commonly cited in North American discussions.