Classified 83k records into 4 tiers by methodology alignment with
Islamic astronomical definitions (Fajr Sadiq, Shafaq al-Abyad).
Tier 1+2 (4,912 records): CCD horizon, naked eye, dark-site SQM, DSLR
-> Fajr baseline: 16.98 deg [95% CI: 16.73-17.22]
-> Weighted R²=0.43, MAE=1.14 deg
-> Well-calibrated at 45-55N (R²=0.74, MAE=0.25 deg via OpenFajr)
-> Extrapolated at equator (no Tier 1 tropical data)
Isha: only 45 Tier 1+2 records. Not enough for a formula.
Key finding: equatorial Fajr angle is the most important missing
measurement. Whether it's 15-16 or 17-18 deg changes everything.
