al-Zarqālah (493 H) said: Fajr is at 18 degrees.

His name is Abū Isḥāq Ibrāhīm ibn Yaḥyā al-Naqqāsh al-Zarqālī (al-Zarqālah) al-Tujibi, and he passed away in the year 493 of the Hijrah, which corresponds to the year 1100. The instrument maker and astronomer from Andalusia. His astronomical instruments and works would continue to be used for another 400 years. The great astronomer Nicolaus Copernicus cited from his works.

The astronomer Ibn al-Zarqālah (493 H) said in the forty-ninth chapter on the knowledge of the twilight (al-shafaq) and the beginning of Fajr (ṭuluʿ al-fajr) in one of his letters:

"You look at the sun, and if it has a northern inclination, then place the end of the Alhidade (rotating measurement device for angle display) at the height of Aries (al-ḥamal) in your country at a quarter of the height, then remove the crossbar from the center of the plate to the side of the sign eighteen... What remains is the time the firmament (al-falak) rotates from sunset to the end of the twilight and also from the start of Fajr (ṭuluʿ al-fajr) to sunrise."