Al-Battānī (317H) said: "Fajr is at 18 degrees."

He is Muḥammad ibn Jābir al-Ḥarrānī al-Battānī (317h) and he was the greatest and most famous astronomer of the Islamic World. His works were significantly involved in the development of science and astronomy in the West. The great astronomers of history utilized his works, among them Nicolaus Copernicus, Johannes Kepler, and Galileo Galilei. He recognized the use of trigonometry in astronomy and laid the milestones for the further development of astronomy.

The astronomer Al-Battānī said in the year 317 H in the twelfth chapter of Al-Battānī's Zīj on the making of the astrolabe:

"If you want to set the arcs (muqantarāt) for the appearance of dawn (ṭuluʿ al-fajr) and the disappearance of twilight (maghīb al-shafaq), place Capricorn (ra's al-jady) at eighteen in the arcs and mark in the counterpart the circle of Cancer (madār ra's al-sarṭān) as a sign. Then place Aries (ra's al-ḥamal) on that arc and mark in the counterpart. After that, place Cancer (ra's al-sarṭān) on it and mark on the counterpart. Then look for a center that brings the three signs together for you and draw a line over them. Then do from the other side what you did in their counterparts so that the arc in the East is the arc of the rising of dawn and the one in the West is the arc of the setting of twilight."