THE TRIGONOMETRIC FUNCTIONS, AS THEY WERE IN THE ARABIC-ISLAMIC CIVILIZATION

In the Greek/lndian period, it is noticeable that different radii were used in connection with the chord. This manner continued in the Indian period with the sine, i.e. different sine tables existed. But throughout the Arabic-Islamic period, there was stability in the radius (for the sine). At the time of al-Battani new terms were introduced, not as functions of angles but as lengths, and again different tables for the same term. Here these terms were not bounded to the circle, and the term miqyas r (measure), which was variable, was used to express "the radius" related to these terms. The trigonometric "functions" at that time were treated as if they were two different groups. While at Abu al-Wafa"s time, there was an advancement by introducing the new terms as functions of angles, and they were immediately bounded to the circle, and instead of having two circles in the same figure, a kind of unity appeared, and again there was stability in the value of r, and