Al-Jayyānī wrote The book of unknown arcs of a sphere, which is considered "the first treatise on spherical trigonometry", although spherical trigonometry in its ancient Hellenistic form was dealt with by earlier mathematicians such as Menelaus of Alexandria, who developed Menelaus' theorem to deal with spherical problems. However, E. S. Kennedy points out that while it was possible in pre-Islamic mathematics to compute the magnitudes of a spherical figure, in principle, by use of the table of chords and Menelaus' theorem, the application of the theorem to spherical problems was very difficult in practice.
Al-Jayyānī wrote "The book of unknown arcs of a sphere", which is considered "the first treatise on spherical trigonometry".  Menelaus of Alexandria, an earlier mathematician, dealt with spherical trigonometry in its ancient Hellenistic form. However, E. S. Kennedy points out that while it was possible to compute the magnitudes of a spherical figure using the table of chords and Menelaus' theorem, applying the theorem to practical spherical problems was difficult.