Al-Khwarizmi confined his discussion to equations of the first and second degrees. He also wrote an important work on astronomy, covering calendars, calculating true positions of the sun, moon and planets, tables of sines and tangents, spherical astronomy, astrological tables, parallax and eclipse calculations, and visibility of the moon. His astronomical work, Zij al-sindhind, is also based on the work of other scientists. As with the Algebra, its chief interest is as the earliest Arab work still in existence in Arabic.