It became at once the great authority of Saracen astronomy.

From this basis the Saracens had advanced to the solution of some of the most important scientific problems.

They had ascertained the dimensions of the earth; they had registered or catalogued all the stars visible in their heavens, giving to those of the larger magnitudes the names they still bear on our maps and globes; they determined the true length of the year, discovered astronomical refraction, invented the pendulum-clock, improved the photometry of the stars, ascertained the curvilinear path of a ray of light through the air, explained the phenomena of the horizontal sun and moon, and why we see those bodies before they have risen and after they have set; measured the height of the atmosphere, determining it to be fifty-eight miles; given the true theory of the twilight, and of the twinkling of the stars.

They had built the first observatory in Europe.

So accurate were they in their observations, that the ablest modern mathematicians have made use of their results.
Thus Laplace, in his "Systeme du Monde," adduces the observations of Al-Batagni as affording incontestable proof of the diminution of the eccentricity of the earth's orbit.