The root of the disagreement lies in the following: it was once widely held that the fixed stars traverse one degree of the celestial sphere—which is divided into three hundred and sixty degrees—every hundred years. When fresh observations were made in the time of the caliph al-Maʾmūn, some observers claimed that the correct rate is two-thirds of that span, namely every sixty-six and two-thirds years. In truth, the earlier observations are sound. This has been verified by comparing the planetary positions recorded by several of the ancients in their books and on their astrolabes with the planets’ positions today, all of which confirms that the ancient reckoning is correct and that the stars do indeed cover a single degree in one hundred years.

Multiplying that figure by three hundred and sixty produces, on the first reckoning, thirty-six thousand years, and on the second, twenty-four thousand. The one who spoke of three hundred and sixty thousand years was referring to a single revolution within ten grand revolutions, while the one who said twelve thousand years simply allotted a thousand years to each sign of the zodiac. Yet none of them possesses any proof that this span marks the duration of the heavens’ existence.

Their entire edifice rests on a faulty premise: the notion that every event is caused by the motions of the celestial spheres.

Set against them are those who deny any influence whatsoever for heavenly motions upon occurrences here below. On this fundamental question the two camps confront one another, the philosophers on one side and the innovators among the scholastic theologians on the other; the latter maintain that none of the heavens’ movements exerts any effect in this world and that no such motion can serve as the cause of anything that comes to be.