the physical nature of heaven: a survey of the concept of falak in hayʾa works

physical explanation of the planets’ place in the heaven is one of the principal questions in the ancient astronomy which the authors of hay?a (cosmology) works tried to find an answer for it. their opinion is in fact a compilation from aristotle’s “fifth element” theory and ptolemy’s “prisms” theory. aristotle’s fifth element theory assumes that the substance of the heavens is different from the other four substances; i.e. fire, air, water and earth, and aristotle named this substance fifth element. moreover since emptiness doesn’t exist in the world it should be the fifth element that fills the heavens. this substance is the origin of the circular motion and its motion must be uniform but it doesn’t appear and the heavenly bodies have different motions in different periods. because of this, we must accept the existence of several falaks (orbits) for the heavenly bodies. ptolemy has discussed about the falaks place and their motions in his planetary hypotheses too. muslim scientist’s attention to this subject in hay?a books shows their attempt to answer these questions. this article wants to survey the viewpoints of aristotle, ptolemy, islamic philosophers and astronomers about the falaks of the heavenly bodies.