When did Celestial Orbs become Hard? An overview of the history of the concept Orb in the Islamic Civilization

This paper investigates the evolution of the concept orb in order to give a rational explanation for the concept's introduction and dissolution. This concept, we will argue, was initially introduced in the Greek culture, while suffering from some conceptual ambiguitites. In particular, two geometrical and physical understandings were associated with the concept and they were indistinguishable among different passages. For Ptolemy, the concept of orb had both geometrical and physical realizations in different treatises. But such conceptual ambiguity was gradually eliminated in the Islamic age of science. We believe that this concept articulation was initiated from the early tenth century by the works of Ibn al-Haytham and Al-Bīrūnī and by the end of the thirteenth century ultimately the concept was used with its highest clarity. We will enumerate some key conceptual characteristics of the concept in this period in order to show how these components marked the beginning of its dissolution in the sixteenth century. In particular, Tycho Brahe and his fellow astronomers in the sixteenth century decided to abandon the concept for some contradictions between these conceptual components and their own empirical observations.