The pre-modern history of cometary theories, in large part, is the history of Aristotle’s theory of comets. Although Aristotle is not the first philosopher who developed a theory on comets, he is the first known to have employed various observational facts to elaborate a consistent theory of comets within a structured cosmology. Aristotle’s theory is a physical theory in which the material, mechanism of formation, and motion of the comets are all explained. The cometary theory of Aristotle is a part of his coherent theory of the cosmos wherein the categorized objects of the universe are arranged in a distinct configuration. To build such a harmonious picture, Aristotle defined some fundamental concepts based on observation and logic. Dividing the entire universe into two distinct regions was the most basic hypothesis. Aristotle separated the heavens from the earth and defined the realm of each. This demarcation of the celestial and terrestrial regions, however, was not merely a determination of borders; it was an introduction of two completely different sets of phenomena, which should be understood by two different sets of physical principles. As modern astronomers and physicists found that they needed to define the realm of ‘outer space’ at the threshold of the space age, a typical ancient natural philosopher needed to answer the basic question “Where does the sky begin?” Although lightning, meteors, the moon and sun, comets and stars are perceived at the same distance on the ‘celestial sphere’, it is clear that some of these phenomena are closer than others. As a matter of fact, human eyes are not ideal measurement tools to estimate depth in the sky. Beyond a certain range, our eyes are not able to evaluate the linear distances of objects accurately. What is perceived is the relative distance, which is a judgment about proximity or distance of objects. When one object obscures a part of another, it is perceived closer than the obscured one.


Cometary Tail Summer Solstice Celestial Equator Lunar Anomaly Terrestrial Region 
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