For over two thousand years, philosophers believed Aristotle’s theory that comets were burning meteors on the earth’s atmosphere. Although some philosophers such as Paolo Toca-Nelli (1397-1482) and Georg Peurbach (1423-1461) had attempted to dispute this view, it is only after the comet of 1577 that significant shift from Aristotle’s ideology. The observation of the comet of 1577 by Tycho Brahe changed the view that comets were below the moon, instead of being above it. Similarly, there was a realization that a comet’s tail fire was not because the comet was following its source of fuel as a flame in a wick. Accordingly, the motion of comets around starry spaces coupled with the opposition of the tails to the sun made the Aristotelian-Ptolemaic theory untenable.
The Great Comet of 1577
Source: Fine Art America
The comet of 1577 played an important role in the revolution of cosmology and astronomy. In the earlier Aristotelian geocentric cosmological scheme, the world was divided into two distinct regions: the heavens and the earth. The heavens were perfectly unchanging and reached from the sphere of the moon to the fixed stars. Further, the heaven’s motions were perfectly circular. The world of corruption and change was the one below the moon. The earth was the center of the universe and all heavy bodies. It was surrounded by spheres of the elements of air, fire, and water, which reached to the moon.
Since the Aristotelian geocentric cosmological scheme believed the heavens were unchanging, an event that involved change was a sublunary one. The main challenge of the Aristotelian view was, although the heavens moved around the earth infinitely in a circular motion, comets only appeared suddenly and moved around constellations for only a brief period before disappearing. Due to their irregularity and lack of pattern in their appearance and motion, their location was believed to be below the moon.
The Comet of 1577
The observation of the bright comet, which first appeared in late 1577, by many philosophers including Tycho Brahe, led to a change in the Aristotelian geocentric cosmological scheme. The huge 1577 comet had a tail that spread in a great arc across the sky. Observers including Tycho Brahe made measurements of the comet’s changing positions, which resulted in an extensive literature on the position of comets. Brahe made a revolutionary discovery that comets were located in the ‘heavens’ when he used its parallax to determine its distance from the earth. Parallax is simply a measurement of an object’s distance by observing the object in different locations at the same time. In particular, Brahe compared his observations in Denmark with that of other observers in Europe, and using these readings he calculated the distance of the comet from the earth. His findings revealed that the comet was further into the ‘heavens’ than the moon. Accordingly, it was part of the moon, an observation that contradicted the Aristotelian geocentric cosmological scheme.
By placing of comets in heavens, Brahe introduced a new question on the path that comets followed. According to Brahe, the 1577 comet formed an arc as it passed over the earth. By the seventeenth century, the movements of comets were mainly based on the mechanical philosophy. For example, Isaac Newton, in his Mathematical Principles of Natural Philosophy, argued that all matters attract other matters. Therefore, comets attracted the sun just as other planets since they were made of matter. Newton concluded that comets path was a conic section because of their rectilinear inertia and a centrally directed force. Accordingly, by the beginning of the eighteenth century, comets were regarded as obeying natural laws of ‘new physics’ and astronomy, and not the Aristotelian physics and Ptolemaic cosmology.
Modern Concept on Comets
Although comets were in the past believed to be celestial objects, modern science has proven comets to be dusty snowballs. These snowballs are made of ammonia, methane, and water. When the snowball gets close to the sun, the ice evaporates and releases fine dust. The released dust are the ones that make the tail to point away from the sun. This phenomenon occurs because the dust particles try to stay with the parent body, but the solar winds try to blow them away. Comets normally always originate from two populations of icy bodies surrounding the solar system, the Kuiper belt and the Oort cloud. The Kiper belt lies just beyond Pluto, whereas the Oort cloud is a chaotic assemblage of icy debris that swirl around the solar system from a distance. When large bodies, even stars, pass near these icy bodies, they deflect their paths from the outer space towards the inner parts of the solar system, where we can see them. In most cases, these icy bodies are consumed by the sun. In some cases, they hit planets, notably Jupiter. In June 16-22, 1994, the Shoemaker-Levi 9, which crashed Jupiter. Similarly, there Earth experienced a similar impact in June 30, 1908, with the Tunguska event.
C/2013 A1 Siding Spring
Source: NASA Science.
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