De-Tailing Comets
May 26, 2011

Science does not always progress. A major setback in the astronomy of the Graeco-Roman world was the widespread adoption of the Aristotelian worldview.

One of many fields affected by the rise of Aristotle’s star was the theory of comets. With much verve, Aristotle advanced his specious argument that comets do not exist in space, in the arena dominated by planets and stars, but are restricted to the atmosphere – or what the philosopher would call the element of ‘air’.

Lacking any solid substance, comets were simple ‘fires’ occupying the same phenomenological slot as what are known today as meteors, aurorae and even the Milky Way, when the comet stands on its own, or as haloes, when it forms as a ‘reflection’ of the antics of a star or a planet. In asserting this palpably false opinion, Aristotle marked a signal departure from a bevy of pre-Socratic thinkers who had lumped comets and planets together.

Yet curiously, from a modern perspective these early theoreticians appear to have had a better grasp on comets than Aristotle, who actually understood heads nor tails of the phenomenon.

According to Aristotle, 'some of the so-called Pythagoreans say that a comet is one of the planets, but that it appears only at long intervals and does not rise far above the horizon'. Hippocrates of Chios (±470-±410 BCE), whose ideas displayed strong Pythagorean streaks, and his disciple Aeschylus 'maintain that the tail does not belong to the comet itself, but that it acquires it …' The pair also reasoned that the comet 'appears at longer intervals than any of the other stars because it is the slowest of all in falling behind the sun …'

Another contemporary, Diogenes of Apollonia, classified comets as ‘stars’, a term that generally included the planets. Apollonius of Myndus (Fourth century BCE) is on record with his belief that 'many comets are planets … a celestial body on its own, like the Sun and the Moon'.

The conviction that comets are physical bodies in their own right facilitated the early theory, vigorously brushed aside by Aristotle, that they result from a conjunction of planets. Democritus of Abdera (±460-±370 BCE), for example, opined that comets are a 'coalescence of two or more stars so that their rays unite.' Thinking along similar lines, Anaxagoras of Clazomenae (±500-428 BCE), too, held that 'two or more stars being in conjunction by their united light make a comet', 'when they appear to touch each other because of their nearness'.

Leucippus of Miletus (±480-±420 BCE) was cited to a similar effect: 'Comets are due to the near approach to each other of two planets'. Much later still, the spiritual father of Stoicism, Zeno of Citium (Third century BCE), judged 'that stars come together and combine their rays, and from this union of light there comes into existence the image of a rather long star'.

To a modern cometologist, proto-scientific speculations of this sort seem far more up to speed than Aristotle’s vapid postulate that all comets are disturbances in what we would call the earth’s atmosphere. In grouping comets with planets, the pre-Socratics listed above anticipated the modern understanding of comets as bodies in the Solar System, whose paths may intersect with those of planets. Their suspicion that comets are planets orbiting at extremely long intervals is vindicated by current knowledge of the periodicity of many comets – including, of course, Comet Halley’s well-known cycle.

More recent discoveries have tended to corroborate even more of the ‘hunches’ Aristotle so vociferously opposed. Observations of the rocky cores of some comets, such as Tempel 1 in July 2005, suggest a common ancestry not only with asteroids, but with the rocky planets occupying the inner region of the Solar System themselves – a far cry from Aristotle’s doctrine that only the element of fire can exist in the superlunary realm.

Conversely, detection of the plasma tails of Venus, Mercury and the earth, and the sodium tails of Mercury and the moon has elicited frequent comparisons to the tails of comets in cutting-edge scientific reports. Even solar prominences, now documented in extensive detail, might suggest a comet-like appendage to the intellectually adventurous.

Aristotle’s opinion, which was to dominate scholarly consensus in the western world until the Seventeeth century, was informed more by the preconceived axiom that everything in the planetary and starry heavens is perfect and immutable than by actual, unbiased observation. Bodies moving on seemingly erratic paths and exhibiting unpredictable behaviour would upset the mathematical elegance Aristotle and his Platonic and Pythagorean colleagues detected in their growing models of planetary orbits.

Aristotle’s banishment of comets from the serene stage of perpetually unerring motion was really a striking demonstration of a so-called topdown theory – quite unlike the bottom-up methodology that has been cultivated since the onset of the intellectual revolution in Renaissance Europe. To be sure, Aristotle did cite observational evidence in his treatment of the subject; however, his predilection for ‘pure reason’ shows when he changes tack to confront competing views with the claim that 'the theory can be shown to be wrong on purely logical grounds'.

The careful reader of his Meteorology also perceives a certain laziness in the pundit’s efforts to explore the pre-Socratic theories of comets to their fullest extent. Aristotle’s refutation of the view that comets are akin to planets may look superficially plausible, but really rests on a tacit but erroneous assumption that comets, like planets, ought to move on the ecliptic plane. In reality, they are free to roam the precincts of the planets under any angle they see fit.

If Aristotle’s cardinal error was to accord greater status to supposedly undefiled reason than to the tested method of construing theories by deduction from sets of observation, the reverse appears to be equally true for his maligned predecessors. The puzzling idea that comets are the product of ‘planets’ in conjunction derived at least in part from direct observation; Democritus, for one, 'has defended his view vigorously, maintaining that stars have been seen to appear at the dissolution of some comets'.

In support of that, the Greek historian, Ephorus of Cyme (Fourth century BCE), claimed that a comet once observed by all mankind ‘split up into two stars, a fact which no one except him reports’. The reference was evidently to the splitting of cometary nuclei, as frequently recorded in modern times. Meanwhile, Hippocrates’ perspicacious argument that the tail is an accessory to the comet could easily have suggested itself if so-called ‘tail disconnection events’ had been observed in Antiquity.

Current knowledge of the pre-Socratic contemplation of comets amounts to little more than the few surviving snippets cited above. The loss of an entire body of literature precludes the possibility to determine exactly which observations led to the remarkably precocious hypotheses that preceded Aristotle.

The Pythagorean penchant for information of Babylonian extraction agrees with Apollonius’ intimation that the scholars who analysed comets as astral objects were ‘Chaldaeans’. While Assyriologists have been able to furnish only meagre support for that statement, it is certainly conceivable that Babylonian astrologers passed on a body of traditions, perhaps never committed to writing, that would have firmly pointed towards a deep affinity between comets and planets.

A larger incidence of comets in the early Holocene, for which some have argued, would naturally have aroused more interest in cometary diversity and nature. As discussed elsewhere, memories of a prehistoric time when Venus’ plasma tail appeared within the visible spectrum seem to have persisted in a variety of cultures, including late 3rd-millennium BCE Mesopotamia.

It has also been argued that planets in conjunction may have produced fireworks if, at times of electrical instability, their pointed tails lined up, brushing against each other. In Seneca’s words, it is then that 'the space between the two planets lights up and is set aflame by both planets and produces a train of fire'.

One of the last echoes of the pre-Socratic idea that comets ensue when planets approach each other may have been Plato’s pithy reference to the mythical Phaethon as a past agent of catastrophe towards the end of a ‘Great Year’. Did Plato think of Phaethon as an earth-bound comet spawned as all known planets arranged in a linear conjunction? Whatever the answer may be, Plato and his precursors unquestionably count as greater trailblazers in cometology than Plato’s pupil Aristotle, who threw caution into the wind along with the comets.


Rens Van Der Sluijs

http://mythopedia.info

Books by Rens Van Der Sluijs: