Wayward Comet:. Martin Beech
his observations and thoughts relating to the comet of 1607 (C/1607 S1), and three bright comets observed in the fall of 1618. Kepler first saw the comet of 1607 on September 26 – after a firework display he had been enjoying while in Prague. The comet was widely observed from across Europe and Kepler, forcing the path to be rectilinear, attempted to map out its path through the heavens (figure 1.15). While Kepler acknowledge that he had problems in forcing the comet’s path to be a straight line, the ultimate irony of this particular story is that the comet of 1607, was in fact, Halley’s Comet. In 1618 Kepler became the first person, as far as is recorded, to see a comet through a telescope. Observing comet C/1618 Q1 on September 6th, Kepler described it as being “large and resembling a cloud”. Perhaps surprisingly Galileo, the great self-promoter, did not bother to observe any of the three comets seen towards the close of 1618. True, Galileo was apparently ill and confined to bed at the time of interest, but it does not seem unreasonable to suppose that if he had felt that comet’s were worthy of his study, he would have done so. True to form, however, while Galileo made no observations of the comet himself he produce a vitriolic attack upon the observations and ideas relating to the comet’s appearance published by Jesuit astronomer Horatio Grassi. Being generous, it could be said, Galileo played the role of Devil’s Advocate, but one rather suspects that he just liked nothing better than a good old public scrap. While he denounced Grassi’s observations, and even brought into question Brahe’s parallax observations relating to the comet of 1577, Galileo offered no particularly new or useful insights to the study of cometary phenomena.
Figure 1.15. The rectilinear path for the comet of 1607 [which was actually Halley’s Comet] as derived by Johannes Kepler. The comet’s path takes it through the orbits of the Earth, and Venus. Image from Kepler’s De Cometis Libelli (1619).
The great French philosopher René Descartes outlined a detailed vision of the stellar realm in his highly influential text Le Monde (published in 1633). Espousing a mechanical philosophy, Descartes brought order to the primordial chaos by invoking the formation of a swirling broth of agitated particles – a space-filling, particulate ocean of ever moving circular eddies and tourbillions (vortices). At the center of each cosmic vortex was a star at rest, with any accompanying planets, trapped like so much flotsam in the ocean swell, being carried around in their orbits by the outer circular flow (figure 1.16). “Picture the bend in a river, where the water coils itself, tuning in circles, some major, some minor,” writes Descartes in his Principles of Philosophy (first published in 1644), “Things floating in this current, we notice, are carried along by it and turned around and around, even heavy objects, some of which revolve about their own centers. Those objects nearer the center of the eddy containing them complete their revolution sooner than do those farther from the center. Finally, although the eddies always turn in circles they hardly ever describe a perfect circle …. Let us likewise imagine the same things happening to the planets. And this is all we need in order to explain all of their phenomena”. Descartes analogy catches the minds-eye, his arguments sound reasonable, the dynamics can be envisioned, and the picture appears to encapsulate the observed properties of planetary orbits. Unfortunately for Descartes, however, as Isaac Newton was eventually to show, Nature is not so easily explained – more than just a good picture is required of a philosophy, and planetary behaviors in particular are more directly explained by mathematics and physical principles.
While falling short on physical realism and lacking mathematical detail, Descartes cosmos was, none-the-less, far from static, and he argued that adjacent vortices would compete for space, expanding and pushing against each other in a tumultuous battle for dominance. As with all battles there would be winners and losers, and in the cosmic battle a star that lost control of its vortex was destined to become a wandering comet. Such star-comets would drift and curve through space skimming around the outer edges of the various vortices that it chanced to encounter. In Descartes’ philosophy, comets followed a trail similar to that of a meandering stream, randomly turning this way and that; sometimes passing close to another vortex-centered star and at other times drifting haplessly through the depths of interstellar space.
It was the appearance of two bright comets in 1664 and 1665 that caught the imagination and eye of Polish astronomer Johannes Hevelius. Turning both his telescope and his positional instruments to these comets, he traced their path across the heavens, and wrote of their behavior in a short text, simply titled, Prodomus Cometicus - Historia Cometae anno 1664 et 1665 (Published by S. Reininger, Gedani, 1665). What sets this particular text apart from the many others written at the same time is that on 4 February 1665, Hevelius set out to estimate the horizontal parallax of the comet C/1664 W1. Remarkably, he determined a “sensible parallax” of 41 arc seconds, and this indicated that the comet was at least 5000 Earth radii away. This distance, much larger than the 230 Earth radii deduced for the comet of 1577 by Tycho Brahe, clearly placed comets within the planetary realm, and the question as to their origins and true paths became paramount.
Figure 1.16. Descartes vortices, with a star located at each center (Y, f, F, S, L, D). The meandering path of a comet (starting at point N in the lower center and continuing to the upper right) works its way from one outer vortex boundary region to another.
Hevelius continued to study comets and eventually published his magnum opus on the subject Cometographia in 1668. Within this tome he attempted to describe and classify cometary tails, and posited that comets move along paths corresponding to conic sections (recall figure 1.10) with the Sun located at a focal point (although he did vacillate somewhat on this latter condition). Since comets were still believed to be ephemeral, one-off displays, Hevelius suggested they were some kind of spurious planet, formed from material exhalations derived from either one of the outermost planets Jupiter or Saturn – the planet of origin being determined by the comet’s color. The frontispiece to Cometographia reveals the Hevelius viewpoint on comet orbits and origins (see figure 1.17). The image shows Hevelius flanked by the ghost of two philosophers past: Aristotle and Kepler. To his right is Aristotle, ignored and pointing somewhat vaguely to a diagram showing comets as atmospheric phenomena constrained to the sublunar region, their tails pointing in any and all directions. To the left of Hevelius, in more earnest stance is Johannes Kepler, who holds his diagram revealing the rectilinear motion of cometary paths; their tails streaming away from the Sun. Hevelius, however, cannot hold the eye of Kepler, and with a gentle, staying wave of his left hand points emphatically with his right to the new interpretation. Comets move along gently curving paths (parabolic or hyperbolic – but not elliptical). Hevelius essentially truncated the long and gently curving (interstellar) pathways depicted by Descartes (figure 1.16); downgrading comets thereby from vortex skimming stars of unknown origin, to locally birthed planet-born progeny.
While justly famed for his lunar and star mapping work, Hevelius, in later life, was overwhelmed by the scientific and instrumental advancements that had developed around him. The beginning of the end for Hevelius began, in fact, with his observations relating to the comet of 1664, for which he presented sky positions that were at odds with all other observers. Then in 1674 Robert Hooke, in his Animadversions, launched into a long critique of Helvelius’s Machina Coelestis (published in 1673) – chastising Hevelius for relying on the eye to make positional observations when telescopic sights offered a much more precise means of measurement. Further misfortune followed in September of 1679 when his observatory in Danzig, Stellaburgum, burnt to the ground. And finally, with the appearance of the comet of 1680, and Newton’s 1687 triumphant description of its orbit as a parabola, with the Sun at the focal point (recall figure 1.5), Helvelius’s notion of comets moving along gentle curving paths was laid to rest. Hevelius died in 1687, the last of the great Tychonic School of naked-eye observers, being spared the news of Newton’s work and the 1705 publication of A Synopsis of the Astronomy of Comets by Edmund Halley in which the periodic nature of the comet of 1682 was revealed.