for the history of astronomy, however, Brahe was able to combine observations of the comets motion made from different locations, and this enabled a parallax distance estimate to be made. Incredibly, the comet was located at least 230 Earth radii away, and its coma was nearly ¼ the size of the Earth. At the deduced distance, the comet was situated well beyond the orbit of the Moon, which sits at a distance of about 65 Earth radii away. Having placed the comet in the celestial realm, however, Brahe argued that it was nonetheless a temporary object that would eventually fade from all view. Brahe additionally suggested a mechanism for the appearance of the comet’s tail – it being the result of sunlight passing trough the rarified and porous matter that constituted the comet’s head.
Figure 1.14. The Comet of 1577 placed (and labeled X) in its own circular orbit (STVX) about the Sun (C) – as deduced by Tycho Brahe in his De Mundi, published in 1588.
Brahe’s master work that included his analysis of the comet of 1577, De Mundi Aetherei Recentiorbus, was published in 1588, and here he placed the comet upon a circular orbit about the Sun just beyond the orbit of Venus (figure 1.14). Brahe’s planetary system, although being post-Copernican in date, was (seemingly) unusual for having the planets Mercury through to Saturn orbiting the Sun, with the Sun and Moon orbiting a stationary Earth [5]. The comet is drawn as moving along a circular orbit, but Brahe additionally suggested the path might be more oval in shape. Since Brahe’s planetary model was never widely accepted the question concerning the shape of cometary paths was not considered settled. Indeed, later on, in the 1630s, Jeremiah Horrocks suggested that the comet of 1577 might have been a temporary ejection of material from the Sun, moving initially along a straight line (rectilinear) path, but then, being strongly affected by the Sun’s magnetic field, it eventually fell back to the place of its origin. The idea that comet’s might move along rectilinear paths had actually been developed earlier by Johannes Kepler in his De Cometis Libelli Tres, published in 1619, although Kepler based his arguments upon the comets seen in 1607 and 1618.
“When something unusual arises in the heavens, whether from strong constellations or from new hairy stars [comets], then the whole of nature, and all living forces of all natural things feel it and are horror stricken”. So wrote Johannes Kepler on comets. Recipient of Brahe’s observational data and Royal patronage, upon the latter’s death in 1601, Kepler, as we have seen, played the vital role of both confirming and correcting the Copernican hypothesis. Planets, as Kepler showed, moved along elliptical paths about the Sun, but comets, Kepler believed, were ephemeral, formed spontaneously from various rising vapors and seen only the once. On this basis Kepler assigned the comets not an elliptical path about the Sun, but a straight-line, rectilinear motion through the celestial realm (figure 1.15). Kepler’s magnum opus De Cometis Libelli, was published in 1619 – the same year as his (perhaps) more famous text Harmonius Mundi in which his third law of planetary motion was first articulated. In De Cometis Libelli, Kepler presented 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
