Wayward Comet:. Martin Beech

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      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.