Wayward Comet:. Martin Beech

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      Figure 1.17. The title page from Hevelius’s Cometographia (1668). To the left in the image is Aristotle, centrally seated is Hevelius and to the right is Kepler. In the center background a crowd of onlookers, some with arms raised to the sky, contemplate the approaching comet. Meanwhile, lofted above the superstitious plebeians, are seen the “astronomers” at work. Three large sextants (characteristic of the design promoted by Tycho Brahe) can be seen in the image, along with an observer viewing the comet through a telescope. The dividers placed on the table next to Helvelius’s right hand are a sign of authority and exactness.

      Kepler, Galileo, and Helvelius (and indeed all astronomers prior to their time) accepted that comets were large ephemeral objects, quasi-planets perhaps, but objects with a limited lifetime. This notion began to change, however, with the realization, through Edmund Halley’s investigations, that at least some comets moved along elliptical paths and were in principle at least long-lived and subject to repeated returns to the inner solar system. Newton’s thoughts on the topic were discussed in his Principia (1687) and in his second magnum opus, Opticks (first published in 1704). In these works Newton suggested that while comets had a solid and durable nucleus, they were additionally surrounded by a tenuous atmosphere. Newton additionally suggested an alchemical link with respect to the utility of the vapors emanating from cometary tails, arguing that planets needed to accrete such matter in order to replenish the fluids vital to the process of vegetation and putrefaction [6]. Indeed, in his Opticks Newton asked, “to what end are comets?” - answering in effect that it was only by the accretion of cometary material that sustained life on Earth was possible. With Newton, the swirling vortices of Descartes were laid to rest. “The motions of the Comets”, Newton writes in his Principia, “are exceedingly regular, are governed by the same laws with the motions of the planets, and can by no means be accounted for by the hypothesis of vortices. For comets are carried with very eccentric motion through all parts of the heavens indifferently, with a freedom that is incompatible with the notion of a vortex”. While Newton freed comets from the random buffetings supplied by Descartes swirling tourbillions, he none-the-less entrapped them within the ever searching web of gravity. By this act, Newton placed comets within the realm of direct calculation; their paths, at least in principle, being described by mathematical formula and direct computation.

      With the mathematical techniques put in place by Isaac Newton, the 18th Century saw ever more calculations of cometary orbits being made. With, in principle, their orbits now being understood and numerated, the issue as to their origins and purpose became a matter of greater interest. It was now clear that comets, for at least some of the time, moved in a realm beyond the planets. Indeed, Halley’s Comet, at aphelion, is located three times further from the Sun than Saturn, the outermost of the known planets until the discovery of Uranus, by William Herschel, in 1781. One of the first diagrams to clearly illustrate cometary orbits and contrast them against those of the planets was that constructed by Thomas Wright of Durham. Writing in his text Clavis Cœlestis: being the explanation of a diagram entitled a Synopsis of the Universe (published 1742). Wright notes that, “Besides the Planets, which all move round the Sun from West to East near the Plain of the Ecliptic; there are other surprising Bodies in the System call’d Comets, whose Motions are perform’d in very different Plains, and in all manner of Directions, both direct and retrograde”. Lavishly illustrated, and massive in scale, Wright shows in the supplementary diagrams that accompanied Clavis Cœlestis the paths deduced for Halley’s three ‘periodic’ comets near to the Sun (figure 1.18), but leaves as a mystery their more distant peregrinations. Other diagrams of the time simple stop once the cometary distance extends beyond that of the orbit of Saturn (figure 1.19) – in many ways, rather than invoking the terrestrial map maker’s “here be dragons”, the astronomers responded with the notion that in the realm beyond Saturn, “there be comets”.