January General Meeting Speaker: Michael Mattiazzo from the Astronomical Society South Australia His subject was “A History of Comets”
The modern definition of a comet is an object in the Solar system that exhibits a coma and/or a tail of debris pointing away from the Sun. It hasn’t always been like that, Michael told us. The word comet itself (now used in all European languages) is a derivation from the Greek kometes (long haired) and the Latin, stella cometa (hairy star). Ideas about the nature of comets abound from the time of the rise of Hellenistic natural philosophy at about 550 BCE when the Pythagoreans considered comets to be a kind of (wandering) planets that were seen rather infrequently and mostly near the horizon in the morning or evening sky. Aristotle in his Meteorology (ca. -330) relegated comets to the lowest, `sublunar' sphere in his system of spherical shells and described them as `dry and warm exhalations' in the upper atmosphere. There is no mention of comets in Ptolemy's Almagest, presumably because they were not considered of celestial origin, but he described them in astrological terms in his Tetrabiblos. Detailed prognostication can be made, provided you know how to read the signs aright: ‘They show, through the parts of the Zodiac in which their heads appear and through the directions in which their tails point, the regions upon which the misfortunes impend. Through the formations of their heads they indicate the kind of the event and the class upon which the misfortune will take effect; through the time which they last, the duration of the events; and through their position relative to the Sun likewise their beginning.’
The Aristotelean view on comets was dogmatically upheld during the following millennium; the first doubts on the nature of comets seem to have been expressed by Thomas Aquinas and also by Roger Bacon in his Opus Tertium from 1267, but like their predecessors they still strongly believed comets to be omens. A bright comet does look a lot like a portent, an omen, a prophetic indication and it is not surprising that people always regarded them as such (see Comet C/2006 P1 McNaught at the beach. Photo Michael Mattiazzo). Writing 2000 years ago, the Roman astrologer Marcus Manilius summed up the then prevailing opinion: ‘Heaven in pity is sending upon Earth tokens of impending doom’. Included in his list of cometary ills were blighted crops, plague, wars, insurrection, and even family feuds. In short, anything could be blamed on comets, and it usually was. “When beggars die there are no comets seen” is how Shakespeare puts it in Julius Caesar, “the heavens themselves blaze forth the death of princes." The appearance of Halley's Comet in 1066 (it was not called that then) was recorded on one of the Bayeux Tapestry panels, leading to the demise of King Harold. The caption, ISTI MIRANT STELLA, was translated in a 1966 National Geographic article as "These men wonder at the star." (see fragment of the famous Bayeux Tapestry). Such beliefs have not entirely died out even today: Comet Hale-Bopp heralded the death of Princess Diana in 1997, and 39 cult people committed mass suicide in the belief of hitching a ride on it through heaven’s gate.
Isaac Newton in Principia (1687), applied his new theory of gravitation to show that the 1680 comet moved in an elliptical orbit and that it passed only about 0.0016 AU above the surface of the Sun. Edmond Halley (1705) computed the orbits of a dozen well-observed comets and demonstrated the periodical nature of the bright comet of 1682. `Halley's Comet', as it was from then on called, was telescopically recovered in December 1758 by Johann Palitzsch, proving conclusively the validity of Newton's law of gravity out to the distance of the comet’s aphelion at 35 AU. Mark Twain (American author and humorist Samuel Langhorne Clemens) is famously quoted as saying in 1909: I came in with Halley's Comet in 1835. It is coming again next year, and I expect to go out with it. The Almighty has said, no doubt: 'Now here are these two unaccountable freaks; they came in together, they must go out together. Twain died on April 21, 1910, one day after the comet's closest approach to Earth.
The history of cometary astronomy is naturally divided into five major periods, the transitions being marked by important new insights. Before 1600, comets were essentially considered to be meteorological phenomena in the terrestrial atmosphere. Then followed two centuries of mostly positional measurements with emphasis on the motions and the orbits, lasting until the early 19th century, when the era of cometary physics was inaugurated, in particular by the passage of comet Halley in 1835. The next major step forward occurred in 1950 with the sudden emergence of the modern picture of comets as being essentially very old solar system objects from the Kuiper Belt or the Oort Cloud, generally in unstable orbits and intensively interacting with the solar electromagnetic and corpuscular radiation. The American astronomer Fred L. Whipple proposed in 1949 that the nucleus, containing practically all the mass of the comet, is a “dirty snowball” conglomerate of primordial ices and dust. Finally, the space missions to Giacobini-Zinner in 1985 and to Halley in 1986 provided the first in situ observations of comets and dramatically widened our scientific horizon.
Michael, in his closing remarks said, we have learned through better understanding that the heavenly spectacle of comets is for us to enjoy, not to fear.
Michael Mattiazzo is an avid (and successful) comet hunter and observer, and an accomplished photographer. His knowledge of the sky is backed by years of practical observing experience, doing variable star observations and writing occultation reports. In 2003 he was presented the Bill Bradfield Astronomy Award by Bill Bradfield himself, and was President of the ASSA for 2006. Michael’s previous talk at the ASV was in September 2006, when he spoke on ‘Amateur Contributions to Comet Science’. See also his unique Southern Comets Website www.yp-connect.net/~mmatti/ for information about comets you can see at present, helpful observing tips and some great photos.