Development of Cometary Thought

Development of Cometary Thought

PART - II

Subodh Mahanti

On the other side,Incensed with indignation, Satan stood Unterrified, and like a comet burned, that fires the length of Ophiuchus huge. In the arctic sky, and from his horrid hair. Shakes pestilence and war

John Milton ( 1608-74) in Paradise Lost.

As it was mentioned in the first part of this article the first work on the comet of 1577 to be published was that of Mastlin, whose observations showed the comet to be a celestial object. Mastlin assumed that the comet moved on a circular path (the true orbit for this comet was parabolic) slightly outside the orbit of Venus. Though Mastlin's observations indicated the celestial nature of the comet but he himself believed it to be a new and horrible prodigy. He discussed its portents in detail. Mastlin believed that while the mysterious nature of the comet was beyond the comprehension of human but once they are created (by God) they became celestial phenomena and they could be treated accordingly.

Nicholaus Copernicus

Nicholaus Copernicus (1473-1543) observed the comet of 1533. He wrote a brief treatise about this but it was published posthumously in 1878. In his magnum opus, De revolutionibus (1543), Copernicus mentioned comet only once. He also considered comets as terrestrial objects and from their motion he concluded that the highest region of the air follow celestial motion. He wrote : "It is said _ that the highest region of the air follows the celestial motion. This is demonstrated by those stars that suddenly appear_I mean those stars that the Greeks called cometae or poganiae. The highest region is considered their place of generation, and just like other stars they also rise and set. We can say that this part of the air is deprived of the terrestrial motion because of its great distance from the Earth".

Tycho Brahe

Tycho Brahe (1546-1601), whose systematic and meticulous observational data enabled Johannes Kepler to formulate his planetary laws, wrote a German treatise on the comet of 1577, which he first noticed on November 13, 1577, and continued to observe for the next two and a half months. This important treatise though written in 1578 was first published by Johann Louis Emil Dreyer (1852-1926) in 1922. Before this it existed only in the form of two manuscripts. This was translated into English in 1979 by J.R. Christianson. Tycho rejected the Aristotelian notion of comets by stating that the comet was certainly above the Moon's sphere. However, Tycho also believed in astrological implications of a comet. He wrote : "Although this comet appeared in the west and will realise its greatest significance in those lands that lie toward the west, yet it will also spew its venom over those lands that lie eastward in the north, for its tail swept thence". Tycho was the first to suggest that a comet's orbit may not be circular.

Johannes Kepler

At the beginning of seventeenth century the telescope was first used for cometary observation. Though this was a significant forward step in the development of cometary science its development was eclipsed by the backward views one comet held by Galileo and Kepler, the most influential astronomers of the period. Johannes Kepler (1571-1630), the founder of celestial mechanics, firmly believed in the straight line motion of comets. Kepler's ideas on comets consisted of a basic Ptolemaic framework interwoven with his own brand of mysticism. Kepler believed in the ominous significance of comets. According to him the cometary influence was due to a disruption in the sympathy of nature. According to Kepler comets were spontaneously created from impurities or fatty globules in the ether. Kepler also observed that at the time of the creation of the comet, a special spirit and intelligence also formed to guide it. But then on the physical nature of comet, Kepler was much ahead compared to his contemporaries.

On the cometary tail formation Kepler wrote : "The head is like a conglobulate nebula and somewhat transparent; the train or beard is an effluvium from the head, expelled through the rays of the sun into the opposed zone and in its continued effusion the head is finally exhausted and consumed so that the tail represents the death of the head".

Galileo Galilei

Galileo Galilei, (1564-1642), the founder of experimental physics, pointed out that comets were not periodic. He believed that comets originated from the Earth _ cloud of vapour moving vertically upward from the Earth's surface at a uniform rate and in a rectilinear fashion. Galileo ridiculed Tycho's observation on comet.

Although the authoritative views of Galileo and Kepler had adverse effect on the contemporary cometary ideas but notable scientists like Willebrod Snel (1580-1626), Pierre Gassendi (1529-1655), Seth Ward (1617-1689) and Rene du Perron Descartes (1596-1650) advanced their enlightened viewpoints.

Frontispiece from Johannes Hevelius Cometographia.Three allagorical figures showing the Aristotelian idea that comets are sublunar(left) the, the Keplerian notion that comets move on straight line paths(right) and the idea of Johannes Hevelius (centre) that comets originate in the atmosphere of Jupiter and Saturn and move about the Sun on a curved trajectory

Before Newton essentially solved the dynamical behavior of the comets, two schools of cometary thought existed. One school constituting Jean-Dominique Cassini (1625-1712), Andrien Auzout (1622-91), Pierre Petit (Ca. 1594-1677), Giovanni Alfonso Borelli (1608-79) and others believed that comets were permanent celestial objects and hence they had circular or at least close orbits. Johannes Hevelius (1611-87), Christian Huygens (1629-95) and others who constituted second school took comets as transitory objects and their intrinsic motions as uniform and rectilinear. According to Aristotle transitory objects have rectilinear paths and permanent bodies have close orbits. And almost everybody believed Aristotle and so to decide the orbit of the comets (rectilinear or close circular) it was to decide whether comets were transitory or permanent bodies. Newton, based on his three observations of the comet of 1680 and also drawing upon the ideas of Kepler, Johannes Hevelius(1611-87), Robert Hooke (1635-1702) and John Flamsteed (1646-1719) developed a technique for determining the parabolic orbit of the comet. Using his orbit determination technique of the comet of 1680, he successfully fit the observations of flamsteed and others. Newton argued that comets were actually seen in the region interior to Saturn's orbit.

While suggesting this he was perhaps influenced by Descarte's suggestion that comets were located beyond Saturn. Newton suggested that the nucleus of a comet is a compact object whose light is derived from the Sun. While not dismissing Kepler's notion of tail particles carried along by the action of solar rays suggested his own theory for the formation of the tail of the comet of 1680. He suggested that it was a very fine vapour which cometary nucleus emitted after being heated near perihelion. While Newton successfully solved the dynamical behavior of the comets (which was subsequently refined) the physical nature of comets remained obscure. Using the modified Newton's method Halley computed parabolic orbits for 24 well-observed comets and he predicted the return of the comet of 1607.

Edmond Halley's ideas that comets were bound to the Sun and return after very long periods of time were expressed in his 1705 work but for the next two hundred years there was no definite clue about the birthplace of the comets. He had also demonstrated that none of 24 comets for which he computed orbits had obvious hyperbolic motions.

In 1755 Immanuel Kant (1724-1804), the German philosopher, published his cosmology. Kant envisaged the solar system as a part of a vast system of stars making up the Galaxy. According to Kant comets also formed with the planets from the solar nebula. But the comets formed at the much greater distances from the sun. As the comets formed in the remotest regions of the solar system they were composed of the lightest particles and this also explained their vapour and tails. The importance of kant's work lies in the fact that he anticipated though in a qualitative way the nebular theory of Pierre Laplace and after it is referred to as the Kant-Laplace theory. Unfortunately in the eighteenth century kant's book was not available as the many of the freshly printed copies were impounded after the printer went bankrupt. Laplace in his Exposition of the system of the world published in 1796 proposed his nebular hypothesis. Laplace argued that the solar system evolved from the rotating mass of a gas that had condensed to form the Sun. As the rotating solar nebula flattened and formed rings of material as it contracted which eventually coalesced into planets. In a similar fashion the extended rotating atmospheres of protoplanets themselves formed rings of material that eventually became their satellite system. However, Laplace's nebular theory soon ran into trouble with the discovery of retrograde orbits in the solar system.

Christian Huygens (portrait kept in Paris observatory)

Sir Frederick William Herschel (1738-1822) who expressed his cometary ideas in 1808 and 1812 (and which were published in the philosophical Transactions of the Royal Society of London), considered comets objects of interstellar origin. While the comets travelled through interstellar space they collected nebulous matter and transferred it to stars they passed, thus replenishing the fuel used in making light. As the comets grew older they lost most of their nebulous matter and produced much shorter tails. As the comets pass from one stellar system to another they become more consolidated and dense. This way a comet loses all of its nebulous materials and form a planet.'

The extremely eccentric and highly inclined orbits of comets could not be fitted into Laplacian hypothesis. Taking note of Heinrich Olbers' (1758-1840) idea that the asteroids originated from a fragmented planet between Mars and Jupiter, Comte Joseph Louis Lagrange (1736-1813) decided to investigate the conditions under which explosive events on the planets would lead to produce bodies travelling on a comet-like orbit around the Sun. He came up with formula giving the velocity required to eject a body into a comet-like orbit at any given inclination. Lagrange thought bodies could be ejected from the planet's surface due to extreme pressure from the hot interior.

Leonhard Euler as he appeared on a Swiss 10 Franc note

In the eighteenth century scientists focussed their attention on the dynamics of comets. The techniques for computing cometary orbits, originally initiated by Isaac Newton and Edmond Halley, were successfully developed by Rudjer J.Boscovic (1711-87), Leonahrd Euler (1703-83), Achille-Pierre-Dionis du Sejour (1734-94), Pierre -Simon Marquis de Laplace (1749-1827), Heinrich Wilhelm Matthias Olbers (1758-1840) and Carl Friedrich Gauss (1777-1855). The general belief of the astronomers was that the comets were solid, permanent, celestial bodies whose motion could be followed by the application of the same Newtonian mechanics as applied in the case of planets.

The nineteenth century astronomers also concentrated their attention on the dynamics of comets. After the return of comet Halley the next successfully predicted return was comet Encke named after its discoverer Johann Franz Encke (17980-1865), in 1822, followed by comet Biela named after its discoverer Wilhelm von Biela (1782-1856) in 1832. The orbital size and period of encke's comet found to be decreasing with time, thus it deviated from purely Newtonian motion. Only a force acting against its orbital motion can decrease the period of a comet. Encke believed that the opposing force resulted from the friction of comet moving through a resisting medium surrounding the Sun. So while Encke's comet revealed a dynamic behaviour which was inconsistent with Newtonian mechanics the comet Biela which disappeared after disintegrating showed that comets were not permanent celestial bodies. The comet Biela was observed to split into two pieces in 1847. The double comet reappeared in 1852. The connection between cometary debris and meteor shower became obvious in the nineteenth century.

Friedrich Wilhelm Bessel (1784-1846) based on his observation of sunward jets or emanations from the nucleus of comet Halley in 1835 suggested that reactive force acting on the comet itself could reduce its period with time.

Laplace expressed his views on the origin of comet in 1813. Like Herschel, Laplace also subscribed to the theory of interstellar origin of comets. According to Laplace comets existed in an interstellar field, outside the sphere of influence of the Sun. Laplace calculated the sphere of influence of the Sun as 100,000AU. The velocities of comets may range from zero to infinity, each velocity being equally probable. As the field of interstellar comets moved pass, the Sun attracted comets whose velocity was nearly zero. While most of the comets would reach the sun on nearly parabolic orbits but in few cases the size cometary orbits may decrease because of planetary perturbations or some resisting medium. Thus Laplace had anticipated the mechanism by which near parabolic path of comet become short-period orbits. Laplace did not take into account the Sun's motion with respect to the interstellar stars and comets. In 1783 William Herschel had shown that the stars are in motion with respect to one another. Herschel not only assumed that the Sun had a so-called proper motion with respect to other starts, but he also correctly anticipated that the Sun was moving towards a point in the constellation Hercules. Ernst Julius Opik (1893-1985), the Estonian astronomer, based on his study of the orbits and perturbations of comets predicted in 1932 that comets would remain bound to the sun at distance of one million AU. Opik's work demonstrated the feasibility that comets may reside in a cloud extending to interstellar distances and they would bound to the Sun even after taking into account the perturbing effects of the passing stars.

Heinrich Mattias Olbers

The English astronomer Raymond A. Lyttleton published one of the few theories put forward during the period 1948-51 that attempted how and where comets formed. Lyttleton argued that comets were formed when the sun passed through an interstellar cloud of dust. Sergey Vsekhsvyatskij, the Russian astronomer, resurrected Lagrange's hypothesis. In 1930 and 1931 Vsekhsvyazskij put forward the idea that major planets were the source of comets. Then in 1951 Vsekhsvyatskij argued that the satellite systems of the planets mainly of Jupiter as the better sources of comets. This is because the escape velocity from those smaller satellites is only a fraction of that is required to escape Jupiter. According to him while short period comets continue to form but long period comets formed millions of years ago during cataclysmic eruptions from the outer, major planets.

In 1948 Adrianus Jan Jsper Van Woerkom brought out the inconsistencies with the picture of interstellar comets being captured into long-period comets by repeated interaction of Jupiter. He discussed the objections to the interstellar origin of comets in detail and while doing so he casually pointed out that a cloud of comets moving permanently with the Sun may be free from some of the objections raised by him.

In 1950 the Dutch astronomer Jan Hendrik Oort (1900-92) in a paper entitled 'The structure of the cloud of comets surrounding the solar system and a hypothesis concerning its origin' and published in the Bulletin of the Astronomical Institute of the Netherlands concluded that the observed distribution of cometary orbits could be explained by assuming a cometary cloud of some one hundred ninety billions comets orbits the Sun at a distance of between 50,000 to 150,000 AU. The cloud is not free from the effects of passing stars. In 1951, that is one year after the publication Oort's hypothesis about the formation of comets, Gerard Peter R. Kuiper (1905-73) pointed out that comets were likely to be composed of icy materials but if comets formed as close to the Sun as the asteroid belt then ice could not be a chief constituent of comets. He was of the view that comets formed between 35 and 50 AU from the Sun and they were the condensation products of the outer solar system. Oort's work brought nearly universal agreement about the source, if not the origin of comets.

Pierre-Simon Marquis de Laplace

Carld Friedrich Guass

Johnn Franz Encke

Friedrich Wilhelm Bessel

Wilhem Von Biela

Here we summaries the ideas on different aspects of comets that are generally accepted.

A comet is a small body composed of ice and dust. The cometary ice is frozen water, plus some methane (CH4), carbon monoxide (Co), and carbon dioxide (Co2). Other Carbon containing compounds detected in comets are : formaldehyde, (H2CO), hydrogen cyanide (HCN) and methyl cyanide (CH3CN).
The nucleus of the comet, a small solid body, composed of frozen water and gas plus embedded dusty material at the centre of a comet's head contains essentially all the mass of the comet. It is the source of activity in a comet. When very distant from the Sun, a comet's nucleus is inert. However, within 3-4 AU solar heating leads to gas sublimation and production first of a coma, then a tail
The part of a comet containing dust and gas released from the comet's nucleus is called cometary tail. Tails are always directed away from the Sun. Tails do not normally develop until a comet is within about 2AU of the Sun. Comet tails have two components. The type I or gas tails consist of ionised gas carried away from the head of the comet by the solar wind. Dust tails can reach 108 km or more in length. The type II or dust tails consist of micrometre size solid particles which are pushed away from the head by radiation pressure on hyperbolic trajectories. Dust tails are usually shorter but still can reach 107 km.
The envelop of gas and dust that surrounds the solid nucleus of an active comet is called cometary coma. The coma often appears as tear drop being largely shaped by the solar wind flowing around the comet. The coma form when the comet is within 3-4AU of the Sun. Near perihelion the coma may be 100,000 km wide.
Comets were formed in the region of Uranus and Neptune. They were formed as the icy debris from the outer planet formation process. They are believed to be left over from the formation of the outer planets.
The primordial comets, which are on near-circular orbits, comprise the Kuiper belt.
Long - term planetary perturbations increase the ecentricities of the orbits of the primordial comets.
When their perihelia are raised well beyond Neptune's orbit the comets are placed in the inner comet cloud which may begin just beyond the orbit of Neptune and merge into the Oort cloud. Once the comets are in the inner comet cloud they are immune from planetary perturbations.
Some inner cloud comets are eventually lifted into the outer Oort cloud by continued galactic plane perturbations that is the perturbation due to an unequal gravitational attraction of the galactic disk on the Sun and a particular comet.
From the outer Oort cloud comets can be striped away by galactic tide perturbations and close stellar encounters. In fact it is assumed that more than 50 per cent of the comets present at the time of formation of the Solar system have been lost from the Sun's sphere. It is assumed that presently there are approximately 1011 to 1012 comets in the outer Oort cloud.
The inner Oort cloud of comets or simply the inner cloud of comets begins with a flattened disk of comets just beyond the planetary region and it extends to approximately 10,000 AU. The long axis of the outer Oort cloud which is directed towards the centre of the galaxy is roughly 200,000 AU from end to end. The shorter axis is approximately 160,000 AU wide.
There is no definite idea about the relative positions of the inner and outer comet clouds. In fact there is little more than hints that the inner cloud exists at all.
It is likely that the comet belt, as well as the inner and outer comet clouds have merged to form one continuous distribution of comets extending beyond Neptune's orbit to the very edge of the solar system.
Comets thrust towards the planetary system from the outer Oort cloud have long-periods or nearly parabolic orbits.
The source of short-period comets may be a flat belt of comets just beyond the planetary system.
Currently around 900 comets are known of which 75% are long periods comets.
No comet with definite hyperbolic orbit has been observed. It tends to indicate that comets are not now arriving from interstellar space.
Observed long-period comets do not evolve into observed short-period comets and the reverse is also not true.
During their passage through the inner Solar System comets can have their orbits altered by gravitational influence of the planets notably Jupiter. One spectacular example was comet shoemaker Levy, which hit Jupiter in 1994.
The ultimate fate of the observed comet is not well understood. It can not be said with certainty that the comets eventually end up in inactive asteroid like bodies. Many long-period comet are hurled into interstellar space and some short-period comets disintegrate into dusty debris.

Immanuel Kant

Ernst Julius Opik

Jan H. Oort

Today we know a great deal of comets but there are many things about them yet to be known . To quote Lucius Annaeus Seneca (4B.C.-AD 65). "The day will yet come when posterity will be amazed that we remain ignorant of things that will to them seem so plain. Men will some day be able to demonstrate in what regions comets have their paths, why their course is so far removed from the other stars, what is their size and constitution. Let us be satisfied with what we have discovered and leave a little truth for our descendents to find out'.

For further reading

1. Arago, Dominique F. 1861. A popular Treatise on Comets. London : Longman, Green and Roberts

2. Armitage, Angus. 1966. John Kepler. London: Farber and Farber.

3. Biermann, Ludwig and Lust, Rhea 1963. 'Comets: Structure and Dynamics of tails' in The Moon, Meteorites and Comets. edited by B.M. Middlehurst and G.P. Kuiper. Chicago University of Chicago Press.

4. Burke, John G. 1986. Cosmic Debris : Meteorites in History. Berkeley : University of California press.

5. Carusi, Andrea, Valsecchi, Giovanni B. eds. 1985. Dynamics of Comets: Their origin and Evolution. Dordrecht, Netherlands: D.Riedel.

6. Chambers, George F. 1909. The Story of Comets Simply Told for General Readers. Oxford : Clrarendon Press.

7. Delsemme, Armand H.1989. 'Whence Come comets' Sky and Telecope 77: 260-264.

8. Gillispie, Charles C., ed., 1970-1980. Dictionary of Scientific Biography. 16 volumes. New York: Charles Seribner's.

9. Hellman, Clarice Doris. 1971. The Comets of 1577: Its Place in the History of Astronomy. Reprint of the 1944 edition with addenda, errata and a supplement to appendix. New York : AMS Press

10. Jervis, Jane L. 1985. Cometary Theory in Fifteenth Century Europe. Dordrecht, Netherlands : D.Reidel publishing Co.

11. Johnson, Francis R.1968. Astronomical Thought in Renaissance England. Reprint of 1937 edition. New York: Octagon Books

12. Needham, Joseph. 1559. Science and Civilisation in China, Vol 3. New York : Cambridge University Press.

13. Newburn, Jr., Ray L., Neugebauer, Marcia M. and Rahe, jurgen. eds. 1991. Comets in Post Halley Era Dordrecht, Netherlands : Kluwar Academic Publishers.

14. Robinson, James H. 1916. The Great Comet of 1680: A Study in the History of Rationalism. Northfield Minnesota : Northfield News Press.

15. Ronan, Colin A. 1970. Edmond Halley, Genius in Eclipse. London : MacDonald & Co.

16. Thorndike, Lynn. 1923-1950. A History of Magic and Experimental Science 8 volumes. New York: Columbia University Press.

17. Westfall, Richard S. 1980. Never at Rest : A Biography of Isaac Newton. Cambridge: Cambridge University Press

18. White, Andrew D. 1910. A History of the Wartfare of Science with Theology in Christiandom. 2 volumes. New York : D. Appleton and Co.

19. Whipple, Fred L. 1976. Background of Modern Cometary Theory. Nature 263 : 15-19.

20. Wilkening, L.L ed. 1982. Comets. Tuscon : University of Arizona Press.

21. Yeomans, Donald K. 1991. Comets : A Chronological History of observation, Science, Myth and Folklore. New York: John Wiley & Sons Inc.

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Letter to the Editor

I have been reading with much interest your valuable Monthly Newsletter of Vigyan Prasar "DREAM 2047". I have had several copies of the article on Earthquakes photocopied and have circulated then to several teachers and students. The article is so well written. Every reader has not only enjoyed reading it but has learnt much from it.

The second article I have enjoyed is an Bhaba Centre for Science Education. Our Trust runs tutorial classes for 750 children of Government Schools (education slums) located in the most down trodden areas of Mysore City. The results of this supplementary education has proved most gratifying. We have not only found blossoms in the dust but the school attendance has gone up from 29% to 89%. We have made every effort to raise the knowledge base fo our teachers but have helped them to create a vision to generate creativity among children. It we can get the address of Homi Bhabha Centre for Scientific Education we would like to get their literature on how the inculcate secular scientific knowledge among children to emancipate them from rituals, superstitions and obscurantism of which our society is a victim. Kindly send us their address.

Dr. H.A.B. Parpia

Formely Director of central Food Technological Research Institute, Mysore

cwhabp@bgl.vsnl.nen.in