Date: Mon, 7 Jun 93 05:14:46 From: Space Digest maintainer Reply-To: Space-request@isu.isunet.edu Subject: Space Digest V16 #693 To: Space Digest Readers Precedence: bulk Space Digest Mon, 7 Jun 93 Volume 16 : Issue 693 Today's Topics: Electronic Journal of the ASA (EJASA) - June 1993 [Part 1] Welcome to the Space Digest!! Please send your messages to "space@isu.isunet.edu", and (un)subscription requests of the form "Subscribe Space " to one of these addresses: listserv@uga (BITNET), rice::boyle (SPAN/NSInet), utadnx::utspan::rice::boyle (THENET), or space-REQUEST@isu.isunet.edu (Internet). ---------------------------------------------------------------------- Date: Sun, 6 Jun 1993 19:09:46 GMT From: Larry Klaes Subject: Electronic Journal of the ASA (EJASA) - June 1993 [Part 1] Newsgroups: sci.astro,sci.space,sci.misc,sci.classics,sci.archaeology,alt.sci.planetary THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC Volume 4, Number 11 - June 1993 ########################### TABLE OF CONTENTS ########################### * ASA Membership and Article Submission Information * Marcus Manilius and Ancient Astronomy - Ian Bacon * Perseids 1993: Shower or Storm? - Peter Brown * Cometary Conundrums - M. Leon Knott ########################### ASA MEMBERSHIP INFORMATION The Electronic Journal of the Astronomical Society of the Atlantic (EJASA) is published monthly by the Astronomical Society of the Atlantic, Incorporated. The ASA is a non-profit organization dedicated to the advancement of amateur and professional astronomy and space exploration, as well as the social and educational needs of its members. ASA membership application is open to all with an interest in astronomy and space exploration. Members receive the Journal of the ASA (hardcopy sent through United States Mail - Not a duplicate of this Electronic Journal) and the Astronomical League's REFLECTOR magazine. Members may also purchase discount subscriptions to ASTRONOMY and SKY & TELESCOPE magazines. For information on membership, you may contact the Society at any of the following addresses: Astronomical Society of the Atlantic (ASA) P. O. 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Please send your on-line articles in ASCII format to Larry Klaes, EJASA Editor, at the following net addresses or the above Society addresses: klaes@verga.enet.dec.com or - ...!decwrl!verga.enet.dec.com!klaes or - klaes%verga.dec@decwrl.enet.dec.com or - klaes%verga.enet.dec.com@uunet.uu.net You may also use the above addresses for EJASA back issue requests, letters to the editor, and ASA membership information. When sending your article submissions, please be certain to include either a network or regular mail address where you can be reached, a telephone number, and a brief biographical sketch. Back issues of the EJASA are also available from the ASA anonymous FTP site at chara.gsu.edu (131.96.5.29). Directory: /pub/ejasa DISCLAIMER Submissions are welcome for consideration. Articles submitted, unless otherwise stated, become the property of the Astronomical Society of the Atlantic, Incorporated. Though the articles will not be used for profit, they are subject to editing, abridgment, and other changes. Copying or reprinting of the EJASA, in part or in whole, is encouraged, provided clear attribution is made to the Astronomical Society of the Atlantic, the Electronic Journal, and the author(s). Opinions expressed in the EJASA are those of the authors' and not necessarily those of the ASA. This Journal is Copyright (c) 1993 by the Astronomical Society of the Atlantic, Incorporated. MARCUS MANILIUS AND ANCIENT ASTRONOMY by Ian Bacon with the assistance of Dr. Jane Bellemore Courtesy of Paul Dickson (Dickson@SYSTEM-M.AZ05.BULL.COM), Editor of the Saguaro Astronomy Club's newsletter, SACNews, in Phoenix, Arizona. Marcus Manilius, a Roman poet of the early Imperial period, is one of the least known astronomers of the ancient world. His work, "The Astronomica", is largely derivative, conveying little new information on ancient astronomy. Not withstanding this, Manilius does present areas of interest for scholarship. In this paper will look at his life and work in some detail. In order to do that we shall first place Manilius and ancient astronomy into their cultural backgrounds. All ancient civilizations - the Chinese, Babylonians, the Greeks, and the Romans - had some interest in the sky. This interest ranged from the mundane tasks of maintaining the calendar and assisting navigation to that of astrological predictions. However, it was the Greeks who first applied deductive logic to the examination of the material Universe and distinguished astronomy from astrology. Beginning in the Fifth Century B.C. and continuing to the First Century B.C., the Greeks founded and made large-scale progress in the science of astronomy. This progress included knowledge of the sphericity of the planet Earth, the existence of the Zodiac/ecliptic, the movement of the planets, explanation of and limited prediction of solar and lunar eclipses, not inaccurate measurements of the relative sizes of Earth, the Sun, and the Moon, the creation of a stellar positional system based on the ecliptic (an ecliptic coordinate system with celestial latitude and longitude) with a one to six stellar magnitude system (still with us today), and of course, a calendar. The Greeks established the basics of astronomy. However, they and subsequently the Romans were hampered by their conceptualization of the Universe, that of a "perfect" heaven with perfect circles. This, with an Earth-centered, geocentric Universe, resulted in increasingly complex models to explain planetary behavior. Each planet (and the Sun) was made to move around Earth in a perfect circle. To explain discrepancies between theory and observation, circles were added to circles in an ascending order of complexity. Increasingly complex constructs of epicycles, eccentrics, and equants formed which persisted unchallenged through the European Middle Ages, the Arab world, and into the Sixteenth Century European Renaissance. Greek and Roman science was not only restricted conceptually but also restricted by a low level of technology. There were few instruments and their accuracy was poor. Ancient mathematical systems were limiting. There were two mathematical systems used by Greek astronomers, one of which did not have a place value, nor did it recognize the importance of zero, nor could it manipulate fractions well. The other system, inherited from Mesopotamia and the Babylonians, was a place value system with a zero but a base sixty mathematical system (the basis for our 360 degrees, with sixty minutes and seconds of arc and time). Added to this, there were no aids to calculation; even the humble logarithm was not developed till the Sixteenth Century. These factors placed a limit on scientific progress. This is typified by one of Greece's greatest astronomers, Hipparchus. Living in the Second Century B.C., Hipparchus was essentially the last of the original thinkers of Greek science. Only one of his minor, earlier works directly survives, but his research is much discussed and used by his successors. Hipparchus is known to have constructed the first model of lunar and solar motion based on observational evidence, discovered the precession of the equinoxes, possibly the first to use trigonometry in his calculations, and determined the length of the tropical year to 365.25 days with an error of plus or minus five minutes. Hipparchus had a passion for observational accuracy and the use of mathematics to solve problems. This is demonstrated by his refusal to construct a solar system theory, as he believed there was insufficient data and mathematical tools to do so accurately. However, within their limits, the Greeks made greater scientific progress than any of their cultural predecessors. The Greeks invented the concept of deductive and inductive logic, and rational thought, one half of the scientific method paradigm of theory and empirical research. From this flowed progress in all fields of human endeavor: Science, medicine, mathematics, architecture, and more. As with astronomy, much of this was not equaled until the Sixteenth and Seventeenth Centuries. By the First Century B.C., the Roman world of Italy and western Europe had fallen under the spell of Greek and Near Eastern cultural influences: Upper-class Romans spoke and read Greek, their children studied in Athens, Greek words entered the Latin language, and Greek art and philosophy were necessities in any house of distinction. One of these new Eastern influences entering Rome were the joint beliefs and knowledge of astrology and astronomy. Astronomy was by and large not important to the Romans beyond its ulitarian functions. Even then the Roman calendar was an active example of political neglect until Julius Caesar, returning from Egypt and Cleopatra, corrected it. In the field of astronomical observation and theory, the Romans did little, relying on earlier Greek theories and observations as the basis for any speculation. Astrology played an ambivalent role in the Roman world. It was practiced by many prominent Romans, including the Emperor Tiberius (14-37 A.D.). The Second Century A.D. Roman historian Suetonius, writing in THE TWELVE CAESARS, tells us that the future Roman Emperor Augustus, as a young man, visited an astrologer to uncover his fate: The stars foretold his greatness. Suetonius continues with a similar anecdote in which another astrologer, the Roman Publius Nigidius Figulus, also made the same prediction concerning Augustus. Despite some public acceptance, astrology was harshly dealt with by the Roman authorities when its usage caused public disorder or threatened political security. At various intervals, usually related to political upheaval, astrologers were expelled from Rome. There are nine recorded expulsions of astrologers between 139 B.C. and 93 A.D.. The supposed cause of these expulsions was public disorder caused by their astrological predictions. After the transformation of Rome from a Republic to an Empire, ruled by an Emperor, the rationale behind official action against astrology changed somewhat. Political concern centered around the longevity of the Emperor and his potential successor. In 11 A.D., the Emperor Augustus (in his seventy-fourth year, three years before his death) forbade the prediction of a person's death and published his own horoscope - both, presumably, to minimize speculation as to the time of his own passing. The aim of these expulsions, which also included that of philosophers, mystics, and diviners, was not to discredit or destroy astrology but to curtail the politically significant activities of its practitioners. Part of the official concern was that astrology, and other means of divination, would be used to predict the Emperor's death. This is not to suggest that the Roman culture gave astrology full faith and credence, but a prediction used at the appropriate moment could destabilize an Emperor's position or encourage a revolt. The mere claim that a prominent person had asked these question of an astrologer became grounds for treason. It is important to put ancient astrology into its cultural context. The ancients were not technologically advanced. While many today heap scorn on those who believe and practice astrology in the modern world, we should remember that those in the ancient world lived surrounded by forces of nature that they neither understood nor controlled. We should not criticize the ancient astronomers for their belief and practice of astrology but remember that the great names of modern astronomy - Copernicus, Brahe, etc. - also practiced astrology to pay their bills. It is with this cultural background that we turn to the main topic of this paper, Marcus Manilius. All that we know of Manilius comes to us from his work, a poem explaining the "science" of astrology to his audience. The poem is known as "The Astronomica". It is divided into five "books", essentially chapters, totaling approximately twenty-five thousand words. Books Two through Five discuss astrology in great detail. However, Book One forms an introduction to the later astrological text by providing an astronomically-oriented summary of the heavens. This book tells us something of Manilius' perception and knowledge of the Universe. Unfortunately, while he does describe much in detail, he gives no specific, datable events. What we have gleaned from Manilius' work is that he lived and wrote in the reigns of Augustus, the first Roman Emperor, and Tiberius, Augustus' successor. Their combined reigns covered the period circa 30 B.C. to 37 A.D.. Manilius evidently had a good education and the leisure and income to spend years pursuing his goal. His motivation for writing "The Astronomica" is as hard to fathom. He may have felt he had a message to give to the Roman world. It is not known if Manilius wrote other works since none appear to have survived. It is also unknown how successful his work was to his contemporaries. Internal evidence suggests that Manilius' wrote between circa 10 to 20 A.D.. One translator of Manilius, G. P. Goold, gives us indications that contemporary authors may have used Manilius as a minor source. There is no major reference to his work until the Fourth Century. He was not considered one of the great authors of antiquity. After the Fifth Century A.D., Europe entered the early Middle Ages, where little or no science of any description was practiced. During this period, "The Astronomica" languished in monasteries. In the Tenth Century, "The Astronomica" was mentioned twice, once by the Roman Catholic Pope Sylvester II and once in a catalogue of books. There are now about twenty copies in existence preserving three different versions of the original text, all with minor gaps. These texts date from the Eleventh and Fifteenth Centuries, when they were themselves recopied from earlier works. It has been the work of classicists in the Nineteenth and Twentieth Centuries to analyze and produce a reliable version of Manilius' original work. The time in which Manilius wrote was the culmination of a tumultuous period of Roman history: The transformation of the Republic, a landed oligarchy (with democratic elements), into the autocratic Roman Empire, ruled by an Emperor. This transformation was implemented at great cost in human life and suffering. Manilius and his contemporaries looked back upon this period before the Empire as a period of social decay and anarchy. They feared its return and considered Augustus and the Imperial system he created their protection against its return. Manilius supported the Imperial regime created by Augustus. His work is dedicated to Augustus and where possible takes the opportunity to praise the accomplishments of Rome's first Emperor. In addition to being an astrologer, Manilius was an adherent of Stoic philosophy. Stoicism had been founded in Athens four centuries earlier. It came to Rome during the Second Century B.C., finding acceptance amongst a portion of the Roman population. Its credo of duty to the state and personal morality found appeal amongst a population undergoing cultural and political transformation. The full social ramifications of Rome's conquest of the Mediterranean in the Second Century B.C. affected Roman society. Later, during the Imperial period, Stoics were, by and large, passively supported by the state. One of Stoicism's tenets was acquiescence to authority - a worthy principle in an autocratic state. Other Stoic tenants perhaps held less appeal, since Greek Stoicism contained a theoretical belief in sexual equality and the abolishment of slavery. Even the Greeks observed these more in the breach than the observance. The Romans adapted Stoicism to their own more practical culture. Stoic philosophy provided a guide to all aspects of life and thought. Within this design there was an astronomical belief system. Stoics believed in a "living", cyclic Universe that periodically destroyed and then reconstituted itself. The "elements" of fire, air, water, and earth formed the basis of this Universe. God, as a rather abstract concept, permeated and was part of the entire Universe. Earth was the center of the Universe with the Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, and the stars (in that order from our planet) revolving in perfect, circular paths around Earth. Due to the interrelationship between all components of the Stoic Universe, a change in one part was thought to cause a change in another. This axiom gave astrology a rational basis: A change in the heavens foretold a change on Earth. Manilius wrote his astrological work from a Stoic perspective. In creating his work, it seems that Manilius was in part refuting a rival philosophy to Stoicism, that of Epicureanism. Founded by the Greek philosopher Epicurus three centuries earlier, Epicureanism was a philosophy oriented less towards disciplined service to the state than towards a life of self-fulfillment and enjoyment. In one letter, Epicurus wrote that "We say that pleasure is the beginning and end of living happily". This found less acceptance in the Imperial period where unstinting (and unquestioning) service to the state was viewed with favor. Writing approximately fifty years earlier than Manilius, the Roman poet Lucretius - himself an active practitioner of the Epicurean lifestyle - expounded this philosophy to the Roman audience in his work, "De Rerum Natura" (Concerning the Nature of Things). Two of the prime axioms of Epicureanism that reach us in the surviving work of Lucretius are that the Universe does not have a divine origin and that reason alone must be used to examine the Universe. These, and an "atomic" theory (that all matter is composed of small, eternal, and indivisible particles), endeared this materialistic philosophy to scientists of the Seventeenth Century A.D. and onwards, while creating continual polemic with the Stoics of antiquity. There are a number of passages by Manilius which seem to refute precise passages written by Lucretius. There are also similarities in the two works as to the layout of some arguments. It is a reasonable assumption that Manilius identified arguments in Lucretius and set out to discredit them in his own work. The slow pace of philosophical debate in the ancient world makes a fifty-year exchange of academic criticism possible. By praising Augustus and criticizing Epicureanism, Manilius is playing his part in the maintenance of the Imperial order, and countering a rival philosophy. There are parallels between Manilius and another earlier author, Aratus of Soli, a Greek Stoic poet who wrote an astronomical poem, the "Phaenomena", circa 275 B.C.. His work gives astrology a minor role. What makes Aratus' work of particular interest is that it is allegedly based on the work of one of the greatest Greek astronomers, Eudoxus of Cnidos (circa 390 to circa 340 B.C.). Regrettably, none of Eudoxus' works survive directly. We rely on the reporting of others for information on his accomplishments. Aratus' work was popular in ancient Rome. There are four extant translations of it from Greek into Latin. As a noted Stoic author, it is not surprising that Manilius used Aratus as a template for his own work. Proof that he assiduously followed the format of Aratus is his continuation of an erroneous point made by his Greek exemplar. While describing the celestial poles, Manilius refers to them as "fixed and unchanging". This is also the description given by Aratus. In circa 140 B.C., the Greek astronomer Hipparchus discovered the precession of the equinoxes, invalidating Aratus' earlier statement, but Manilius had not incorporated this discovery into his own work. What this lapse tells us is uncertain. It must be remembered that Manilius was writing an astrological work of art, not an astronomical treatise. It is not impossible that he simply disregarded the empirical materialist Hipparchus as irrelevant to the true aim of his work and followed the popular Stoic Aratus. In a similar fashion, modern astrologers still use a geocentric Universe essentially unchanged from the time of Manilius as the basis for their predictions. It is reasonable in any case to conclude that Book One of Manilius' work, which is a astronomical introduction to the full work, is based on the work of Aratus. Manilius' work is aimed at a sophisticated audience. It is not a practical "hands on" guide to casting a horoscope but the equivalent of an academic discussion of astrology. In this it seems to follow the imperial aversion to predictive astrology. Manilius begins Book One by telling his readers about the background of astrology: How men in "primitive" times did not know the "why" of the Universe but after careful research knowledge increased until the peak of learning was mastered - that of astrology. Manilius firmly identifies with astrology and regards astrology as the culmination of human scientific progress. Manilius next describes various cosmological theories, asking: Does the Universe have a beginning or end; was it born from chaos; is it composed of atoms or of the four elements (earth, water, air, and fire)? His greatest effort is reserved for a detailed explanation of his own Stoic philosophy. There is a curious parallel between these theories and modern cosmological thought. However, rest assured, that any perceived similarity between Manilius' suggestion that the Universe has no beginning nor end and the Steady State Theory is purely coincidental. In Book One, Manilius is at pains to prove to the reader that Earth is a sphere. He mentions Earth's shadow during lunar eclipses. He particularly notes the star Canopus, which was not visible from Rome but visible further south from Rhodes. This continued emphasis on the spherical Earth suggests a hidden motive. Lucretius wrote that a spherical Earth where people and animals walk about "upside down" was ridiculous. In this case Manilius refutes Lucretius and Epicureanism. It is also reasonable to assume that he was attempting to prove the fact of a spherical Earth to a skeptical lay audience. Next the constellations were described. The constellations we possess today are those known by the Greeks and Romans. However, the ancient constellations did not cover the entire sky. Some stars were outside any constellation. Such stars were "associated" with a neighboring constellation. Manilius begins his description of the constellations with the Zodiac, continues with the northern constellations, and then concludes with the southern. Manilius lists forty-six constellations, two of which are errors on his part. The furthest south of these is Centaurus. At the end of his list of constellations, Manilius states that "they are the roof of the Universe". The stars, in his opinion, formed the sphere most distant from Earth. They were at the "top" of the sky. In Manilius' time there existed a sky slightly different from our own. Two thousand years of precession lies between us. For Manilius the first point of Aries was in Aries. The north celestial pole lay near Beta Ursa Minor, twelve degrees from its present position near Polaris. Polaris was just another second magnitude star. From southern Egypt the Southern Cross was visible just above the horizon. Manilius relates how seafarers, particularly Phoenicians (the ancient inhabitants of what is now Lebanon), used both Ursa Minor and Major (the Little and Big Bears) for navigation, but preferred Ursa Minor as it was the more accurate, though fainter, guide. Manilius does refer to the theoretical existence of southern constellations. This is partly given as additional proof of the sphericity of Earth. He tells us that the southern sky will be filled with "ordinary" constellations and not strange or unusual figures. It is Manilius' belief that the southern sky has constellations similar to the northern. He suggests this for he believes the two halves of the heavens should "balance". It may be that he is striving to suggest to his readers that the southern sky is essentially the same as the northern. At this point in history no living Mediterranean dweller had reported seeing the far southern sky. As always, the great unknown generated fantasies of mythical existences which Manilius tried to refute. Manilius' description of the constellations varies. Some are merely listed ("...these the Crab follows, then the Lion, then the Virgin...") while others, usually the brighter and larger ones, are described in more detail ("...Orion may be seen stretching his arms over a vast expanse of sky and rising to the stars with no less huge a stride. A single light marks Orion's head, which is impeded in high heaven with his countenance remote. It is Orion who leads the constellations as they speed over the full circuit of heaven..."). Many constellations have a description of their mythological background. It is clear from this form of description that Manilius was not attempting to write a quantitative catalogue of stellar positions. If we look at the stellar catalogue of Ptolemy Claudius, written almost two centuries later, we see a list of stars containing celestial latitude, longitude, and a visual magnitude. Manilius' work is very different. As a philosophical and poetical artist writing at a conceptual level, Manilius may have regarded such formal enumeration of the sky as irrelevant to this work. A curious lack in Manilius is the limited discussion of the role of the planets. The planets play a large role in astrological prediction, both in modern and ancient astrology. Yet Manilius mentions them only in passing, in the context of discussing other celestial phenomena. This gap in Manilius' work has been used to suggest that part of it is missing, that it was not completed, or that possibly Manilius attributed a lesser role to the planets. This question deserves further research. Manilius does give some information on the planets. He states the relative distance of the planets: Furthest from Earth is Saturn, followed by Jupiter, Mars, Venus, Mercury, and the Moon. Planetary retrograde motion is described as strange and "backward moving stars (planets)". Manilius continues his narrative by describing the circles of the sky. Into this category he places the Zodiac, the Celestial Equator, the Milky Way, the Tropics of Cancer and Capricorn, and the Arctic and Antarctic Circles. In his description of the Zodiac we receive a catalogue of the constellations and prominent stars. The Zodiac is "from which the whole scheme of destiny is derived", a clear indication of its importance to an astrologer. On several separate occasions, Manilius refers to the Zodiac as having "twice six" constellations. This form of enumeration harks back to the Babylonians, demonstrating the ultimate source of Manilius' information. In the later books, describing astrological calculations in detail, the zodiacal constellations receive a far more in-depth appraisal. Lastly, Manilius describes comets. Comets were considered evil and bringers of misfortune in ancient and Medieval society. The Roman author and Stoic Seneca, writing fifty years after Manilius during the reign of Emperor Nero, devotes a book/chapter of his "Natural History" to comets. He reports how comets are associated with Earthly disaster. Even English author William Shakespeare's tragedy "Julius Caesar" mentions the ill effects of comets. Caesar's wife advises her husband, on the Ides of March (March 15), that comets foretell "the death of princes". Caesar was murdered that day in 44 B.C.. Manilius tells us of the various theories of the origin of comets: That of Aristotle where comets are inflammable Earth vapors ignited by dry air; of Diogenes Apolloniates, where comets are wandering stars; or the gods' means of warning humans of impending tragedy. The last is also Manilius' belief. One very interesting point to be noted from book one of "The Astronomica" is a reference to the color of the star Sirius, the Dog Star. A question as to the ancient color of Sirius is occasionally discussed by modern astronomers. Due to its brightness, Sirius received a degree of popular prominence in ancient society. As its helical rising occurred in mid-July, it came to be associated with summer heat. There are references to Sirius scorching fields, causing rabies (a "burning" disease), and being the bane of farmers. In reality, the star was over eight light years from Earth. Evidently Sirius' helical rising was regularly observed, for its appearance allegedly foretold the health of the coming year. Due to scintillation, Sirius' appearance at these times was that of a bright, flickering red star. This visualization of a "red" Sirius entered into the popular consciousness. A number of ancient authors, including Homer, Horace, Cicero, Seneca, and Ptolemy, have called Sirius red in their works. Manilius, however, clearly called Sirius "blue-white" - its true color. Manilius also provided us with an outline of Roman Stoicism and Roman astrology. Astronomy places a secondary role. In "The Astronomica", we uncover a different version and interpretation of ancient astronomical and astrological thought, rather than new knowledge. If Manilius' work had not survived, little would have been lost. This and his Latin have meant that his work have been little examined by classicists. However, there are so few astronomical works surviving from the ancient world that each must be closely examined to wring from it what information we can. After Manilius there were few remaining classical astronomer of note until the surviving work of Ptolemy Claudius, circa 150 A.D.. His work, while of questionable veracity and partly astrological, summarized the astronomy of the ancients and was the basis for Medieval Arabic astronomy. Ptolemy's work was not superseded until Copernicus and Kepler. The failures of Manilius as an astronomer in themselves tell us to what standing astronomy had declined from the earlier Greek period. Manilius can at least reflect on the work of other, more prominent authors of his period. Based upon my investigations into Manilius, I feel that he was at least an active observer of the heavens. His work, while not great in itself, has some merit and worthwhile information. It represents a large expenditure of time and energy. Its scope and complexity required many years of observation and composition. His work also allows us to better measure the works of other classical scientific figures. Further Reading - General Discussions of Ancient Astronomy: A well quoted authority is A HISTORY OF ASTRONOMY, by A. Pannekoek, Interscience Publishers, Inc., 1961. A more exacting look at ancient astronomy comes from THE EXACT SCIENCES IN ANTIQUITY, by O. Neugebauer, 1957. A detailed look at the "scientific" era of ancient astronomy can be found in EARLY GREEK ASTRONOMY TO ARISTOTLE, by D. R. Dicks, Thams and London, 1970. For a "light" general text, look at ASTRONOMY OF THE ANCIENTS, edited by K. Brecher and M. Feirtag, MIT Press, 1980. For those interested in reading further on Greece and Rome: FROM SOLON TO SOCRATES, by Victor Ehrenberg, Methuen and Co. HISTORY OF ROME, by Michael Grant, Weidenfeld and Nicolson. An introduction to Stoic philosophy as it relates to astronomy can be found in THE STOIC TRADITION FROM ANTIQUITY TO THE EARLY MIDDLE AGES, Colish, ML., pub. E. J. Brill, 1985. For a description of the Epicurean scientific philosophy, read Lucretius' "De Rerum Natura", translated by W. Leonard, and S. Smith. For a critique of Ptolemy's work, read THE CRIME OF PTOLEMY CLAUDIUS, by R. Newton, The John Hopkins University Press, 1977. Related EJASA Article - "Astronomy in Ancient Mesopotamia", by Stacey Abrams - September 1991 About the Author - Ian Bacon is a Masters student in Classics at the University of Western Australia. His Master's topic is an examination of Manilius from an astronomical viewpoint. Ian possess degrees in computer science, information technology, and Classics. Ian has worked at the Perth (Australia) Astronomical Observatory and as a computer programmer. Taking time to complete a Classics Masters is a long- term ambition of Mr. Bacon and one he thoroughly enjoys. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Ian Bacon Department of Classics & Ancient History Masters Student University of Western Australia Email: ibacon@uniwa.uwa.edu.au Nedlands, Australia, 6009 P.O. Box 166, Scarborough, Australia, 6019 fax: +61 (09) 380 1009 +61 018 950 126 (private mobile telephone) voice: +61 (09) 380 2165 Disclaimer: "What, take my comments for those of the University? Ha, ha, ha, ha, haaaaaaa!" "He will never amount to anything." - School report on Albert Einstein -=-=-=-=-=-=-=-=--------------------------------------------=-=-=-=-=-=-=-=- PERSEIDS 1993: SHOWER OR STORM? by Peter Brown Courtesy of Paul Dickson (Dickson@SYSTEM-M.AZ05.BULL.COM), Editor of the Saguaro Astronomy Club's newsletter, SACNews, in Phoenix, Arizona. After many years of speculation, the parent comet of the Perseid meteor stream returned to the neighborhood of the Sun in the last months of 1992. The original predictions for the comet placed its perihelion passage in the first years of the 1980s. Much anticipation surrounded this event and many people reported noticeable increases in Perseid activity, particularly in 1980. In all likelihood, the returns around 1980 were ordinary. The few observers who noted high activity became the "standard" quoted ZHRs (Zenith Hourly Rates) for many years and therefore a self-fulfilling prophecy developed with respect to high Perseid activity. In addition, these years still saw vastly different methods of reduction and analysis of visual data, so that comparisons between different groups and even individuals with varying perceptions were unrealistic. The returns after 1980/1981 were generally quoted as weaker in activity in direct proportion to the interest in the stream and the belief that comet P/Swift-Tuttle had arrived unseen or not at all. Beginning in 1988, the International Meteor Organization (IMO) implemented global analysis of the stream using standardized reduc- tion techniques and from data with uniform collection parameters. Additionally, the IMO introduced computer-calculated ZHR procedures, allowing flexibility to researchers in choosing the methods of reduction and therefore permitting accuracy checks of the final results. This initial global analysis produced a surprise: A double maxima! The result was widely criticized, for the statistical significance of the new structure could not be objectively determined and the reduction procedures, though based on the best available techniques at the time, were still somewhat new. The new peak appeared some twelve hours before the "normal" Perseid peak. Initial explanations ranged from differences in perception between different groups of observers to a simple statistical "blip" in the data. Considering that more than 53,000 meteors had been used in the analysis, the latter explanation seemed doubtful. Then the same double peak structure was found in the 1989 data at the same location, again separated by twelve hours from the primary "stable" maximum. The data from 1989 were even higher quality than in 1988, with about the same number of meteors. The analysis techniques had been refined through experience with other shower global analysis and the conclusion seemed inescapable: A double peaked structure for the Perseids existed. The double peak profile had not been conclusively observed previously and a few explanations for the structure were given. The newer peak was felt to consist of younger particles than the main peak some twelve hours later (a conclusion that was to ultimately prove true). The authors of the 1989 analysis speculated that the material might be from a passage of P/Swift-Tuttle in the early 1980s. The 1990 return was destroyed by the full phase of the Moon and no reliable analysis could be attempted with such bad data. The next year, observers around Earth had been alerted to the possibility of enhanced activity due to the new peak some twelve hours before the main maxima. In 1991, the new peak would favor observers in Japan - and favor them it did! The Japanese observers witnessed one of the strongest displays of the Perseids in the last century with ZHRs over four hundred. This was clearly stronger activity than had been witnessed in the past few returns and it seemed that the stream was changing. Shortly after the Japanese announced the heightened activity, Brian Marsden pointed out that in a paper he published in 1973 he discussed the possibility that P/Swift-Tuttle might actually return in 1992 if it was the same comet observed in 1737. While he had ranked the possibility as slight that the 1737 comet was P/Swift-Tuttle in 1973, the enhanced Perseid display in 1991 revived the remote chance that the comet might return in 1992. The telling sign would be Perseid activity in 1992. Unfortunately, a full Moon would compete with the meteor show and make data analysis very tricky. As the data from the previous returns showed that Europe would be the best place to observe the early peak, much preparation was made there to capture the event. Unfortunately, meteor showers - unlike eclipses - have an inherent unpredictability resulting from our lack of knowledge regarding the dust distribution about the parent comet. The 1992 display showed this maxim perfectly: The new peak shifted some two to three hours earlier than what had been observed in past years. As a result, Asian and Russian observers were in the best locations to witness the display. After much analysis of the available observations, it appears that the 1992 activity was higher than in 1991, perhaps with a peak ZHR of order five hundred, though this peak value will remain highly uncertain due to the effects of lunar interference. This brings us to the next logical stage of the "act", the 1993 display. With P/Swift-Tuttle recovered shortly after the 1992 display (and with elements close to those predicted by Marsden in his 1973 paper), it became apparent that the geometry between the comet and Earth could make the 1993 display very strong. Indeed, our geometry with the comet is very similar to that between Earth and comet P/Tempel-Tuttle in 1833. This is suggestive that a strong return is in store for observers in 1993. However, P/Tempel-Tuttle is *not* P/Swift-Tuttle and the dust distribution about the latter is unknown. While there is much circumstantial evidence favoring a storm, nothing can be certain. Keeping these cautionary notes in mind, what might be predicted for 1993? Based on the node of the comet and the maximum activity in 1992, one would expect peak activity to be at 1 UT (Universal Time) on August 12, 1993. Some have suggested that the shift in activity between the 1991 and 1992 displays suggest that we can expect another 0.1 day advancement of activity in 1993, closer to 22 UT on August 11. While this is possible, I consider the shift unlikely. Meteor storms usually occur very close to the node of their parent comet as the 1992 display did relative to P/Swift-Tuttle. Basing an estimate of this sort on two data points (the 1991 and 1992 maxima) is a bit question- able at least, so I see little reason to suppose a further 0.1 day shift will occur. What sort of display are we likely to encounter? The past meteor storms for which reliable observational data exist suggest that newly ejected cometary material is rich in faint meteors. This seems to be the best guess of what will be seen in 1993. That is not to say that there will be little or no large particles encountered, but the proportion of faint meteors to bright meteors will be higher than in regular Perseid displays. The central questions - how long will the display last and what will be the maximum activity - are very difficult to predict. Meteor storms generally last for a few hours at most. Some historical records suggest that large displays can carry on for days, but these records are very open to interpretation. Data from more recent storms seems to suggest that several hours (two to six) are a good guess for the longest time for which unusually high activity might be observed. The 1833 Leonids, for example, showed strong activity for nearly six hours. The peak rates are complete unknowns. The largest meteor storms on record for the last few centuries produced activity on the order of 100,000 meteors per hour for intervals shorter than about one hour. Ancient records do little to pin down peak rates of meteor storms earlier than about 1800. Everyone's guess is equally valid in this instance. Whatever the 1993 display produces it will go down in history as one of the most waited for showers ever. The International Meteor Organization would be interested to receive your observations, whether you see unusual numbers of meteors or not. Please follow the techniques outlined in the August 1993 issue of SKY & TELESCOPE magazine and send the completed summaries to the addresses given therein. For readers in North America, the IMO may be contacted through the author, Peter Brown, North American Secretary - IMO, Dept. of Physics, University of Western Ontario, London, Ontario, N6A 3K7, Canada. (E-Mail - PETER@CANLON.PHYSICS.UWO.CA) Readers outside North America should contact the Secretary-General of the IMO, Paul Roggemans, Pijnboomstraat 25, Mechelen, Belgium, B-2800 Observations should be sent to: Rainer Arlt, Berliner Strasse 41, D-O-1560, Potsdam, Germany (E-Mail - 100114.1361@compuserve.com) About the Author - Peter Brown is a graduate student in physics studying meteor ------------------------------ End of Space Digest Volume 16 : Issue 693 ------------------------------