2Department of Physics and Astronomy Brigham Young University Provo, Utah 84602
JournalofCosmology.com, July, 2010
The regularity of the orbits of comet Halley has made possible the determination of its visits backwards in time through the Middle Ages to antiquity. Computer models have provided correlations between reports of comets back to the second and third centuries BC and astronomical records of the Babylonians and Chinese. So far the earliest probable sighting is the return of 240 BC, confirmed by Chinese observers. Thus far ancient Greek records, which do not contain systematic diaries of heavenly events, have not been considered in this connection. One famous event recorded by Greek philosophers and historians is the fall of a meteor in northern Greece in 467/6 BC. At the time of the meteor, a comet was visible. This coincides with the retrodicted appearance of comet Halley in the summer of 466 BC. Using computer models we examine the probable path of comet Halley on that return and find it is consistent with reports about features of the observed comet. The philosopher and scientist Anaxagoras is said to have predicted the fall of the meteor. One ancient source corrects this confusion and allows us to see how the Greeks combined theory and observation in this case.
Keywords: Halley’s Comet, Ancient Greeks, Aristotle, Anaxagoras.
Halley’s comet is one of the most prominent visitors to the inner solar system. Using Newtonian theory, Edmond Halley in 1705 predicted the return of a comet seen in 1682, which duly returned in 1758 (after Halley’s death) to make the predictor posthumously famous and acquire his name; the comet also provided a spectacular confirmation of Newtonian physics against rival theories, which soon faded from the competition. Its returns have been eagerly anticipated ever since. But the success in predicting returns has also allowed astronomers to project backwards the visits, first to 1607 and 1531, as Halley himself had done in developing his prediction, and then to earlier times. Study has revealed Halley’s comet to be the harbinger of disaster in the Norman invasion of England in 1066, portrayed in the Bayeux Tapestry. Its appearances were recorded with fear an awe throughout antiquity and the Middle Ages.
Calculating appearances of comet Halley is not easy, since its orbits sometimes bring it close to planets whose gravity can modify its course. Its orbital period typically varies between 75 and 76 years, but can return in as few as 74 or as many as 79 years. Computer models made to predict or retrodict its visits must take into account gravitational interactions between the comet and the planets. Using computer models calibrated for actual and presumed alterations in the comet’s orbit, its visits have been extended back into the third century BC. Detailed records kept by Babylonian observers on one side of the world, and Chinese observers on another, have allowed researchers to identify appearances of the comet in 87 and 164 BC (Stephenson et al. 1985). Chinese records make probable the sighting an earlier appearance in 240 BC (Kiang 1972), now the earliest known sighting of the comet. Yeomans and Kiang (1981) have made projections of earlier returns of the comet back to 1404 BC, when a near pass to Earth renders further calculations problematic without observations to correct them. They made their calculations “to allow possible identifications of ancient cometary observations with this famous comet” (p. 644). Thus it is in principle possible to find earlier sightings in ancient records, but such sightings are difficult to obtain because of the fragmentary remains of early astronomical records.
We wish here to propose a possible sighting drawn from Greek records that have not been considered in this connection. Whereas Babylonian and Chinese observers kept meticulous records of daily phenomena in the heavens for centuries, the Greeks do not seem to have kept similar records. While they made important advances in cosmological theory and in theoretical astronomy, they had no native bodies of data to draw on. Greek astronomers such as Hipparchus and Ptolemy seem to have drawn mainly on Babylonian records for the data they used in their theories (Neugebauer 1957, 1975; Hunger and Pingree 1999; Rochberg (2004). Hence it is not surprising that the Greeks have not furnished observational records with which to check the appearances of comets and the similar phenomena. Any records found in Greek sources are likely to be accidental in the sense of not arising from systematic habits of observation and record-keeping.
Notwithstanding the challenges posed by Greek sources, there is one event that left a mark on Greek astronomy and cosmology which offers the possibility of furthering our knowledge of comet Halley. In 467/6 BC a meteor fell in northern Greece which caused a sensation in the Greek world. Records of the event note that the meteor fell while a comet was burning in the sky. Ancient (and modern) commentators have mostly concerned themselves with the meteor while treating the comet as an interesting coincidental feature of the situation. We shall briefly rehearse the information about the meteor and then focus on the comet.
The meteor fell during daylight hours in the vicinity of Aegospotami (“Goat Rivers”) in the Hellespont region of northern Greece. The meteorite was about the size of a wagon, and became a wonder to the inhabitants of the area–in fact it was a tourist attraction for over five hundred years (see reports cited below). It immediately became associated with the name of the philosopher and cosmologist Anaxagoras, who was said to have predicted the meteor’s fall. Greek philosophers had been speculating about the structure and composition of the cosmos for more than a century before this event (Furley 1987; Wright 1995; Graham 2006; Gregory 2007; Sedley 2007). But there is no record of any awareness of meteors earlier than Anaxagoras. Early thinkers including Anaxagoras theorized about shootings stars, but tended to view the streaks of light they saw as meteorological events rather than as passages of meteors (Diels 1879; Mansfeld and Runia 1997, 2009). From this time forward, theories of meteors became common. This fact creates something of a quandary: how could Anaxagoras predict a phenomenon that was by nature unpredictable, and do it when the phenomenon itself was unknown? If the connection between Anaxagoras and the meteor is based on misconceptions, how reliable can the reports be? What is remarkable about the meteor is that, in a time when mythological thinking was common (philosophers proposed scientific explanations but presumably remained a small intellectual elite), the prodigious event of a meteor falling to Earth was never associated with any mythological action, but only with a scientific theory. This tends to suggest that Anaxagoras’ theories were known and were seen to connect with the event. Anaxagoras was a member of Pericles’ circle in Athens, and so was well known among the intelligentsia (Graham and Hintz 2007). According to his cosmological theory, the heavens consisted of naturally heavy stony or earthy bodies kept aloft by something like the centrifugal force of a cosmic vortex. Most other previous theories described heavenly bodies as fiery or cloudlike manifestations that were lighter than air. In this historical context, the fall of a stony body to earth would be seen as a confirmation of Anaxagoras’ theory against rival models of the heavens (Curd 2007).
For our purposes the importance of the meteor arises from its dating. It fell in the year 467/6; or according to another report in 468/7 (The Marmor Parium –a marble stone from Paros with inscriptions of important events, composed in 264/3 BC– in inscription 57 dates the meteor to the archonship of Theagenides, or 468/7 BC). The event was remarkable enough that it appeared in early chronicles and was passed on by thinkers interested in astronomy. Its date is fairly well set by the reports to within 468-66 (that is: summer 468 to summer 466). We should note here that early Greece had no standard calendar, with each city state using its own system of lunar months which had to be supplemented irregularly by intercalary months to keep in step with the solar year (On issues of chronology see Bickerman 1980). Hence there is room for some discrepancy in just translating an early local report to a later one using years of the Olympic games cycle or Athenian archons (magistrates) as reference points. Consulting Yeomans and Kiang’s Table 4 (p. 643), we find an expected return of comet Halley in July of 466, which makes the meteor-comet event potentially interesting to modern astronomy. The obvious question becomes: is the comet observed in connection with the meteor of Aegospotami, which we shall call “Anaxagoras’ comet,” identical with comet Halley?
Let us look at three (of four) ancient reports of the comet: When the stone fell from the air at Aegospotami, having been supported by the wind it fell down during the day. It so happened that at the same time a comet was visible in the west. (Aristotle Meteorology 344b31-34)
The Greeks report that Anaxagoras of Clazomenae, in the second year of the 78th Olympiad [467/6] predicted, by his knowledge of astronomical writings, within what days a stone would fall from the sun, which happened in the daytime in a region of Thrace on the Aegos river. The stone is still exhibited, being about the size of a wagon load, of a burnt color, which fell while a comet was seen burning in the night (Pliny Natural History 2.149 = DK 59A11).
Some say the fall of the stone was an omen of this event [the Athenian defeat in a battle at Aegospotami, 405 BC], for an immense stone had fallen on Aegospotami, according to common belief. (2) (It is shown to this day by the inhabitants of the peninsula, who stand in awe of it.) It is said that Anaxagoras predicted that one of the bodies entangled in the heaven might, if there were some slip or agitation, break off and fall or be cast down; and indeed none of the stars is in its natural place; being stony and heavy they shine by resistance of the revolving aether, and being constrained by the angular momentum of the revolution they are dragged by force . . . . (4) In his treatise On Piety Daimachus supports Anaxagoras, reporting that before the stone fell, for seventy-five days a huge fiery body was visible in the sky, like an inflamed cloud, not still but moving with complex and branching motions, so that fiery fragments from its shaking and errant course flew in every direction, flashing like shooting stars. (5) But when it had fallen to earth there and the local inhabitants got over their fright and gathered around it, they saw no activity of fire, not even a trace, but a rock lying there, large indeed, but representing no appreciable fraction of that fiery mass above (Plutarch Lysander 12.1-5 = A12).
These reports provide good evidence for the event. Aristotle, a leading researcher and scientist as well as philosopher, writing a little over a century after the event, notes the synchronism of the meteor and comet and the daytime descent of the meteor. The comet was at the time visible in the west. Pliny gives the dating and confirms that the meteorite is still an attraction, writing five hundred years later (in the mid-first century AD). Plutarch, writing in the late first or early second century, himself a well-read scholar familiar with astronomical theories in general and Anxagoras’ in particular, gives a brief summary of Anaxagoras’ theory and repeats a description of the comet from an earlier historian. He makes it clear that it was Anaxagoras’ theory that predicted the possible fall of a meteor; Anaxagoras did not personally predict this particular meteor. West (1960) has suggested that the presence of the comet might have led Anaxagoras to anticipate the fall of a piece of it. There are problems with this suggestion, however, because on Anaxagoras’ theory comets are just conjunctions of planets. Hence we need not disqualify other less careful reports of the event, like Pliny’s, as making patently absurd claims. Daimachus tells us that the Anaxagoras’ comet was visible for seventy-five days and gives a vivid description of a major comet sighting (without using the word “comet”).
The Greek reports lack the astronomical details that are found in Chinese and Babylonian diaries and that might help verify the path and exact timing of the sightings. Nevertheless, they do provide some important information, including the period of visibility. While this information cannot demonstratively confirm that Anaxagoras’ comet was indeed Halley’s comet, we can reconstruct the likely path of the latter to see whether it agrees with the observations recorded. Based on the orbital elements determined by Yeomans and Kiang we determined the orbital path of the comet through the inner Solar System. From the 1986 passage we determined the magnitude scaling based on the comet-Sun and comet-Earth distances. In addition we used the first naked eye observations from November 1985 to set a cut-off magnitude of 4.2 as the faintest possible level at which the comet could be seen (See Morris and Green 1982). Based on these assumptions we arrive at a reconstruction of the passage of comet Halley in 466 BC. In Fig. 1 we show the orbits of Mercury, Venus and the Earth from an overhead view. The path of the comet is then shown in black. The positions of Venus, Earth and the comet are shown for each day between these dates. Clearly the comet is in the appropriate retrograde orbit. In Figure 2 we show the X-Z motion of the comet with respect to the Earth to give a more complete picture.
From these figures we can piece together a picture of the comet’s passage. On June 4th a close conjunction of Venus, Jupiter, and the moon occurred in the sunset. Comet Halley could have become visible soon after, on June 5th or 6th early in the morning very near the Pleiades. It would have grown in brightness each day, with the tail becoming more elongated. On June 4th the comet had passed from below the Earth’s orbital plane to above it. Then a few days later on June 8th the comet would have passed inside the Earth’s orbit. This could very easily have left material in for the Earth to pass through later in September. Later on June 28st the comet would have passed inside the orbit of Venus. (At no point does the comet pass inside Mercury’s orbit.) On August 6th the comet would have passed very close to Venus on its way back out of the Solar System. Then on August 27th the comet would have moved beyond the Earth’s orbit.
The almost parallel tracks of the Earth moving counter-clockwise and the comet moving clockwise would have kept them in close proximity for an extended period of time. On July 12th the comet would have made its closest approach to the Earth, 0.46 AU, followed quickly on July 18th by the perihelion passage. The comet should have reached its brightest magnitude of -0.4 during this time (see Fig. 3, plotting magnitude as a function of date). Because of the nearness of our planet and the comet, and the position of the comet in the sky, it might not have disappeared at the time of its conjunction with the sun, early on July 18th, but simply have moved from the morning sky east of the sun to the evening sky west of the sun, with its tail at least highly visible extending away from the sun and upward. The Earth would have moved under the tail of the comet, but might have intersected with the debris field from below the tail itself. The tail might have described a spectacular arc above the sun like that in figure 4. Since the tail of a comet can extend more than 1 AU, at the comet’s closest approach to Earth the tail may have stretched more than 0.5 AU over and beyond the Earth.
On July 21, soon after its perihelion passage, the comet would have been bright (0.3) and appeared right above a conjunction of Venus and Saturn. On August 9 the comet would appear near Venus, with the planet appearing at almost -4 magnitude and the comet at 2.7. The comet would become fainter and lower in the western sky until about August 25th it would disappear. Accordingly, it is possible that the comet was visible for about eighty days, depending on atmospheric conditions and the darkness of the sky. Fortunately, the summer is the dry season in Greece, when clear skies are the rule.
What we can say based on a computer model of the return of 466 BC is that the reconstructed appearance is consistent with the ancient reports we have. Comet Halley could have been visible for seventy-five days running. Unlike most comets, Halley would not have disappeared from the sky for several days around the perihelion, but might well have remained visible just before and just after the solar conjunction. This is important because, absent an adequate theory of comets, many ancient observers would take a comet appearing in the west at evening several days after an appearance in the east in the morning as a different comet (Anaxagoras understood comets as planetary conjunctions. See Aristotle Meteorology 342b27-29, Aetius 3.2.2. Earlier Xenophanes had seen comets as luminous cloud formations: Aetius 3.2.11).
Around the time of conjunction the tail should have been large and impressive as the Earth passed under it (see Fig. 4). While the Earth did not pass directly through the tail, it may have intersected with the debris field from it. This would have generated small meteors, consistent with the shooting stars reported by Daimachus as preserved in Plutarch.
[Insert Fig. 4 here.]
Aristotle also claims that the meteor associated with the comet was held aloft by winds.In general Aristotle holds that meteors and similar phenomena arise from exhalations in the atmosphere (Meteorology I.4); he does not, however, associate them specifically with winds, but rather with either combustion or compression (ibid. 341b31-342a1). In particular, sideways motion is caused not by wind but by an interaction of forces (342a24-27). In contrast, Aristotle associates comets with windy conditions (344b26 ff.). The summer season, after the solstice, is the time of the etesian or annual winds that blow strongly from the north over the Aegean sea. Aristotle Meteorology he identifies the beginning of the winds with the rise of the Dog-star (Sirius), i.e. in late July.If we take comet Halley to be Anaxagoras’ comet, we should infer that the meteor fell after July 18th when the comet was in the western sky. None of this proves definitively that Anaxagoras’ comet was Halley’s comet.
Nevertheless, given the rarity of cometary events, especially encounters with major comets, we should consider comet Halley to be a strong contender for the comet associated with the meteor of Aegospotami. There remains the possibility of finding or identifying further ancient sightings of comets from Chinese and Babylonian records. The Greeks provide one possible reference point in the search, one that takes us back three orbits and 226 earlier than the earliest previous probable sighting. If Anaxagoras’ comet is indeed identical with Halley’s comet, it would add another important entry to the distinguished resumé of the most famous comet.
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