In 1066, two widely separated men recorded a stunning celestial phenomenon, one a Muslim chronicler named Ibn al-Athir, the other an anonymous scribe in an English monastery. Ibn al-Amir's note read like this:
In the year A.H. 458 [A.D. 1066], during the middle of the month of Jumada II, a comet appeared just before dawn. It was white, and to the eye appeared to be 10 cubits long and one cubit wide [roughly 19 feet by two feet]. It remained visible until the middle of the month of Rajab [that is, for one month], then faded away.
The other chronicler, whose entries would be included in what was later called the Anglo-Saxon Chronicles, reported that King Edward had died, that Harold had succeeded him and that the Normans under William the Conqueror had invaded England - and won the Battle of Hastings. He also noted that a comet - called by some "the long-haired star" - had appeared in the skies six months before the Normans landed:
At that time, throughout all England, a portent such as men had never seen before was seen in the heavens. Some declared that the star was a comet, which some call "the long-haired star": it first appeared on the eve of the festival of Letania Major, that is, on 24 April, and shone every night for a week.
A century or so later, Ibn al-Jawzi, a Muslim historian, found a description of the same phenomenon - as seen from Baghdad:
On 10th Jumada I, a large comet appeared in the sky. It had a tail towards the east approximately three degrees in width and its tail was very long - reaching right to the zenith. It stayed until Sunday night of the last six days of the month. Then it came back on Tuesday evening at sunset with its light folded around it like the moon. People were terrified. As night darkened, it threw its tail towards the south, staying for 10 days before it faded. Later, merchants' books said that 26 ships headed for Oman sank near the coast on the last day the comet appeared. About 80,000 people perished along with all their belongings.
That comet, though no one knew it then, was Halley's Comet, which, last July, edged into telescopic range again for the first time since May, 1910.
Halley's Comet (commonly pronounced "Hail'ee's but also "Hally's" as in "valleys" and, according to the Times of London, "Haul'ee's") is named after Edmund Halley, the 17th-18th century astronomer who first observed it, calculated its orbit and predicted almost precisely when it would appear again. It is also the most famous comet in history; big and brilliant, it is one of the few comets whose orbit periodically brings it into the range of the naked eye.
According to one British scientist, Halley's Comet was sighted as early as 164 B.C., and 87 B.C. in Babylon; both those sightings were recorded on recently deciphered Babylonian clay tablets. By the time it vanishes - again - in May, 1986, it will be the only comet in history to be thoroughly examined, measured and photographed - thanks to the exceptional advances made in astronomy since Galileo first trained his 4.4-centimeter lens (1¾ inches) on the moon in 1609.
As late as 1910, when the comet last appeared, 102-centimeter optical telescopes (40 inches) were the biggest available. But by the 1970's, the U.S.S.R. was building a 600-centimeter optical telescope (236 inches), radar was tracking the ionized-air trails of meteors, and radio telescopes with dish antennae were tracking the stars. Even more important, the United States, Europe and Soviet Russia have explored space, put men on the moon, scattered satellites throughout space and instituted regular space shuttle flights, all of which have enabled astronomers to expand their studies of space - and comets.
On its current visit, consequently, Halley's Comet is being studied more closely than nearly any other celestial body in history. The United States, for example, will devote two space shuttle flights to observation of the comet and has four of the largest telescopes in the world trained on the comet from Hawaii. In addition, the Soviet Union has telescopes from 10 major observatories focused on it, Great Britain has set up a telescope in the Canary Islands and Europe has set up an observatory in Spain. And in 1986, the National Aeronautics and Space Administration (NASA) will send up a shuttle with a telescope mounted on it.
Europe, Russia and Japan, furthermore, have each sent up satellites, to probe the mysteries of space: the Giotto named after the painter who depicted the comet in a 1301 fresco on the walls of a church in Assisi, Italy, Vega 1 and 2 from Russia and Japan's Suisei and Sakigake.
The comet is also attracting excited attention throughout the world. In September, for example, the U.S. Naval Observatory in Washington installed a special telephone line to provide information on the comet - the "Halley Hotline" - and recorded more than 1,600 calls the first week. And thousands of people have booked cruises to the southern hemisphere where the comet will be visible in January and then again in March and April. In the United States, lines are scheduling comet cruises and the famous Cunard Line had booked some 2,500 passengers on three ships as early as November. Most of the ships plan to offer lectures by astronomers, ex-astronauts or science teachers.
In the north, British Airways is offering one hour comet flights over the North Atlantic and its first flight was fully booked in. mid-November. Flying at an altitude of 11,000 meters (35,000 feet) - well above atmospheric haze and city lights - passengers were afforded the opportunity to enjoy a clear, unrivaled view of the comet's spectacular progress.
On the ground, amateur astronomers and others who purchased telescopes and binoculars for the occasion have been trying to spot the comet too, though, with the comet low on the horizon and dim, good sightings have been rare.
It's a far cry, obviously, from 1066, when even learned men, reflecting millennia-old fears, linked the appearance of the comet with the disasters that overwhelmed England that fateful year; then, and for a long time afterwards, comets were thought to presage the death of kings. As Shakespeare wrote:
When poor men die there are no comets seen;
The very heavens blaze forth the death of princes.
Those beliefs, in fact, endured for a long time: until the discoveries of Isaac Newton and Halley disclosed the true nature of comets and gave the coup de grace to the ideas of Aristotle that had blocked a rational approach to astronomy.
Aristotle believed that the earth was the center of the universe and that the planets - including the sun - revolved around the earth attached to very thin, transparent spheres. The last of these spheres, beyond Saturn, was the sphere of the fixed stars, those whose positions do not change with the passage of time. Everything that happens above the sphere of the moon is eternal and unchanging. Change - and transitory phenomena - all take place in the sublunar world, the area between the earth and moon.
Since comets appear and disappear, Aristotle said, they obviously could not exist in the unchanging translunar spheres; they must be phenomena like clouds, mists, lightning and other "corruptible" events of the sublunar world.
There are two other features of the Aristotelian system that influenced the ancient idea of comets. The first is that Aristotle's sublunar world was made up of four elements in varying proportion - earth, air, fire and water - but the translunar world was made up of a fifth, which he called "ether" - an unchanging substance. The heavens and celestial bodies were formed of it - hence their incorruptibility.
The second feature is that motion in the translunar world was circular and uniform, without beginning and without end, while in the sublunar world it was linear, had a beginning and end and was not perfect, that is to say, not uniform. Thus Aristotle's model elegantly explains everything: a stonejalls in a straight line here in the sublunar world, while the planets circle the earth on fixed and immutable orbits.
Later thinkers, like Ptolemy, refined this system and adjusted it by increasingly complex mathematical explanations of observed phenomena - like the eccentricity of planetary orbits. But the basic model was not questioned until the time of Copernicus - in the late 15th and early 16th centuries - and was not proved to be wrong until the time of Newton and Halley.
Comets, for Aristotle, were not astronomical objects at all, but meteorological, and were very close to the earth: in his sublunar world. They were formed under certain conditions, he thought, by accretions of hot air, which rise, and sometimes catch fire. Furthermore, he said, the frequent appearance of comets heralded dry, windy years - thus establishing an association between comets, weather and other natural events that increasingly came to grip men's imagination.
Because those ideas were accepted almost without question for 2,000 years, few scientific observations of comets were ever undertaken; if comets were transitory meteorological events, like clouds, the only point of observation was to predict good and bad harvests, epidemics - and other disasters.
Genghis Khan, for instance, is said to have taken the appearance of Halley's Comet in 1222 very seriously. Interpreting the first sighting as an ominous sign from his pagan gods, he halted his invasions of Eastern Asia and turned instead on the Muslim world to the west - bringing devastation and destruction.
Even the Muslim world, however, accepted Aristotle's belief that comets and disasters were somehow linked. In the early ninth century, for example, astrologers in the Abbasid Court in Baghdad warned Caliph al-Mu'tasim (795-842) not to go to war with the Byzantines in Asia Minor because of a bad portent: in 837 Halley's Comet had appeared again.
As it turned out, the caliph went to war anyway and won, in spite of his astrologers' warnings, and a Damascus-born poet, Abu Tammam, greatly moved by his leader's courage, wrote a famous poem about the battle and al-Mu'tasim's defiance of his astrologers.
The sword which can decide the outcome of events is more reliable than books (of astrology).
Truly the white sheets (swords), not the black sheets (papers) are apt to clear all doubts and uncertainties.
The true news is better told by the glimmer of lances, not the seven planets.
They frightened the people (with omens of) a horrible catastrophe that would strike when the comet appeared from the west.
They arranged the heterogeneous constellations in accordance with their whims.
They speak on behalf of the stars while the stars move in their orbits or axes unaware of their talks.
Hence their prophecy and the fabricated tales could not stand the test of time.
In the Muslim world, as elsewhere, Aristotle's view was accepted widely even though Muslim astronomers made some important adjustments of detail. The popular 10th-century encyclopedia, The Epistles of the Brethren of Purity, for example, classifies comets along with atmospheric phenomena such as winds, thunder, lightning, snow, frost and rainbows. The encyclopedia also says that comets are composed of vapors that are condensed by the action of Saturn and Mercury and then become as transparent as crystals - which is why the sun's light can be seen through them. Furthermore, says the Epistles, comets turn with the heavens until they dissolve - and herald both good news and bad.
There were dissenters. Seneca, the Roman philosopher, for example, had giver, an amazingly modern account of comets in his Natural Questions as early as the first century:
Do you really believe that in this immense and splendid universe, among the innumerable stars which adorn the night in diverse ways, without leaving the slightest part of the heavens empty or inactive, only five stars have the right to move freely and that the rest must all stay fixed, an immovable crowd? He who believes that those things which are most common are the only ones possible is ignorant of the power of nature. Comets are rarely seen because Nature has assigned them another place, other times, movements different from those of the other stars ... Why, then, should you be so surprised that comets, a spectacle so rare, should be subject to fixed laws and that we should not know the beginning or the end of a journey whose return occurs only after long intervals of time?
This extraordinary passage - which clearly indicates that Seneca thought comets were astronomical rather than meteorological phenomena, and even more amazingly appears to indicate that he thought they followed periodic orbits – anticipated Edmund Halley by 1,640 years. Furthermore, it was immediately followed by an equally astonishing prediction: "There will come a time when a careful and centuries-long study will throw light on these natural phenomena."
But Seneca and other dissenting voices were ignored and Aristotle, rigorously logical if totally mistaken, reigned supreme in most of the West and East. One exception was China - and thus Chinese celestial observations helped uncover the truth about comets.
Since the Islamic world - geographical intermediary between the Latin west and China - had access during the early Middle Ages to the scientific heritage of Sassanid Persia and India, as well as Greece, at least one Muslim astronomer also questioned the Aristotelian dogma concerning comets. In 1602, for instance, a curious passage in Astronomia Instauratae Progymnasmata by Tycho Brahe, a famous Danish astronomer, credited an Arab with having been the first scientist to disprove Aristotle. Brahe said Abu al-Ma'shar observed comets in the translunar spheres; he was referring to the 10th-century Arab astronomers' personal observations of comets "beyond the sphere of Venus."
One of the difficulties in assessing the Muslim world's contribution to "cometology" is the fact that many key texts remain unpublished, and that many others have not survived. There is no doubt that Islamic astronomers saw Halley's Comet, but they knew little or nothing about its nature. Ibn al-Athir's sighting in 1066 is one example, and Ibn al-Nadim's 10th-century bibliography, the Fihrist, includes two monographs by the ninth-century astronomer and scientist al-Kindi on the appearance of what is surely Halley's Comet. The titles are: The Star Which Appeared and Was Observed for Some Days, until it Disappeared and What Was Observed about the Great Sign during the year A.H. 222. That year, Anno Hegira 222, is equivalent to A.D. 837, the year in which Halley's Comet appeared and Caliph al-Mu'tasim defied his astrologers and went to war against the Byzantines.
Muslim historians and annalists, however, recorded sightings of comets and, like their European counterparts, typically linked them to disasters. A Mas'udi, a 10th-century historian with more than a passing interest in science also spotted a comet in A.H. 299 (A.D. 912)
A great hailstorm, with stones weighing as much as a Baghdadi ritl, afflicted Kufa at the same time as a heavy wind, in the month of Ramadan; many houses and buildings were knocked down. This sinister event was followed by an earthquake which cost the lives of a large number of inhabitants."These disasters took place in Kufa in 299 A.H. The same year saw an earthquake in Egypt and the appearance of a comet.
This passage from the Meadows of Gold is typical of most notices in Arab histories, and is very similar to that in the Anglo-Saxon Chronicles. Although the comet he referred to was not Halley's, al-Mas'udi associated it with meteorological phenomena and with the earthquakes in Kufa and Egypt, just as Aristotle would have done.
The value of such notes as those in Meadows - and the sighting by Ibn al-Athir – should not be underrated; modern astronomers have been able to derive important information by collecting and studying them. Even Halley, who was able to prove Newton's theory of universal gravitation by collecting information about the appearance of comets in the past, inferred the "negative acceleration" of the moon from a historical study of eclipses.
Modern scientists, furthermore, using the same method, but with access to Chinese, Babylonian and even Mayan records of eclipses, have been able to not only confirm his inference (that the moon is slowing down), but that the earth too is rotating about its axis more slowly than in the past. [Three hundred million years ago, the lunar month had 31 days. It now has 29.5. The moon, since classical times, has been decelerating, in relation to the earth, at the rate of six centimeters per year.]
The fact that the length of the day has not remained constant can be graphically seen if we consider that the total solar eclipse visible in Mesopotamia in 136 B.C. would have been seen in Europe if the rate of the earth's rotation had remained fixed.
Islamic records of eclipses, recording the exact times they occurred and where they were visible, have - when taken in conjunction with Chinese, Mayan and European observation - given scientists a very precise knowledge of when, where and at what time eclipses have occurred during the past millennium and a half.
Such observations have not only allowed scientists to draw the startling conclusions they have, but have proved of great use to historians. Since almost all ancient and medieval chronicles mention total - and sometimes even partial - solar and lunar eclipses, as well as the appearance of comets, they provide a sort of clock against which to check dates. Sometimes this is complicated by the fact that there was an understandable tendency in the past to link eclipses too with great events, such as the death of kings, wars, famines and epidemics. By using the precise tables compiled by scientists, historians can often sort out the chronological confusions of the past.
Since the shadow cast by the moon upon the earth during a total solar eclipse only covers an area of the earth 270 kilometers across (167 miles) and is only visible outside the region for a few hundred miles on either side of the band - the record of such an eclipse can often help locate where a chronicle must have been composed, or at least where the chronicler was at the time.
Efforts are constantly being made to push further back into time - the earliest recorded eclipse seems to be 2095 B.C. Serious efforts are now being made to correlate eclipses, comets and ancient records of meteorological phenomena in hopes of discovering if there is a periodicity in the long run of cycles of plenty and famine, wet and dry periods. Islamic chronicles have much to contribute to these efforts, and as yet have been scarcely used.
Despite Aristotelian theories - they endured for centuries - European astronomers did make progress. In the 15th century, Toscanelli, a Florentine scientist, became the first European to make precise observations of comets; he observed five, between 1433 and 1472, marked their positions accurately and correctly drew them with their tails facing away from the sun. A German contemporary, Regiomontanus, observing the comet of 1472, suggested, but could not prove, that comets were celestial rather than sublunar phenomena. And in the 16th century, scientists noted again that the tails of comets always faced away from the sun - an observation that planted a seed of doubt; it implied a relationship between comets and the sun which Aristotle had ruled out.
But it was Tycho Brahe, in 1588, who was the first to prove that comets were not sublunar. In his book De Mundi aetherei recentioribus phaenomenis Liber secundus, qui est de illustri Stella caudata ab elapso fere triente Novembris anno MDLXXVII usque ad finem Januarii sequentis conspecta, he showed that a luminous comet which appeared in 1577 - not Halley's - was at least six times farther away than the moon. Tycho Brahe, interestingly, had built an observatory in 1577, the year he saw the comet, and it was reportedly similar to that just opened by Taqi al-Din Muhammad ibn Ma'ruf in Istanbul: a main building with a library and quarters for the astronomers and a smaller building for the instruments. This was no coincidence; the Muslim influence in the study of astronomy was so strong in 16th-century Europe that Brahe placed a portrait of al-Battani next to that of Copernicus in his office at the observatory.
Even earlier, however, in 1543, the Polish astronomer Nicolaus Copernicus had published his revolutionary De Revolutionibus Orbium Coelestium, which placed the sun in the middle of the universe, with earth and the other planets orbiting around it. Next came Johannes Kepler, a disciple of Tycho Brahe, who showed that the orbits of the planets were ellipses and set down his famous three laws of planetary motion, which explained the form of planetary orbits, how they move along them and how the motion of the planets is related to their distance from the sun. But even Kepler could not explain why those laws should obtain. This was left to Sir Isaac Newton, one of the greatest scientists in history, and, indirectly, Newton's friend Edmund Halley who not only pushed Newton to publish his findings - he paid publishing costs himself - but predicted the great comet whose reappearance would prove Newton's theories.
It is a disturbing thought that without Halley, Newton might not have published a work which explains basic principles of the universe. Born in 1656, Halley had been interested in astronomy, since he was 16, at which age he went up to Oxford, "well versed in Latin, Greek, and Hebrew," according to John Aubrey. A geographer as well as an astronomer, Halley explored the Atlantic and penetrated the Antarctic before he was appointed Royal Astronomer in 1720. Halley went on to describe the system of trade winds and monsoons, and discover the Law of Inverse Squares.
Halley's contributions to science, however, must also include his financing the publication of Newton's epoch-making Philosophiae Naturalis Principia Mathematica in 1687, in which Newton explained Kepler's three laws and stated the principal force at work in the universe: gravity.
As early as 1666, Newton, reflecting on Kepler's Third Law - that the farther planets are from the sun the more slowly they move in their orbits - deduced "that the forces which maintain the planets in their orbits must vary as the inverse of the squares of the distance from the centers around which they move." At the same time, he found that the force necessary to hold the moon in its orbit was the same as the force of gravity on the surface of the earth - ideas elaborated in the Principia, as the Law of Universal Gravitation: "two bodies, with mass respectively and attract one another with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance r that separates them."
Even Newton, however, could not apply this outside the solar system. Did the same law operate outside it, throughout the universe? The answer, of course, was yes, and the proof was Halley's Comet.
In the Principia, Newton stated that comets were beyond the moon, and showed how three precise observations of a comet in different positions could be used to describe its orbit, which could be parabolic, hyperbolic or elliptic. In the first two cases, the curve followed by the comet is open, and the comet passes through our solar system never to return. In the latter case, when the orbit is elliptical - that is, closed - the comet will return. Newton also made a number of deductions about the physical properties of comets which still hold good: that the heads were solid and durable and surrounded by their own atmosphere, that the tails were made up of gases, were always opposed to the sun and shone because they reflected sunlight.
Modern observations have added considerable detail to Newton's deductions. In the 1950's, for example, F.L. Whipple proposed the "dirty iceberg" theory: that when comets were far from the sun they were composed of solid blocks of frozen gases, plus such meteoric material as metals, silicons, oxides and carbon particles. As the comet nears the sun, and the "iceberg" defrosts, the gases and meteoric materials form the "coma" – which are then diffused by the "solar wind," and form the tail of the comet. On September 11, 1985 this theory was apparently reconfirmed when a spacecraft flew through the tail of another comet: the Giacobini-Zinner comet 71.2 million kilometers from earth (44 million miles.)
These gases, which, with dust, glow in sunlight, make up the great fiery tails of the comets that once terrified ancient peoples. In 1910, for example, the tail of Halley's Comet was 100 million kilometers long (60 million miles).
It would have been impossible to prove Newton's theory, however, if he had not also provided the means to calculate cometary orbits. Realizing that the physical appearance of comets - their size, brilliance, length of tail - was not adequate to identify them, Newton showed that each comet, like each planet, has its particular orbit, and that its orbit, not its physical appearance, identifies it, and that the orbit can be determined if six things are known: (1) the inclination of the comet's orbit with respect to that of the earth; (2) the orientation of the "nodes" - the two places where the comet's orbit intersects that of the earth; (3) the longitude of the "perihelion," the point at which the comet is closest to the sun; (4) its eccentricity; (5) the major semi-axis of the orbit; and (6) the time at which the comet passes the perihelion. In addition to these, the velocity of the planet or comet must be known. Thus notes like that in Anglo-Saxon Chronicles or in al-Mas'udi's work are not enough to identify a comet; astronomers must have precise information and in the pre-Newtonian era, the only people in the world who could provide this sort of detailed information about comets were the Chinese.
Because the Chinese believed that celestial and meteorological events were warnings to the emperor, China had official astronomers attached to the court for 2,000 years: from 200 B.C. to 1912. Since their task was to record the smallest changes in the heavens, the imperial observatory was supplied with instruments to measure the positions of the planets and the stars, and nightly observations were recorded each morning in a log book. Furthermore, five mathematicians spent every night looking at the heavens – one at the zenith, the others at the four quadrants - noting changes in the direction of the winds, rain, eclipses, meteors, conjunctions of planets, and, of course, comets. Thus, unhampered by Aristotle, China recorded all celestial events - data that would prove invaluable to later researchers.
In 1623, two Jesuits, one a former student of Galileo named G. Schreck, arrived in Peking with the first telescope seen in China, as well as news of the recent discoveries in Europe by Kepler and Copernicus. Their success in predicting an eclipse in 1629 - discomfiting the Arab and Chinese astronomers at the Ming court-led to a close relationship between European and Chinese scientists and in 1759, another jesuit sent a translation of Chinese sightings of comets to the Royal Observatory in Paris, observations which eventually helped Europeans trace former appearances of Halley's Comet back to 240 B.C.
That same year - 1759 - was the year in which Halley had predicted the comet sighted in 1682 would return, if Newton's theory on orbits and his own calculations were correct. This comet, he had also said, was the same comet sighted by Apiano in 1537 and by Kepler in 1607.
Since Halley's prediction, scholars all over Europe had busied themselves trying to predict the exact date that the comet would appear in 1759 - and trying to establish the dates of previous sightings so that the periodicity of the comet could be established. With the help of the dates from China, they decided that it would not be exactly 76-year periods between visits - and that proved to be correct. The period between that comet's appearance in 1531 and 1607 was 76 years and 2 months, while that between 1607 and 1682 was 74 years and 11 months.
It even came earlier than Halley predicted: on Christmas night, 1758, a farmer and amateur astronomer named Palitzsch, who lived near Dresden, sighted the long-awaited comet. But the sighting nevertheless fulfilled Halley's prediction and conclusively proved Newton's law of Universal Gravitation.
Since then, the comet has been seen in 1835, 1910 and is visible right now as 1986 gets underway, having reached its aphelion in 1948 - the point when it was farthest from the sun after the 1910 visit - and having begun to return at an initial speed of 910 meters (2,985 feet) per second.
In its 1910 appearance, interestingly enough, the ideas of Aristotle seemed to emerge again: in the American South, pills were sold against its evil effects, in Russia special masses were celebrated to protect the people and in Italy bottled air was sold to protect people against the evil miasma the comet was thought to bring in its wake. Some people, terrified by the comet's approach, spent 1910 in caves, mines and underground tunnels, and even some scientists were concerned. One French astronomer, Camille Flammarion, warned that the comet's tail of "poisonous gas" could wipe out life on earth.
This year there is another link with Aristotle. As part of the world's massive welcome, Fred Hoyle and Chandra Wickramasinghe, two of Britain's leading astronomers, hope to test their theory on the origins of life: both believe that life originated in outer space and came to earth as microorganisms that bombarded the earth in the wake of comets - causing, over millions of years, evolutionary changes not explained by Darwin's theory. It is interesting - and probably would have amused Aristotle - that Wickramasinghe arrived at that theory by studying influenza epidemics; he thinks they are caused by microorganisms from outer space. Maybe Aristotle and the learned men of the Middle Ages were not so wrong after all.
