|The Planet Mars: A History of Observation and Discovery|
source ref: ebookmars.html
On September 5, 1877, Mars came to a perihelic opposition in the constellation Aquarius, approaching to within 35 million miles (56 million km) of the Earth. Since the last perihelic opposition in 1860, the 26-inch (66-cm) refractor of the U.S. Naval Observatory, located at Foggy Bottom on the banks of the Potomac River in Washington, D.C., had gone into operation. In 1877, Asaph Hall was in charge of it and planned to use it to search for Martian satellites (fig. 4).
Hall was the son of a failed clock maker. He and his wife, Angelina, were working as schoolteachers in Shalersville, Ohio, in the 1850s when Hall decided that he wanted to become an astronomer. Though he had not received much formal training (he had stayed only a year at the University of Michigan at Ann Arbor before leaving because of a lack of funds), he applied to become an assistant at the Harvard College Observatory. William Cranch Bond, the observatory's director, was, like Hall, the son of a clock maker. He too had begun his astronomical career without many advantages, and he duly hired the young man to assist himself and his son, George Phillips Bond.
The position was not a lucrative one, and Hall later recalled that when he first met G. P. Bond, who had been away from the observatory when Hall arrived, Bond "had a free talk with me, and found out that I had a wife, $25 in cash, and a salary of $3 a week. He told me very frankly that he thought I had better quit astronomy, for he felt sure that I would starve. I laughed at this, and told him my wife and I had made up our minds that we were used to sailing close to the wind, and felt sure we would pull through."1
Hall left Harvard in 1863 for the U.S. Naval Observatory in Washington, D.C., and took charge of its great refractor, the first of the great refractors made by miniature painter--turned-optician Alvan Clark, in 1875. For two years it had been in the hands of Simon Newcomb, who was more interested in mathematical astronomy than in observing, and his assistant, Edward Singleton Holden. Hall later recalled finding "in a drawer in the Eq[uatorial] room a lot of photographs of the planet Mars in 1875. From the handwriting of dates and notes probably Holden directed the photographer, but whoever did the pointing of the telescope had . . . satellites under his eye."2
Hall later retraced the steps that led him to undertake his own search for Martian satellites:
|In December, 1876, while observing the satellites of Saturn I noticed a white spot on the ball of the planet, and the observations of this spot gave me the means of determining the time of the rotation of Saturn, or the length of Saturn's day, with considerable accuracy. This was a simple matter, but the resulting time of rotation was nearly a quarter of an hour different from what is generally given in our text books on astronomy: and this discordance, since the error was multiplied by the number of rotations and the ephemeris soon became utterly wrong, set before me in a clearer light than ever before the careless manner in which books are made, showed the necessity of consulting original papers, and made me ready to doubt the assertion one reads so often in the books, "Mars has no moon."3|
On looking further into the matter, Hall learned that William Herschel had looked unsuccessfully for satellites in 1783, and that the director of the Copenhagen Observatory, H. L. d'Arrest, had done so in 1862 and 1864. (He did not mention Holden's photographic search with the great Washington refractor.) Of these searches, d'Arrest's had been the most thorough. He had been guided by rough calculations of the distance from the planet at which a satellite could exist before it was wrenched away by the Sun into its own planetary orbit, and had set this limit at a distance corresponding to 70' of arc from the planet at greatest elongation. Hall, on redoing the calculation, realized that the actual limit ought to be more like 30' of arc, and that Martian satellites were likely to be found even closer than that to the planet. He began to suspect, therefore, that d'Arrest, for all his thoroughness, had not paid sufficient attention to the inner space near the planet.4
When Hall began his quest in early August, he naturally wanted to work alone, so as to receive full credit in the event of a discovery. By great good luck, Holden, his assistant, was invited by Henry Draper to Dobbs Ferry, New York, "at the very nick of time."5 Hall began by scrutinizing faint stars at some distance from Mars itself, but each one soon dropped behind the planet, proving it to be an ordinary field star.6 Next he pressed the search closer, "within the glare of light that surrounded [Mars]," using special observing techniques to reduce the glare, such as "sliding the eyepiece so as to keep the planet just outside the field of view, and then turning the eyepiece in order to pass completely around the planet." On the night of August 10, the first on which Hall attempted to examine the inner space near Mars, he found nothing, but the seeing on the banks of the Potomac was horrible that night, and the image of the planet appeared "very blazing and unsteady." He was on the verge of giving up, but Angelina encouraged him to have one more try, and the next night, at half past two, he found a suspicious object which he referred to in his notebook only as "a faint star near Mars." He scarcely had time to secure its position before the fog began rolling in from the Potomac. The next few nights were cloudy. On August 15, the sky cleared at eleven o'clock, but the atmosphere, Hall noted, was still "in a very bad condition." Not until August 16 did he again find the "star near Mars," which proved, in fact, to be the outer satellite. That night he showed the object to another assistant, George Anderson, but told Anderson to "keep quiet" about it. On August 17, while waiting for that satellite to reappear, he discovered the inner one. In closing his observing notes for the night, he remarked: "Both the above objects faint but distinctly seen both by G. Anderson and myself." Hall had by this time "spilled the beans" to Simon Newcomb, and on August 18, Hall and Anderson were joined in the dome by David Peck Todd, Newcomb, and William Harkness. Todd noted: "Seeing extremely bad: still I saw the companion without any difficulty. `Halo' around the planet very bright, and the satellite was visible in this halo." Only then did Hall announce the discovery of the two satellites. Newcomb tried to gain a share of the credit for himself, implying in an article that appeared in the New York Tribune two days after the discovery was announced that Hall had not fully appreciated what he had found until he---Newcomb---had worked out the period of revolution from the preliminary observations.
Meanwhile, in New York, Holden and Draper were also getting into the act. On August 28, Holden announced that they had used Draper's 28-inch (71-cm) reflector to discover a third satellite, and on returning to Washington, Holden claimed to have found yet a fourth. Hall was skeptical and wrote to Arthur Searle of Harvard: "I think it will turn out that the Draper-Holden moon and the recent Holden moon do not exist."7 He attempted to confirm these alleged discoveries with the Washington refractor without success, and later computations showed that Holden's moon did not even obey Kepler's laws of motion. "Its existence was therefore a mathematical impossibility," Hall wrote to Edward C. Pickering of Harvard Observatory, adding bitterly: "If I were to go through this experience again other people would verify their own moons."8
Rumors of Holden's spurious moons would continue to circulate in the astronomical community for years, and Holden became known as the man "who had set all Washington astronomers laughing by detecting a . . . satellite of Mars with an impossible period and distance, and remaining deceived by it for months!"9 But Holden, in Hall's view, at least, had behaved more admirably than Newcomb. As late as 1904 Hall was still bitter about Newcomb's attempt to usurp credit for the discovery of the satellites, and wrote to S. C. Chandler, Jr.: "Newcomb was greatly excited over my discovery. Holden was away, and Draper made a blunder, and afterwards Holden behaved very well. Newcomb felt disappointed and sore, and something is to be allowed for human nature under such circumstances. He was always greedy for money and glory."10
In response to a suggestion by Henry Madan of Eton, England, Hall named the satellites Phobos (Fear) and Deimos (Flight), after the attendants of Mars mentioned in the fifteenth book of Homer's Iliad: "He spake, and summoned Fear and Flight to yoke his steeds." Hall continued to watch the new satellites until the end of October, and his observations gave him the information he needed to work out the mass of Mars---the amount of matter it contains---from its effect on the moons' motions. It was 0.1076 times that of the Earth (a value very close to the currently accepted value of 0.1074). We will examine the satellites in greater detail in chapter 14.
Phobos and Deimos were seen not only by Hall but by viewers using much smaller instruments---indeed, Deimos was glimpsed by Hall, John Eastman, and Henry M. Paul with the U.S. Naval Observatory's own 9.6-inch (24-cm) refractor. This only goes to show that the discovery of the satellites of Mars owed quite as much to Hall's insight---his imagination and willingness to doubt conventional wisdom---as to the size of his glass. As he later wrote, "All that was needed was the right way of looking, and that was to get rid of the dazzling light of the planet."11 He was confident that with the right way of looking, the satellites could have been found "very easily" even with Harvard's 15-inch refractor in 1862.
There were other important studies of Mars in 1877 as well. Nathaniel Green, an amateur astronomer and professional portrait artist who at one time had given lessons in painting to Queen Victoria, made a careful study with his 13-inch (33-cm) Newtonian reflector on the eastern Atlantic island of Madeira, a site renowned for its pellucid skies. Green drew up a map and also noted brightenings at the limb and terminator which he identified correctly as morning and evening clouds (fig. 5).12 Another valuable set of observations was made by Englishman Henry Pratt, using an 8-inch (21-cm) reflector. Pratt reported that "in moments of the finest definition the markings have exhibited a stippled rather than a streaked character, and glimpses were obtained of a structure so complicated and delicate that the pencil cannot reproduce it. . . . Frequently what at first sight appeared as a broad hazy streak has been, by patient watching for the best moments, resolved into several separate masses of shading enclosing lighter portions full of very delicate markings."13
But by far the most important development of the memorable 1877 opposition---after Hall's discovery of the satellites, of course---was the landmark study of the planet begun by the Italian astronomer Giovanni Virginio Schiaparelli, who would become the leading expert on the planet for the next two decades (fig. 6).
Schiaparelli was born on March 14, 1835, in the town of Savigliano, in the Piedmont region of northwestern Italy, not far from the French border.14 The town lies among the foothills of the Alps and is overlooked by an ancient Benedictine abbey. His interest in astronomy was first awakened when his father, a furnace maker, took the four-year-old Schiaparelli outside on a clear and serene night and pointed out some of the constellations. "Thus, as an infant," Schiaparelli later wrote, "I came to know the Pleiades, the Little Wagon, the Great Wagon. . . . Also I saw the trail of a falling star; and another; and another. When I asked what they were, my father answered that this was something the Creator alone knew. Thus arose a secret and confused feeling of immense and awesome things. Already then, as later, my imagination was strongly stirred by thoughts of the vastness of space and time."15 Young Schiaparelli's interest was further stimulated by the total eclipse of the Sun of July 8, 1842, which he observed with his mother through the window of the family's casa, and by the instruction of a learned priest of Savigliano, Paolo Dovo, who lent him books and gave him, from the campanile of the church of Santa Maria della Pieve, his first views through a small telescope of the phases of Venus, the moons of Jupiter, and the rings of Saturn.
On completing the rather rudimentary course that was all the elementary schools of Savigliano had to offer, Schiaparelli went on to the University of Turin, from which he graduated in 1854 with a degree in architectural and hydraulic engineering. For a time he employed himself in the private study of astronomy, mathematics, and languages, and in November 1856 he received an appointment as a teacher of mathematics in an elementary school in Turin. But his heart was not in it. Instead, as he later wrote, "without taking into account my almost absolute poverty, I formed the project of devoting myself to astronomy, which was not done without much opposition on the part of my parents."16
His great opportunity came in February 1857 when the Piedmontese government awarded him a small stipend, enabling him to receive training in astronomy. He spent two years studying at the Royal Observatory in Berlin, which was then under the directorship of Johann Franz Encke, and another year at the Pulkova Observatory working under Wilhelm Struve. Then, in 1860, he returned to Italy to take up a post as secondo astronomo under Francesco Carlini at the observatory at the Brera Palace in Milan, which had been founded by the famous Jesuit astronomer Roger Boscovitch a century earlier. The instruments at the observatory were antiquated and woefully inadequate; there was only an old equatorial sector and a meridian circle with lenses of 4 inches (10 cm) aperture. Nevertheless, Schiaparelli made the best use of the modest resources available to him, and in April 1861 employed the equatorial sector to discover the sixty-ninth asteroid, Hesperia. The following year, Carlini died and Schiaparelli succeeded him as director.
The 1860s saw immortal work in Milan: the brilliant investigation in which Schiaparelli showed that the August meteors follow the same orbit as that of the bright comet Swift-Tuttle (1862 III), thereby forging a link between comets and meteors and at the same time answering the question about falling stars that he had put to his father so long before.17 For this work he was awarded the prestigious Lalande Prize of the French Académie des Sciences in 1868.
The fame of his meteor work and the growing national pride of the recently organized Kingdom of Italy brought Schiaparelli a more powerful telescope, an 8.6-inch (22-cm) refractor made by Merz, Fraunhofer's successor, which was installed on the roof of the Brera Palace in 1874 (fig. 7). At first, Schiaparelli used it mainly to measure double stars. Over the next twenty-five years he would make 11,000 such measurements, and the keenness of his eyesight is attested by the close separations of some of the double stars on which he successfully put the wires of the micrometer (incidentally, Schiaparelli, like Dawes, was very nearsighted; again, this did not interfere with his work at the eyepiece). Robert G. Aitken, a highly respected observer of double stars with the much larger telescope at the Lick Observatory, would later say of Schiaparelli's measures of the double star ß Delphini that "the residuals shown in Schiaparelli's measures . . . will not seem very large---in fact it is surprising that measures of such a pair could be obtained at all with so small a telescope."18
In 1877, Schiaparelli began his studies of the planets. Before describing what he found out about Mars, I will summarize some of the results he obtained on the other planets. At the time, the rotations of all the planets from Mercury to Mars were believed to be about twenty-four hours. The rotation of Mars had, of course, been established beyond dispute, and indeed was well known to within a tenth of a second. However, Schiaparelli had little confidence in the rotations ascribed to Mercury and Venus, and he decided to investigate the matter further.
Venus is brilliant but difficult to observe because it usually shows only nebulous and ill-defined surface markings. In December 1877, Schiaparelli made out a pair of bright oval spots near the southern cusp of the planet, and also a shadowy streak. The markings were unusually conspicuous by Venusian standards, and he kept them under observation for two months, during which he failed to detect the slightest change in either their form or their position relative to the terminator. He therefore concluded that Venus's rotation was very slow, between six and nine months, and probably equal to the period of its revolution---224.7 Earth days.19 This announcement received support from some quarters, and disagreement from others. Indeed, the question of Venus's rotation continued to vex visual observers right up to the early 1960s. It was finally settled only by advanced methods of radio astronomy.20
Unlike brilliant Venus, Mercury had received little attention from astronomers; being innermost to the Sun, it is notoriously difficult to observe. Schroeter, in 1800, had offered the only positive determination; he had observed exclusively in twilight periods, and he deduced from the blunted appearance of the southern cusp, which seemed to be unchanged from one night to the next, that the period of rotation must be right around twenty-four hours. Schiaparelli found the twilight conditions generally unfavorable because of the planet's low altitude, and instead decided to try to observe Mercury during broad daylight, when it was higher in the sky. He made the first tests in June 1881, and was encouraged enough to plan a regular study in 1882. That year he made every effort to keep the planet under continuous surveillance, observing it on February 4--10, March 31--April 28, May 24--31, August 5--21, and September 19--30. Although most of the observations were made around the times of Mercury's greatest elongations from the Sun, the planet was near superior conjunction in August, and Schiaparelli succeeded in following the small gibbous disk, then only 4" of arc across, to within 3.5° of the Sun. With his fine refractor he made out markings on the surface which, he wrote, usually appeared "in the form of extremely faint streaks, which under the usual conditions of observations can be made out only with the greatest effort and attention."21
Schiaparelli's best observations were made in 1883--84. He was on the verge of making his results public then, but he decided to wait until he had had a chance to observe Mercury with the observatory's new 19-inch (49-cm) refractor, which was installed in 1886. These studies added nothing substantially new. Finally, in 1889, he was ready to announce his main result: the rotation period of Mercury, he declared, is equal to that of its revolution, eighty-eight days, and thus one side of the planet is always perpetually in daylight and the other in darkness. Though his observations had indicated a definite slow drift of markings across the disk, he explained this as an effect of libration, already well known in the case of the Moon, with its captured rotation with respect to the Earth. The Moon's libration is simply a result of the fact that the constant rate of the Moon's axial spin gets out of step with its changing velocity in its elliptical orbit, thereby producing an apparent rocking movement to and fro. Mercury, because of its very eccentric orbit around the Sun, would be expected to have a very marked libration, amounting to some 47° 21' in longitude. But even granting such a broad allowance for variation in the observed position of the planet's features, Schiaparelli still found discrepancies, and he remarked to the English astronomer William F. Denning that the markings were "extremely variable." Sometimes they seemed to be "partially or totally obscured." Moreover, the planet showed "some brilliant spots which change their position." Thus Schiaparelli concluded that the surface of Mercury was sometimes covered by "veils [of] . . . more or less opaque condensations produced in the atmosphere of Mercury, which from afar presents aspects analogous to those which our Earth would show from a similar distance."22
Other observers, including Henri Perrotin, Percival Lowell, and René Jarry-Desloges, confirmed the eighty-eight-day rotation period. So, most notably, did the skilled Greco-French astronomer Eugène Michael Antoniadi, who between 1924 and 1929 carried out a careful study of Mercury with the 33-inch (83-cm) refractor at Meudon Observatory, near Paris. He agreed with Schiaparelli's rotation period, and also supported the existence of Mercurial clouds. Among the leading observers of the planet, only the French astronomer Georges Fournier was unable to reach a definite conclusion as to the rotation of Mercury.23
In fact, Fournier's diffidence proved to be justified. The true rotation of Mercury was discovered only in 1965 by radio astronomers. Instead of being equal to the period of revolution, 88 days, it is precisely two-thirds of this---58.65 days. How could the visual observers have been so far wrong? It turns out that 58.65 days is not only two-thirds of Mercury's period of revolution around the Sun, it is close to half of the period between its successive appearances in the same phase as viewed from the Earth (the synodic period, which in Mercury's case is 116 days). Thus, when Mercury comes to its greatest elongations from the Sun and is best placed for observation, astronomers in the Northern Hemisphere tend to see the same features; it is natural to conclude from this that the planet always keeps the same face toward the Sun.24 The selective observation of the planet through periodic observing windows has been called the stroboscope effect. The synchronism is not perfect; after about seven years new regions of the planet begin to swing into view---but by the time seven years had passed, Schiaparelli had already made up his mind and had given up regular observing of the planet. As for his successors, their results demonstrate only too clearly that once a definite expectation is established, it is inevitable that subsequent observers will see what they expect to see, refining their expectations in a continuing process until finally everyone sees an exact and detailed---but ultimately fictitious---picture.
Even Antoniadi was misled; ironically, by the time he began his study of Mercury, the same features had returned to the disk that had been present for Schiaparelli's observations. He too kept the planet under observation for only seven years, and thus it is not surprising that (with due allowance made for differences in their drawing styles) Antoniadi's chart should appear almost identical with Schiaparelli's, or that the drawings of later observers appear to have been, in the words of Clark Chapman and Dale Cruikshank, "subconscious reproductions of Antoniadi's chart."25
I hope the reader will forgive me for going into the episode of Mercury's rotation at such great length. It provides, I think, an excellent introduction to the Martian "canal" affair because it clearly illustrates what Antoniadi once referred to as "the snares awaiting the observer at each stage of his work,"26 snares from which no observer, no matter how skillful or cautious, can ever be entirely immune.
These snares are nowhere better documented than in the history of the famous, or infamous, Martian canals. We now return, therefore, to the fall of 1877, with Mars close to the Earth. Schiaparelli was eager to test the 8.6-inch Merz refractor---which he had been using for two years to study double stars---on Mars. It is clear that he did not at first intend to devote himself to a continued series of observations of the planet. Rather, he explained,
|I desired only to experiment to see whether our refractor . . . possessed the necessary optical qualities to allow for the study of the surfaces of the planets. I desired also to verify for myself what was said in books of descriptive astronomy about the surface of Mars, its spots, and its atmosphere. I must confess that, on comparing the aspects of the planet with the maps that had been most recently published, my first attempt did not seem very encouraging.27|
Further study showed, however, that his drawings agreed quite well with the best made at previous oppositions, such as those by Kaiser and Lockyer. Thus, on September 12, 1877, he resolved on a careful study for the purpose of drawing up a new map of the planet. He generally used a magnifying power of 322x on the Merz refractor (and later, when the disk had become very small as the planet receded to a great distance from the Earth, 468´). Meticulous observer that he was, he was not satisfied to rely on eye estimates of the positions of features. Instead, he based his map on micrometric measures of the longitudes and latitudes of sixty-two distinctly recognizable points on the planet. The resulting map was a tremendous advance over anything that had appeared before. It was, Camille Flammarion declared, "a truly remarkable piece of work, and one showing features which the old observers of Mars could never have suspected. It depended for its successful completion on an unflagging persistence, an excellent eye, a rigorous method of observation and a good instrument."28
Precisely for this reason, Schiaparelli found himself faced with a dilemma. At first he had intended to adhere to Proctor's nomenclature, which Flammarion had already adopted for his 1876 map, with a few changes; for example, Flammarion had decided to retain the older name Mer du Sablier, French for "Hourglass Sea," instead of using Proctor's Kaiser Sea. On looking through his 8.6-inch Merz, however, Schiaparelli found that drastic changes were necessary; some names had to be abandoned, and many new ones had to be introduced to describe the numerous features being seen for the first time. Proctor's four main "continents" were actually a multitude of islands, several of his "seas" had disappeared or shrunk to insignificance (Main Sea, Dawes Sea), while still others had opened up. "In order to avoid misunderstandings and mistakes," Schiaparelli wrote, "I had to create a special nomenclature, which served my particular purpose. This nomenclature, which was devised while I was laboring at the telescope and is probably not without many shortcomings, was retained in my memoir only because it described perfectly what had been seen."29
He named the bright and dark areas on Mars after terrestrial lands and seas, and did so without apology, since, he explained,
|in general the configurations seen represented such a clear analogy to those of the terrestrial map that it is doubtful whether any other class of names would have been preferable. Do not brevity and clarity also induce us to use such words as island, isthmus, strait, channel, peninsula, cape, etc.? Each of which provides a name and description which expresses well what could not otherwise be expressed except through long paraphrases that would need to be repeated each time one spoke of the corresponding object. . . . In order to avoid prejudice regarding the nature of the features on the planet, these names may be regarded as a mere artifice. . . . After all, we speak in a similar way of the seas of the Moon, knowing very well that they do not consist of liquid masses.30|
Rather than follow Proctor and use the names of past, and in some cases still living, observers of the planet, Schiaparelli drew his appellations from his intimate knowledge of classical literature and the Bible. The ancient Greek founder of physical geography, Dicaearchus, had drawn a line through the middle of his map of the Mediterranean world running from the pillars of Hercules in the west to the Taurus Mountains in the east, which he had called the "great diaphragm." Schiaparelli drew a similar line on Mars running between the belt of dark markings to the south and the lighter regions to the north. The main dark areas, which were given the names of bodies of water, were, proceeding eastward from the Herculis Columnae (Columns of Hercules) in the extreme west: Mare Sirenum (Sea of Sirens), Mare Cimmerium (Sea of the Cimmerians), Mare Tyrrhenum (Tyrrhenian Sea), Mare Hadriaticum (Adriatic Sea), Syrtis Major (Gulf of Sidra), Sinus Sabaeus, Margaritifer Sinus (Pearl-bearing Gulf, the old name for the rich coast of India), Aurorae Sinus (Bay of the Dawn), and Solis Lacus (Lake of the Sun, recalling the legend according to which the Sun rises "in the baths of the ocean"). The bright areas were named for lands. Thus Ausonia (Italy) was separated from Libya by the Tyrrhenian Sea, and other lands included Hellas (Greece), Aeria, Arabia, Eden, Chryse, Tharsis, and Elysium, names which have since become a rich part of Martian lore.
"I do not ask that the [nomenclature] be approved by astronomers in general, nor do I request the honor of its universal acceptance," Schiaparelli wrote modestly. "To the contrary, I am ready to accept as final whichever one is recognized by competent authority. Until then, however, grant me the chimera of these euphonic names, whose sounds awaken in the mind so many beautiful memories."31 The plain fact of the matter is that Schiaparelli had effectively refashioned Mars with a set of romantic and wistfully evocative names, whose power, despite his stated cautions, was not to be lost on the human capacity to yearn after lost paradises and conjure up nostalgic visions. What Percival Lowell once said about naming is no doubt very true: "Naming a thing is man's nearest approach to creating it."32 In a sense, Schiaparelli's names created a new Mars, or at least a new way of looking at the old Mars. Although it encountered some initial resistance, his nomenclature eventually prevailed. With the single exception of the name Hourglass Sea (Mer du Sablier), which Flammarion continued to prefer to Schiaparelli's Syrtis Major, the French astronomer admitted that Schiaparelli's nomenclature was "euphonic and charming," and he added: "Personally, I hope with all my heart that this ingenious areographical nomenclature will replace all preceding systems."33 So it did.
It is interesting, and no doubt psychologically significant, that after the introduction of this new map of Mars bearing names so apt to appeal to human emotions, the planet began to gather around itself in succeeding years a considerable mythology of its own. Moreover, it is fitting that this map, whose nomenclature put Mars as much in the realm of mythical as of factual places, should also have been the first to include the strange canali, or "canals," which played such an important role in the planet's subsequent mythification (fig. 8).
Indeed, as Schiaparelli continued his observations of the main markings of the planet, smaller details flashed out at him from time to time. There were, for instance, two or three occasions in October 1877 when he witnessed "moments of absolute atmospheric calm. In these circumstances it seemed as if a veil were removed from the surface of the planet, which appeared like a complex embroidery of many colors. But such was the minuteness of these details, and so short the duration of their visibility, that it was not possible to form a stable and sure impression of the thin lines and minute spots therein revealed."34
To Schiaparelli's eye these fine details were predominantly linear features, for which he adopted the convenient term that Secchi had first introduced---canali. In Italian, canali can mean either "channels" or "canals." It is clear that Schiaparelli had completely natural features in mind---indeed, he often used the word fiume (river) as a synonym. Strictly speaking, the term channel would have been preferable, but instead it was canal, with all its connotations of artificial waterways, that was adopted in English, with far-reaching consequences.35
Schiaparelli's earlier training in draftsmanship had given him the ability "to transcribe quickly onto paper the almost cinematic impressions of the figures observed in the field of the telescope."36 However, his eye was "strongly affected by daltonism," or color-blindness; thus, as he himself admitted, he "failed to distinguish gradations of red and green," and he once described the general appearance of the major markings as "almost like that of a chiaroscuro made with Chinese ink upon a general bright background."37 On the other hand, his color-blindness seems to have made him more sensitive to delicate markings at the threshold of visibility; as a record of fleeting impressions, his observations are unrivaled.38
The canals appeared to Schiaparelli only one or two at a time rather than as a whole network. Moreover, and strangely, they were not always best seen when Mars was nearest the Earth but in some cases long afterward; in the words of Percival Lowell, "distance . . . is not, with the canals, the great obliterator."39 Some of Schiaparelli's own notes in this regard are well worth quoting. On October 4, 1877, when the planet's disk was 21" of arc across, he recorded in the yellow region between Margaritifer Sinus and Aurorae Sinus only the broad Ganges canal, even though he enjoyed moments of perfect definition. The same area remained unchanged when he studied it again in early November, but on February 24, 1878, on a disk of only 5.7" arc, he found in this hitherto blank region the Indus, which was now "easily visible."
Though later observers were baffled by these observations, there is a perfectly logical explanation. We now know that dust clouds develop on Mars and are especially frequent during summer in the Martian southern hemisphere, when the planet is near perihelion. Though at times only parts of the surface are covered, at other times the clouds may spread into planet-encircling or even global storms enveloping all the surface features; the latter was the case, for example, in 1971. Schiaparelli made perhaps the first reliable observations of these clouds. Already at the end of September 1877, he had made out a large, bright cloud east of Solis Lacus. On October 10, he found that Mare Erythraeum and Noachis were covered. In beginning his observations that evening, he found that everything appeared normal between 240° and 350° W longitude; he then interrupted his observations to secure a set of measures of a new comet that had been discovered a few days earlier by Wilhelm Tempel at Arcetri. On returning to Mars, with the central meridian now standing at 8° longitude, he wrote in his notebook: "Mars is beautiful. The Mare Erythraeum in large part appears covered by cloud. Noachis is dim. The continent of Deucalion is hardly observable. However, Arabia is plainly in view, and the Sinus Sabaeus stands out as well as ever."40
In addition to these clouds, which stood out in contrast to the dark areas beneath them, there seemed to be still others in the bright areas, where "their presence becomes recognizable only in a negative sense---that is, not from what is seen of them, but from what they hide from view."41 From September to December 1877, the greater part of the planet between the line of the "great diaphragm" and latitude 30° N appeared thus "covered with clouds," including the continental area in which the Indus afterward made its belated appearance. Schiaparelli noted that in the period around opposition, which was shortly after the summer solstice in the southern hemisphere of Mars, clouds and veils were frequent, but that by January, February, and March 1878 the atmosphere of the planet had largely cleared. Thus many of the canali, hitherto veiled by mists and clouds, were revealed for the first time despite the much reduced size of the disk.
Schiaparelli was not the first to see the canali. A few ill-defined streaks appear in Schroeter's drawings, and there seems to be at least one in a drawing by Beer and Mädler; still others were recorded by Secchi, Kaiser, and Lockyer, and Dawes was especially prolific in noting them. But with Schiaparelli the canali became the dominant motif of the planet, as a simple glance at his map suffices to show. Martian research, long dominated by a simple "analogy to the Earth" approach, and moving confidently forward as observers found further correspondence at each successive stage of discovery, had clearly entered a startling new phase.