|The Planet Mars: A History of Observation and Discovery|
source ref: ebookmars.html
Mars is by no means an easy object to study. It is a small world, only slightly more than half the diameter of the Earth, and even at its nearest it never approaches closer than 140 times the distance of the Moon. Moreover, the features on its surface are of low contrast, and thus are difficult to delineate accurately. To study Mars properly, perfect instruments and a steady atmosphere on Earth are required.
The large reflectors used by Herschel and Schroeter showed considerable detail and allowed the first reasonably thorough study of the planet's polar caps, axial inclination, and seasons. These instruments were far from ideal, however; the mirrors, which were made of speculum metal---an alloy of copper and tin---were difficult to figure accurately, and they tarnished easily. (It was only in the late nineteenth century that the more satisfactory method of silver coating glass mirrors was introduced.)
Meanwhile, the refractor had begun to make a comeback. In 1729, an Englishman named Chester Moor Hall showed that Newton had been premature in considering the problem of the refractor insoluble.1 Hall showed that when a concave lens of flint glass was combined with a convex lens of crown glass, the chromatic aberration produced by one lens could be made to nearly cancel that of the other for a certain band of wavelengths; say, the yellow and green spectral region to which the human eye is most sensitive. He had discovered the secret of the achromatic lens, but he himself did not pursue it further, and the importance of his discovery was not fully realized by other opticians. It was not until after 1750 that another Englishman, John Dollond, went into business with his son Peter to make achromatic lenses on a commercial basis.
A good refractor with a 2.5-inch (64-mm) objective lens could now have a length of 20 inches (51 cm) instead of 20 feet (6.1 m), as in the days of the aerial telescopes. Obviously, this was a tremendous improvement; but even so, the early instruments of the type were far from perfect. Their components were not correctly matched, and bright objects seen through them continued to have a troublesome amount of unfocused light (secondary spectrum), so that they were surrounded by green and wine-colored fringes. Later, in order to correct for the secondary spectrum, Peter Dollond began adding yet a third component to his lenses. One of these triplet lenses with a 3.75-inch (95-mm) aperture was purchased by Neville Maskelyne for the Royal Observatory at Greenwich, and another, of 3.8 inches (97 mm), was acquired by the wealthy connoisseur Dr. William Kitchener, who commented that "it shews the disc of the Moon and of Jupiter as white and as free from colour as a Reflector."2 But progress in this direction was limited. Lenses larger than about 4 inches (10 cm) in diameter proved to be almost impossible to make because the disks always developed streaks and veins when the flint was cast.
Schroeter made few observations of Mars after 1800. Instead, the most diligent observer of the period was a Frenchman, Honoré Flaugergues, who is best remembered today as the discoverer of the great comet of 1811. He had a private observatory at Viviers (Ardèche), in southeastern France, whose main instrument was a rather inefficient achromat of 44-foot (13.4-m) focal length, giving a usual magnification of only 90x. Flaugergues began observing Mars at the opposition of 1796, but carried out especially thorough studies at the oppositions of 1809 and 1813, the latter being the first perihelic opposition of the new century. He noted the more prominent patches on the disk, which appeared dark reddish to him, and calculated the times at which the planet's rotation ought to bring the same aspects into view again. He found great inconsistencies, however, and was unable to believe that he was actually observing the solid surface of the planet. Instead, he accepted Schroeter's conclusion that only atmospheric features were on view. "These patches seemed to me to be in general confused and badly defined," he wrote in 1809, "to an extent that it was difficult to distinguish exactly their outlines and their full extent. I can say only that normally the south part of the disk was the region of Mars which contained the principal patches."3 As noted earlier, we now know that Mars is sometimes obscured by dust clouds and veils, and some historians claim that Flaugergues was the first to record them.4 Here I must disagree; Flaugergues simply wasn't a good enough observer, and his telescope was too small and too crude to have produced trustworthy results. It is clear that for all his diligence, he achieved no significant advance over Schroeter or even Maraldi.
In addition to his rather confused observations of the spots on Mars, Flaugergues noticed the rapidity of the south polar cap's melting and remarked that if it consisted of ice and snow, "as everyone believes," this proved that Mars, despite its greater distance from the Sun, must be warmer than the Earth!
Meanwhile, important developments had been taking place in optics. In 1799, a Swiss artisan named Pierre Louis Guinand had discovered that flawless disks as large as 6 inches (15 cm) in diameter could be cast from flint glass if the flint was stirred while it was cooling. Guinand moved to Munich in 1805, and there he joined forces with a brilliant young optician, Joseph Fraunhofer, who under Guinand's supervision became extremely proficient in glassmaking. Moreover, Fraunhofer practically devised the science of correctly designing achromatic objective lenses using only two components. By 1812 he had succeeded in producing a fine achromat 7.5 inches (19 cm) in diameter. Fraunhofer's refractors produced clear, brilliant images of the Moon and planets, and were a marked improvement over the large reflectors used by Herschel and Schroeter. By 1813, the year Schroeter's observatory at Lilienthal was destroyed by the French, the refractor was once more coming to dominate astronomy---indeed, the nineteenth century would become the "century of the refractor."
Among the first to use a Fraunhofer refractor to view Mars was Georg Karl Friedrich Kunowsky. He was a lawyer by profession, and served as Justizrat in Berlin, but Kunowsky was also a keen amateur astronomer. With a 4.5-inch (11-cm) Fraunhofer refractor, he made a number of sketches of the dark patches at the unfavorable opposition of 1821--22.5 Unlike Schroeter and Flaugergues, however, Kunowsky came to the conclusion that the patches were fixed features of the surface. Nevertheless, his results were hardly definitive, and the question---163 years after Huygens had first sketched Syrtis Major---remained unresolved. That being the case, it is easy to understand why no one had seen any point in attempting to draw a map of the planet.
All of this would change with the advent of Wilhelm Beer and Johann Heinrich Mädler, who began the next new era in the study of Mars---a period Flammarion described as the "geographical period." Thus far the study of Mars had advanced by slow and uncertain steps; after Beer and Mädler many uncertainties remained, but henceforth the results rested on a firmer basis. Flammarion, in his incomparable fashion, said it best: "Christopher Columbus was happy when he was halted by the American continent during his voyage of circumnavigation beyond Asia. Mars does not have its Christopher Columbus. He achieved fame by the single fact of touching America; a phalanx of astronomers has been busy for more than a century studying their celestial continent. But Beer and Mädler deserve to be remembered as the true pioneers in this new conquest."6
Mädler was born in Berlin in 1794, the son of a master tailor.7 He had taught himself to read by the time he was four, and a maternal uncle, Paul Strobach, recognizing his ability, pushed for him to get a sound education. At age twelve he was sent to the Friedrich-Werdersche Gymnasium in Berlin. Meanwhile his interest in astronomy had surfaced, inspired by the comet Flaugergues had discovered---the Great Comet of 1811. Mädler proved to be an excellent scholar and seemed destined for an academic career, but then disaster struck---when he was nineteen, an outbreak of typhus claimed both his parents and his maternal uncle, and he found himself weighted with the responsibility of supporting four younger siblings. He abandoned his academic dreams and enrolled in the tuition-free Küsterschen Seminary in order to study to become an elementary school teacher. At the same time he began giving lessons as a private tutor, and in 1819 he founded a school in Berlin for children of parents with limited means---obviously not a highly lucrative way to make a living. Meanwhile, he began attending lectures at the University of Berlin and heard, among others, P. G. L. Dirichlet on higher mathematics and Johann Elert Bode and Johann Franz Encke on astronomy.
A turning point in his life came in 1824, when he met Wilhelm Beer, who had applied to him for private lessons in higher mathematics and astronomy. Beer, a well-to-do banker who had just taken over the family banking business from his father, Jakob Herz Beer, was then twenty-seven years old. His brothers were the poet Michael Beer and the composer Jakob Beer, who styled himself Meyerbeer and went on to become the most successful operatic composer of his day.
After he met Mädler, Beer decided to set up his own observatory. The main instrument was a 3.75-inch (95-mm) Fraunhofer refractor, which was mounted equatorially and equipped with a clock drive allowing it to follow the apparent drift of the stars. Beer set it up near his villa, on a platform in the famous Tiergarten under a rotatable dome 12 feet in diameter whose shutters opened upon a swath of sky 20° wide. The telescope was in place by 1828, and two years later Beer and Mädler began to use it for mapping the Moon, the work for which they are best remembered. (It has long been recognized that most of the actual mapping was done by Mädler; Beer's main contribution was in allowing him to use the observatory!)
They had been mapping the Moon for several months when Mars's perihelic opposition on September 19, 1830, presented them with a great opportunity. For several weeks around this date, Beer and Mädler observed Mars extensively. Their first goal was to precisely determine its rotation period, and at the same time they hoped to establish once and for all whether or not the patches on the surface of Mars were variable.8
Beer's 3.75-inch Fraunhofer refractor, though of modest size, was first class and gave a sharper definition than the larger reflectors of Herschel and Schroeter. Even so, Beer and Mädler found the Martian surface features to be weak and ill-defined---which explains how the sharp disagreement as to their nature and permanence could have gone on for so long. They wrote that
|the use of a micrometer did not seem convenient to us, the thickness of the threads causing more uncertainty in measurement of such fine objects than was produced by estimating by the eye alone. The drawings were executed immediately at the telescope. Ordinarily some time elapsed before the indefinite mass of light resolved into an image with recognizable features. We next attempted to estimate the coordinates of the most distinct points, using the white spot at the south pole for the determination of the central meridian, and only then sketched in the remaining detail. . . . Finally, each of us compared the drawing with the telescopic image, so that everything shown was seen by both of us and hopefully may be considered fairly reliable.9|
Their study left little doubt that the markings were constant. "Our observations," they wrote, "are thus in important disagreement with earlier ones. . . . The hypothesis, that the spots are similar to our clouds, appears to be entirely disproved."10
At the beginning of their observations, Beer and Mädler's attention had been struck by a small round patch "hanging from an undulating ribbon." This round patch had been represented very imperfectly by Herschel in 1783, and on several occasions by Schroeter in 1798, but Beer and Mädler were the first to show it clearly. It lay only 8° south of the equator, and they regarded it as a convenient reference point for determining the rotation period of Mars. Later astronomers concurred in the aptness of their choice---ever since Beer and Mädler, that feature has defined the zero meridian of Mars, and Camille Flammarion later named it the Meridian Bay (Sinus Meridiani). Rather than giving names to the various markings they mapped, Beer and Mädler simply designated them with letters---thus the small round patch was indicated by the letter a, Syrtis Major by efh, and so on. From their observations of the patch a, they put the rotation period at 24 hours, 37 minutes, 9.9 seconds.
In 1830, Beer and Mädler began a careful study of the south polar cap. They followed its rapid shrinking and noted that this continued until the Martian season corresponding to our mid-July. Then the cap began slowly to increase again. These observations lent strong support to the idea that it consisted of ice and snow.
Although Beer and Mädler were able to confirm some of their earlier results at the oppositions of 1832 and 1834--35, they added little that was new. In 1837 they began to use a much larger instrument, the 9.6-inch (24-cm) refractor of the Royal Observatory of Berlin. Despite this superior instrument, however, they were much hampered by Mars's greater distance from the Earth and the "almost unprecedented bad weather" in Berlin. Nevertheless, they were able to achieve some useful results. They revised the rotation period to 24 hours, 37 minutes, 23.7 seconds---very close to the presently accepted value. This was, they noted, 2 minutes shorter than the period published by William Herschel, but they were able to account for the discrepancy after carefully reviewing Herschel's records from 1777 and 1779. During the interval Herschel had used to compute the rotation period, Mars had completed one more rotation than he had realized. When this was factored in, the agreement with their own results was excellent.
In 1830, the rapidly shrinking south polar cap had been tilted toward the Earth; in 1837, it was the north polar cap. The two caps showed markedly different behavior; both were centered within a few degrees of the poles, but the south polar cap grew much larger. At the same time, its retreat was more rapid and complete; the smallest size Beer and Mädler recorded for the south polar cap was 6°, whereas the north polar cap never shrank below 12° or 14°.
There are perfectly logical reasons for these differences, and they have to do with the nature of the Martian seasons. To make this clear, it is useful to introduce a calendar based on Ls, the areocentric longitude of the Sun, which gives Mars's position in its orbit relative to the Sun-Mars line at the northern spring equinox, which marks the beginning of northern spring. This point is defined as Ls = 0°. The northern spring lasts from Ls = 0° to 90°, summer from 90° to 180°, fall from 180° to 270°, and winter from 270° to 360° (or 0°). (As on Earth, seasons in the Martian southern hemisphere are 180° out of phase with those of the northern hemisphere, so that the southern hemisphere has its summer when the northern hemisphere has its winter, and vice versa.) Since Mars's year is almost twice as long one Earth year---it lasts for 668.6 Martian days, or Sols (1 Sol = 24 hours, 37 minutes, 22.663 seconds)---it follows that Martian seasons will be much longer than those of Earth. They are also more unequal---a consequence of the much greater eccentricity of the Martian orbit. The details are given in table 1.
Since Mars's perihelion lies at Ls = 250.87°, the planet passes this point late in the southern hemisphere spring. Mars is then 26 million miles (43 million km) nearer to the Sun and receives 45 percent more solar radiation, than at aphelion (Ls = 70.87°). Thus southern hemisphere springs and summers are shorter but much hotter than northern hemisphere springs and summers, with peak temperatures as much as 30°C higher. Conversely, the southern hemisphere autumns and winters, which occur near aphelion, are much colder and longer. The southern hemisphere is thus a place of extremes; the northern hemisphere is one of relative moderation.
The behavior of the polar caps reflects these idiosyncrasies. The south polar cap grows greatly during the southern hemisphere's long, cold winter and shrinks rapidly during the short, hot summer. The north cap, reflecting that hemisphere's more moderate seasons, does not vary between such wide extremes. There are compositional differences as well, about which I will have more to say later. The northern cap is mainly made up of water ice, the southern cap of frozen carbon dioxide.
In addition to their studies of the polar caps, Beer and Mädler continued to sketch the dark patches on the planet. The dark area surrounding the north polar cap seemed to undergo especially marked changes---in 1837 it was of unequal width and not everywhere equally black, though it was still noticeably darker than the other spots; by 1839 it had become faint and narrow. These changes would be explained, they suggested, if the dark area was marshy soil moistened by meltwater from the retreating snow. There were times in 1837 when Mars was nearly featureless apart from the polar patch, which always remained distinctly in view. Indeed, even the patch efh (Syrtis Major) was by no means well defined. Despite the unsteadiness of the atmosphere at Berlin that year, it is tempting to believe that some of the obscurations may have been genuinely Martian---caused by the dust clouds and veils that are known to develop from time to time.
In 1840, Mädler combined all the observations and drew the first map of Mars ever made. Admittedly, it leaves much to be desired, but it nevertheless represents a tremendous step forward. That same year, Mädler left Berlin to become director of the Dorpat Observatory in Estonia, and at the opposition of 1841 he made only a few observations with the 9.6-inch (24-cm) Dorpat refractor. The experience of 1837 was repeated; though he was able to recover some of the spots of earlier years, he looked in vain for others, including the prominent round patch a. He was no longer so certain of the long-term stability of the markings.
Beer and Mädler tower so far above their contemporaries that there is a distinct danger of forgetting the other observers who were active at that time. There was William Herschel's son John, for example, a great astronomer in his own right. He considered the ocher areas of Mars to be continents and suggested that they might be similar to the red limestones of Earth; the dark areas he regarded as seas since, he noted, water absorbs light more strongly than land. He also recorded greenish tints in the seas, though these he suspected of being illusory---a mere optical effect resulting from contrast with the ocher areas. As we shall see, the colors of Mars would become one of the most debated features of the planet. Another leading astronomer of the period was François Arago, who in 1830 became director of the Paris Observatory. He too remarked on the colors of the planet, finding a rosy tint obvious at low powers; with larger instruments, however, this passed successively to orange, yellow, and finally to lemon.11 He agreed with the younger Herschel that the greenish tint of the dark areas was a contrast effect.
The next perihelic opposition took place on August 18, 1845, and it is memorable for the discovery by Ormsby MacKnight Mitchel, of the Cincinnati Observatory, of the large detached area of the south polar cap (centered at 75° S, 320° W) that begins to separate from the cap's rim at about the same Martian seasonal date each year (Ls about 240°). The final remnants do not disappear for another twenty or thirty days. This feature is still sometimes referred to as the "Mountains of Mitchel," but the name is a misnomer---instead of being mountainous, the region is actually depressed relative to the surrounding terrain.
As better telescopes became more widely available, and more and more people acquired the passion to observe, the number of observations of Mars increased. The improvement in the quality of drawings in the thirty years after Beer and Mädler did their work is dramatic. In 1856, for example, the English amateur astronomer and pioneer photographer Warren de la Rue made several excellent drawings with a 13-inch (33-cm) reflector---in one, Syrtis Major, or the Hourglass Sea, appears very narrow; in another, the conspicuous round patch described by Beer and Mädler is represented as a pointed tongue.
At the opposition of 1858, Mars was extensively observed by the Jesuit astronomer Angelo Secchi, director of the observatory of the Collegio Romano in Rome.12 Secchi used a 9.5-inch (24-cm) equatorial refractor with magnifying powers of 300--400x. In one of his first observations, on May 7, 1858, he described "a large triangular patch, blue in color." This was none other than the well-known Hourglass Sea, but Secchi gave it a different name: he called it the "Atlantic Canale," commenting that it "seems to play the role of the Atlantic which, on Earth, separates the Old Continent from the New." Thus the first occurrence of the fateful term canale, which in Italian can mean either "channel" or "canal." Secchi himself was inconsistent; later he called the same feature the "Scorpion"---a not inapt comparison to its appearance at the time.
Secchi was impressed with the great variety in the tints of the Martian features and even attempted to make the first color representations of the planet in pastels. He described the dark areas surrounding the polar caps as "ashen colored," but most of the other dark areas appeared bluish, with an occasional tint of green. As for the nature of the Martian markings, he wrote:
|It is clear that the variations [in the polar caps] can be explained only by a melting of the snow or a disappearance of the clouds covering the polar regions. These aspects also prove that liquid water and seas exist on Mars; this is a natural result of the behavior of the snows. This conclusion is confirmed by the fact that the blue markings which we see in the equatorial regions do not change sensibly in form, whereas the white fields in the neighborhood of the poles are adjacent to reddish fields which can only be continents. Thus, the existence of seas and continents . . . has been today conclusively proved.13|
At the perihelic opposition of July 1860, Mars was very far to the south, and thus difficult to observe from the Northern Hemisphere of the Earth, where most observatories were located. At the next opposition, in 1862, Mars was more favorably placed, and a number of observers took advantage of the opportunity, including Secchi in Rome, Lord Rosse in Ireland, and William Lassell on Malta. An important series of drawings was made by the director of the Leyden Observatory, Frederik Kaiser, who reworked the rotation period by comparing his drawings with those made by Hooke and Huygens in the seventeenth century---his result was 24 hours, 37 minutes, 22.6 seconds.14 He also compiled the best map of the planet up to that time, continuing to use letters to designate the various features. Another skillful observer was J. Norman Lockyer, an English astronomer destined to play a prominent role in late Victorian science.15 Lockyer used a 6.25-inch (16-cm) refractor, and his drawings of Mars, in the estimation of E. M. Antoniadi, "gave us the first really truthful representation of the planet."16 This English astronomer accepted the basic permanence of the dark areas, though he noted that there were obvious variations over time. Thus, to give but one example, Solis Lacus---the "Oculus," or Eye, as it was then known---which had been depicted as nearly circular by Beer and Mädler, had become distinctly elongated by Lockyer's time. Still other variations Lockyer believed to be accounted for by clouds---indeed, his disks indicate that there were some rather persistent veils over Mare Erythraeum in 1862. Like Secchi, he believed that the greenish areas were seas and the ruddy areas continents.
This belief was by now generally accepted, though Secchi was premature, to say the least, in considering that it had been proved. A few astronomers, at any rate, remained skeptical. An Oxford professor of geology named John Phillips, one of the most active observers of Mars at the opposition of 1862, wrote that "a great part of the northern area appeared bright, and often reddish, as [if] it were land, while a great part of the southern area was of the grey hue which is considered to indicate water, but relieved by various tracts of a tint more or less approaching to that of the brighter spaces of the northern hemisphere."17 Though he hedged regarding whether the dark patches were actual seas or mere gray plains like those of the Moon, he pointed out that if they were actual seas, the Sun's specular reflection ought to be visible on them. According to later calculations by G. V. Schiaparelli, this reflection would appear as bright as a star of the third magnitude. For many years the precise spot where the starlike image should be sought was published in physical ephemerides of the planet, but it was never seen. In the meantime, a different explanation of the Martian surface features was published by Emmanuel Liais, a French astronomer who left the Paris Observatory and moved to Brazil, where he became director of the observatory of Rio de Janeiro. Liais proposed that the ruddy areas were deserts of sand, and the dark patches vast tracts of vegetation, although it must be admitted that his ideas received little attention at the time.18
The opposition of December 1, 1864, was not quite as good as that of 1862---Mars attained a maximum diameter of only 17.3"---but the opposition was nevertheless memorable for the study carried out by Rev. William Rutter Dawes, the son of a mathematics teacher and once an astronomer on an expedition to Botany Bay, Australia. Dawes had studied medicine as a young man and later became a clergyman with a small Independent congregation at Ormskirk, north of Liverpool. After failing health forced him to give up his congregation, he devoted himself entirely to astronomy. In the 1840s he was an assistant at the private observatory of a wealthy businessman, George Bishop, at St. John's Wood, London. After his second marriage---to an Ormskirk solicitor's widow---Dawes acquired the financial independence he needed to set up his own private observatories, first at Cranbrook, Kent, and then, from 1857 until his death in 1867, at Haddenham, Buckinghamshire. He was an exceptional observer noted for the keenness of his sight; but eagle-eyed as he was at the telescope, he was so terribly nearsighted that he could pass his wife in the street without recognizing her!
Dawes had already made some drawings of Mars in 1862 and at earlier oppositions. In 1864, he used an 8-inch (20-cm) Cooke refractor, usually with a magnifying power of 258x. His drawings, wrote Richard Anthony Proctor, "are far better than any others. . . . The views by Beer and Mädler are good, as are some of Secchi's (though they appear badly drawn). Nasmyth's and Phillips', De La Rue's two views are also admirable; and Lockyer has given a better set of views than any of the others. But there is an amount of detail in Mr. Dawes' views which renders them superior to any yet taken."19 Camille Flammarion concurred: "The drawings by . . . Dawes brought a new precision to studies of Mars."20 A case in point: what Beer and Mädler had taken as a small, perfectly round spot (the feature they had designated a) was seen by Warren De la Rue as pointed and by Lockyer as an elongated patch; Dawes, however, resolved it into a bay with two forks, whose extensions, he noted, gave the impression of "two very wide mouths of a river, which however I could never trace. . . . It may be that the sea has receded from that part of the coast, and left the tongue of land exposed."21 This was the famous "Dawes' forked bay"---a name that is still used from time to time.
By the 1860s, Beer and Mädler's chart was hopelessly out of date. Kaiser's was a definite improvement, but he had done nothing to improve on the old lettering system of nomenclature, which was proving more and more inconvenient. Over the years, a few names had come into general but unofficial use for the most prominent or singular features---Hourglass Sea and Oculus, for example---but most of the features remained unnamed.
The first attempt to find a more suitable Martian nomenclature was made by Proctor. He was a prolific writer of popular books on astronomy and, as we have seen, a great admirer of Dawes. In 1867, Proctor drew up a map of Mars based, somewhat crudely, on Dawes's drawings.22 He explained his system of nomenclature by saying, "I have applied to the different features the names of those observers who have studied the physical peculiarities presented by Mars." For later reference, some of his names are here paired with those later proposed by Schiaparelli:
|Kaiser Sea||Syrtis Major|
|Main Sea||Lacus Moeris|
|Herschel II Strait||Sinus Sabaeus|
|Dawes Continent||Aeria and Arabia|
|De La Rue Ocean||Mare Erythraeum|
|Lockyer Sea||Solis Lacus|
|Dawes Sea||Tithonius Lacus|
|Madler Continent||Chryse, Ophir, Tharsis|
|Maraldi Sea||Mares Sirenum and Cimmerium|
|Hooke Sea||Mare Tyrrhenum|
|Herschel I Continent||Zephyria, Aeolis, Aethiopis|
Proctor's nomenclature has often been criticized, mainly because so many of his names honored English astronomers, but also because he used many names more than once---in particular, Dawes appeared no fewer than six times (Dawes Ocean, Dawes Continent, Dawes Sea, Dawes Strait, Dawes Isle, and Dawes Forked Bay). Moreover, as Schiaparelli later complained, Proctor's map did "not even give an accurate representation of the observations of Dawes himself."23 Even so, Proctor's names are not without charm, and for all their shortcomings they were a foundation on which later astronomers could---and did---improve.
Through Dawes's skillful work, the main outlines of the Martian "seas" and "continents"---as most of the astronomers of the day assumed them to be---had been reliably depicted. Moreover, on those same drawings on which the broader features of the Martian disk were so artfully and accurately portrayed, tentative indications of a class of finer details were now rising for the first time above the threshold of perception. From some, though not all, of the pointed extensions of the dark seas there seemed to be prolongations into thin, wispy streaks, which imperceptibly dissolved again into the broad ocher parts of the planet. The same ocher areas, Secchi had noted, were not uniform but seemed to be filled with fine detail, whose nature "it was impossible to depict, or even for the imagination to capture."24
All of this was suggestive. The broader features of the Martian surface may have been completely defined, but there was something more remaining to be discovered---a fine print, in relation to which humankind stood in the 1860s where Christiaan Huygens had stood in relation to the big print two centuries earlier.
The observations of the early pioneers had been limited by the chromatic aberration and diffraction that afflicted their instruments. Diffraction is a consequence of the wave nature of light. Because of it, a telescope can never form an image of a star as a perfect point; instead, the image is a small disk surrounded by a series of rings---the larger the telescope, the smaller the apparent disk and the more closely spaced the rings. In the case of a double star, if the diffraction patterns overlap too much, the components will remain unresolved. (Dawes himself is perhaps best remembered today for working out what is known as Dawes's limit: the aperture of a telescope, determined empirically, needed to just separate the components of close double stars.) Diffraction, of course, enters just as critically into observations of planetary surface detail. A thin line on a planet, for example, becomes widened by diffraction into a band of decreased intensity on both sides. If a line is less than a certain width, its contrast with the background is so much reduced that the eye is simply unable to grasp the weakened tones.
The limits to visibility set by diffraction are invincible; the only remedy is to increase the aperture width of the telescope being used. It is this increase in aperture and the corresponding reduction in the diffraction limit that explain why Beer and Mädler, with a 3.75-inch (95-mm) aperture, were able to make out only a round spot while Dawes, with 8 inches (20 cm), saw a forked bay.
Secchi, Kaiser, Lockyer, and Dawes, the best observers of their day, all used instruments with apertures of less than 10 inches (25 cm). With such instruments it was possible to reach, during the steadiest moments of the atmosphere of Earth, the limits of visibility set by diffraction. Naturally, superior results were expected from much larger instruments. In 1862, the 18.5-inch (47-cm) Clark refractor of the Dearborn Observatory, in Chicago, had just come into service, surpassing the 15-inch (38-cm) refractors of the Harvard and Pulkova observatories, which had been built in the early 1840s. But the era of great telescopes was only beginning. By 1870, the 25-inch (63-cm) Cooke refractor had been set up on the estate of wealthy amateur R. S. Newall at Gateshead, in northern England; and in 1873 the 26-inch (66-cm) Clark went into operation at the U.S. Naval Observatory in Washington, D.C. Unfortunately, the locations for these large instruments could hardly have been more ill-chosen, and it soon became painfully apparent that there are other waves no less pertinent to planetary observation than light waves: atmospheric waves, which impose their own barrier to seeing and are an ever-present foil to attempts to reveal finer planetary surface markings from the Earth. Since a larger telescope takes in larger and stronger areas of atmospheric turbulence, beyond a certain point---about 12 or 16 inches (30 or 40 cm)---the advantages over smaller instruments will be partially or entirely offset by blurring, at least much of the time. It follows that a smaller instrument used in good conditions can surpass the results of a much larger one used in terrible seeing (bad atmospheric conditions). After fifteen frustrating years of trying to use his instrument, Newall wrote, "Atmosphere has an immense deal to do with definition. I have had only one fine night since 1870! I then saw what I have never seen since."25
Thus far the story of Martian exploration has been intertwined with the optical improvement of the telescope. Now we enter a new era, in which the pursuit of the best seeing---the quality of the air, which becomes best when it arranges itself into steady, stable layers above the ground---becomes equally important to the quest for Martian detail.