The Planet Mars:
A History of Observation and Discovery

William Sheehan

Chapter 3
"A Situation Similar to Ours"

When Giovanni Domenico Cassini became blind in 1710 (he died two years later), his son Jacques took charge of the Paris Observatory. Indeed, the Cassinis were destined to become one of the most remarkable astronomical families in history. Jacques (Cassini II) was succeeded by his son, César-François Cassini, who is also known as Cassini de Thury (Cassini III), who in turn was succeeded by his son, Jacques Dominique Cassini (Cassini IV). In politics, the family remained strongly royalist in its sympathies, and Cassini IV was finally forced to resign in 1793, during the height of the French Revolution.

Among the other worthies in this illustrious family's tree was the elder Cassini's nephew, Giacomo Filippo Maraldi (born in 1665), who became an assistant at the Paris Observatory. Although overshadowed by the fame of his brilliant uncle, Maraldi was a competent astronomer in his own right; his work included the compilation of a star catalog and the calculation of cometary orbits. But he will always be best remembered for his work on Mars, which he pursued almost singlehandedly during a period when studies of the planet were generally neglected.

Maraldi made a careful series of observations at every opposition beginning in 1672; his best results were obtained at the perihelic oppositions of 1704 and 1719.1 He described the markings on Mars as not normally well defined even in large telescopes---most of his observations were made with the 34-foot (10.4-m) Campani telescope of the Paris Observatory---and suspected that they were variable not only from opposition to opposition but even from month to month. His sketches show a dusky band near the middle of the disk, which reminded him of one of the cloud belts of Jupiter; it was interrupted in places and occupied only somewhat more than one hemisphere of Mars. At one point this band was joined by another at an oblique angle, and taking this as a reference point, Maraldi was able to determine the rotation of the planet. This point seemed to return to the same location on the disk after thirty-seven days, during which the planet had rotated thirty-six times. He therefore worked out the rotation period at 24 hours, 40 minutes, which agreed exactly with his uncle's result. In addition to the bands just described, he made out a large triangular patch on the Martian surface. The features described by Maraldi are readily identified on modern maps: the band consists of the darkish swath from Mare Sirenum in the east to Mare Tyrrhenum in the west, while the dark triangular patch can be nothing other than Syrtis Major, the "Hourglass Sea." Nevertheless, Maraldi himself was not convinced of their permanence; he thought they were merely clouds.

Though the south polar spot had been sketched by Huygens in 1672, it was Maraldi who began the first thorough study of the Martian poles. He found both poles to be marked by whitish spots, though because the southern hemisphere of Mars was tilted toward the Earth at the perihelic oppositions, the spot near the south pole was markedly easier to observe. When the polar spot was very small, Maraldi noted that it underwent a small revolution as Mars rotated, which meant that its position was slightly eccentric to the pole. Moreover, there were changes in the spot's extent: in August and September 1719 it disappeared entirely, though later it returned. This behavior suggested that the material of which the spot was made underwent physical changes of some kind, although Maraldi himself declined to speculate on what they might be.

After Maraldi's observations of 1719, little work was done on Mars for the next several decades, and of that little, none is deserving of special notice. We pass by, therefore, the wasted perihelic oppositions of 1734, 1751, and 1766, and go directly to the contributions of William Herschel, one of the greatest astronomers who ever lived. Though his most important work concerned the sidereal universe, in his early career Herschel made numerous observations of the Moon and planets.

Friedrich Wilhelm Herschel (who later naturalized his name to William) was born at Hanover, in Germany, in 1738. His family was musical, and at first young Herschel attempted to follow his father into a career as a bandsman in the Hanoverian Guard. In the spring of 1757, however, after the Guard's disastrous campaign against the French during the Seven Years' War (Herschel himself came under fire at the Battle of Hastenbeck), he decided to leave the Guard and seek his fortune in England. England and Hanover had enjoyed close ties ever since 1714, when a Hanoverian prince, George Louis, ascended the throne of England as George I. Many Germans followed the Georges to the court of London, and Herschel was drawn like a magnet. He struggled at first, but by 1766 had established himself as a musician in the dazzling resort city of Bath---he was the organist for the Octagon Chapel, and also composed music and gave private lessons. Though for the next ten years he made his living as a busy musician, his interests turned increasingly to astronomy, and before long he began to cast about for a suitable telescope.

The refractor, as we have seen, had reached an apparent dead end with the aerial telescopes of Huygens and Cassini. Isaac Newton, the architect of the theory of gravitation, was among those who concluded that the problems were insurmountable, and he proposed that a curved mirror be used in place of the lens to collect the light. In the Newtonian reflector---so-called to distinguish it from the many other variations on the basic idea---the mirror reflects the light back up the tube to a small, flat mirror, which is set at a 45° angle so as to redirect the beam through a hole in the side of the tube, where the image can be magnified by an eyepiece. Provided the curve given to the main mirror is a parabola, all the light is brought to a single focus; and since the light never has to pass through a lens until it reaches the eyepiece, chromatic aberration can be avoided. Newton himself actually produced a reflector with a 1-inch (2.5-cm) mirror made of bell metal, which he presented to the Royal Society of London in 1672. Unfortunately, the metals used in the mirrors of the day were difficult to figure and bring to a good polish, and it was not until 1722 that John Hadley was able to produce a reflector capable of equaling the performance of Huygens's aerial telescopes.

In 1773, when Herschel began to take a serious interest in astronomy, he first directed his attention to refractors, but he found the long tubes almost impossible to manage. He next rented a small reflector and, finding it satisfactory, attempted to purchase such an instrument. Unfortunately, those available were beyond his means, and he decided to experiment with mirror making. By this time his sister Caroline and brother Alexander had come over from Hanover and joined him in Bath, and every room in the house in which they were then living soon took on the appearance of a workshop. After a number of failures at mirror making, Herschel finally, in 1774, succeeded in making a small reflector with which he recorded his first observation, of the Orion nebula. He went on to produce larger and larger instruments that were far superior to any others of his day. In 1777, he moved to 19 New King Street near the center of Bath, bringing with him working reflectors of 7-foot (2.1-m) and 9-foot (2.7-m) focal lengths. From the long, south-facing garden in back of the house he made a few observations of Mars at its opposition that year---in particular, he recorded the "two remarkable bright spots on Mars" (the polar caps).

Herschel returned to Mars in 1779, and again in 1781. On the most fateful night of his career---March 13, 1781---he observed Mars with a recently completed 20-foot (6.1-m) reflector and recorded in his notebook that there was a "very lucid spot on the southern limb . . . of a considerable extent." Earlier on that same evening, between 10:00 and 11:00 P.M., he had discovered with his 7-foot telescope a tiny disk among the stars. At first he thought it was a comet, but the disk later proved to be something much more consequential: it was nothing less than a new planet, the first discovered in modern times.

This discovery changed Herschel's life. He was granted a pension by George III that allowed him henceforth to spend all of his time on astronomy. In gratitude, Herschel proposed to call the planet Georgium Sidus, the Star of George, but the name never stuck. On the Continent (and eventually, after Herschel's death, in England also) it was superseded by the name Uranus, which had been proposed by the German astronomer Johann Elert Bode.

In the months after his great discovery, Herschel's attention was not entirely absorbed in observing the new planet. He did return to Mars from time to time as it came to a perihelic opposition on July 27, 1781, and from his observations worked out a new rotation period: 24 hours, 39 minutes, 21.67 seconds, which is exactly 2 minutes too long. He was also able to confirm that the north polar spot, which he observed carefully during the months of June and July, was eccentric to the pole---the same result that Maraldi had earlier established for the south polar spot.2

Before the next opposition of Mars, in 1783, William and Caroline moved from Bath to Datchet, near Windsor. There William rented a dilapidated old house not far from the Thames. It was large and commanded an excellent view of the sky, and it was reasonably close to Windsor Castle in case the royal family should want to look through his telescopes. There William worked incessantly, as his sister later recalled:
The assiduity with which the measurements on the diameter of the Georgium Sidus, and observations of other planets, double stars, etc., etc., were made, was incredible, as may be seen by the various papers that were given to the Royal Society in 1783, which papers were written in the day-time, or when cloudy nights interfered. Besides this, the twelve-inch speculum was perfected before spring, and many hours were spent at the turning bench, as not a night clear enough for observing ever passed but that some improvements were planned for perfecting the mounting and motions of the various instruments then in use, or some trials were made of new constructed eye-pieces, which were mostly executed by my brother's own hands.3
In September and October 1783, Herschel observed Mars extensively---the planet came to a very good almost-perihelic opposition on October 1---and noted that the south polar cap was very small. On October 1, he jotted in his notebook: "I am inclined to think that the white spot has some little revolution. . . . It is rather probable that the real pole, though within the spot, may lie near the circumference of it, or one-third of its diameter from one of the sides. A few days more will show it, as I shall now fix my particular attention on it."4 Later that night he established that the spot had "a little motion, for it is now come farther onto the disk."5 Over the next several nights he carefully measured its position from hour to hour. Later, he used these observations to calculate that the south polar cap was 8.8° from the south pole. This was not unexpected; on Earth, the pole of greatest cold does not correspond with the geographical pole. He was also able to work out the inclination of the planet's axis to the plane of its orbit---28° 42'---and fixed the equinoctial point on the Martian ecliptic (the vernal equinox of Mars) at 19° 28', in Sagittarius. As the planet's axial inclination was very nearly the same as that of Earth, Herschel realized that the Martian seasons must be analogous to ours, though nearly twice as long. From this, he thought he could "account, in a manner which I think highly probable, for the remarkable appearances about its polar regions":
The analogy between Mars and the earth is, perhaps, by far the greatest in the whole solar system. The diurnal motion is nearly the same; the obliquity of their respective ecliptics, on which the seasons depend, not very different; of all the superior planets the distance of Mars from the sun is by far the nearest alike to that of the earth: nor will the length of the martial year appear very different from that which we enjoy, when compared to the surprising duration of the years of Jupiter, Saturn, and the Georgium Sidus. If, then, we find that the globe we inhabit has its polar regions frozen and covered with mountains of ice and snow, that only partly melt when alternately exposed to the sun, I may well be permitted to surmise that the same causes may probably have the same effect on the globe of Mars; that the bright polar spots are owing to the vivid reflection of light from frozen regions; and that the reduction of those spots is to be ascribed to their being exposed to the sun.6
Herschel calculated the diameter of Mars to be 0.55 times that of the Earth, and he found its figure to be just as flattened as that of Jupiter---the ratio of the equatorial to the polar diameter he put at 16/15.

The dark markings on the planet came only incidentally within the course of Herschel's survey, and he made little more than crude sketches of them. Even so, they are far more clearly delineated in his drawings than they are in those of his predecessors. With Herschel's work, Mars studies left the Maraldi era behind and entered a new epoch. The drawings of 1783 are of special interest. One easily recognizes many Martian features: the Hourglass Sea is clearly shown, as are the features that later became known as Sinus Sabaeus and Sinus Meridiani. This alone would be sufficient to show that the Martian surface markings are generally fixed and permanent. However, Herschel's drawings also provide unmistakable evidence of changes. For example, he showed a prominent feature that is no longer present---a triangular patch, somewhat similar to Syrtis Major, which curves down from Mare Cimmerium to end almost in the form of a hook.

In addition to these studies of the surface, Herschel made a few observations with a bearing on the question of whether Mars had an atmosphere. Cassini, in 1672, had observed a fifth-magnitude star (Phi Aquarii) disappear a full six minutes from the disk of Mars, which led him to conclude that the planet must have a very dense atmosphere. Herschel suspected that rather than being hidden by the planet's atmosphere, the star had merely disappeared into the glare around Mars, and eventually he was able to put the question to a test. Using his 20-foot (6.1-m) reflector, which had a mirror 18.7 inches (47 cm) in diameter, he followed two faint stars as they approached Mars without noting the least diminution of their light. This proved that the Martian atmosphere could not be as appreciable as Cassini had supposed. Nevertheless, Herschel did have evidence that an atmosphere existed. "Besides the permanent spots on its surface," he wrote, "I have often noticed occasional changes of partial bright belts . . . and also once a darkish one, in a pretty high latitude. . . . And these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapours floating in the atmosphere of that planet." Thus he concluded that the inhabitants of Mars "probably enjoy a situation in many respects similar to ours."7

After the 1783 opposition of Mars, Herschel began to wrestle with the momentous questions of the sidereal universe and the construction of the heavens, and henceforth paid scant attention to Mars. Nevertheless, his success inspired others---and none more than Johann Hieronymus Schroeter, who became one of the most enthusiastic amateur astronomers of all time and the next important observer of Mars.

Although interested in astronomy from an early age, Schroeter followed the family profession of law. He graduated from the University of Göttingen, which had been founded by George II, and in 1777 was appointed secretary of the Royal Chamber (of George III) in Hanover. Music was another of his many interests, and this brought him into contact with two of William Herschel's younger brothers, Johann Alexander and Dietrich, through whom his interest in astronomy was reawakened. Dietrich helped him to obtain a small telescope in 1779, and Schroeter used it to make a few observations of the Moon and Venus. However, the decisive event of his life took place in 1781, with William Herschel's discovery of Uranus. In a spirit of emulation, Schroeter resolved to dedicate himself henceforth to astronomy and resigned his position in Hanover in order to take on the less demanding post of chief magistrate of Lilienthal, a village on the moor near Bremen. He took up residence in the Amthaus there in 1782. Meanwhile, he had succeeded in obtaining two mirrors, 4.75 and 6.5 inches (12 and 16.5 cm) in diameter, that had been made by William Herschel himself. The larger of the two Schroeter assembled into a 7-foot (2.1-m) reflector that was in every respect identical with the one Herschel had used to discover Uranus; when Schroeter began to use it, in 1786, it was the largest telescope in Germany.

Unlike Herschel, who was mainly concerned with stellar and nebular astronomy, Schroeter's lifelong interest centered almost entirely on the Moon and the planets. He was a compulsive acquirer of telescopes---by 1793, he had erected several in the garden of the Amthaus, of which the largest had a 19.25-inch (49-cm) mirror and a focal length of 27 feet (8.2 m). He was also a tireless observer, and he published, at his own expense, a succession of large tomes describing his observations. One concerned with his lunar work, the Selenotopographische Fragmente, appeared in 1791, followed by a second volume in 1802. There were similar volumes devoted to each of the planets.8

As might be expected, Mars came in for its share of attention. Schroeter's first observations of it were made with the smaller of the two Herschel telescopes in November 1785. He noticed only a "few grey, misty, poorly bounded patches."9 As he followed these patches from night to night, he thought they often appeared to be similar, but he could never quite convince himself that they were identical. At the next opposition, in 1787, he was able to use the larger of his Herschel telescopes on Mars for the first time, and his suspicions about the instability of the Martian surface hardened into an idée fixe. "The spots and streaks on the globe of Mars are always changing," he wrote, "even from hour to hour. But that they are the same regions is shown by the fact that the same shapes and positions develop and pass away again, as one would expect of the variable atmospheric appearances occurring above a solid surface."10 Very strange, since the drawing he made on this particular night shows Syrtis Major in clearly recognizable form! The region is the same one Huygens had figured so well in 1659, so there ought to have been no question whatever of the permanence of the markings in this of all regions---Schroeter's failure to recognize it only testifies to the difficulty of perceiving an unfamiliar object correctly and the strong influence of fixed ideas. Indeed, some of his later drawings actually show the Martian markings in the form of dark belts similar to those of Jupiter. The delusion that the patches on Mars were mere cloud forms pervaded all of Schroeter's work, and though he made careful observations at all the later oppositions---especially the excellent perihelic opposition of August 30, 1798---he never doubted that what he was seeing was a mere floating shell of clouds.11

Schroeter also carried out careful observations of the polar caps, which he considered to be the result of a "dazzling atmospheric precipitation."12 His measures of the diameter of the planet were very close to those of Herschel, though he determined that the equatorial and polar diameters agreed to within 1/81 (in this he was more nearly correct than Herschel; the polar and equatorial diameters are now known to agree to within 1/500). He confirmed Herschel's results as to the obliquity of the axis and the nature of the Martian seasons, and concluded that of all the planets, Mars was the most similar to Earth.13

Schroeter's later days were sad ones. He lived in the turbulent era of the Napoleonic Wars, and the peaceful "Vale of Lilies" was engulfed in 1806 when it came under the control of the French. Henceforth Schroeter was cut off from the financial support he had enjoyed from George III, and the French did not pay him for his work as magistrate, which included collecting taxes for them. He was soon so straitened that he found it difficult to keep up the observatory, and his situation became even worse in 1810 when he was dismissed from his position. By Napoleon's decree, Lilienthal became part of the Department de le Bouche de Weser, which had Bremen as its capital. Nevertheless, Schroeter carried on as best he could, and he was able to observe the Great Comet of 1811.

The worst was yet to come. In April 1813, as the French were reeling back from their disastrous winter campaign in Russia, a skirmish took place near Lilienthal between a French detachment and a small band of Russian Cossacks. A French officer was wounded and reported that his detachment had been fired upon by the local peasantry. Without further notice, the French general, Vandamme, gave orders to set fire to Lilienthal. A strong wind fanned the conflagration, which destroyed the government buildings where Schroeter kept many of his books and manuscripts. Schroeter himself was "obliged to fly with my family, in our night dresses, to my farm at Adolphsdorf." His observatory escaped the inferno but was broken into and plundered by French troops several days later. They "with a fury the most unprovoked and irrational destroyed or carried off the most valuable clocks, telescopes, and other astronomical instruments."14

Soon afterward the French were expelled from Germany, and Schroeter, reinstated as chief magistrate, attempted with all the strength left in him to rebuild Lilienthal. But it was too late for him to try to rebuild his observatory. He kept despair at bay by writing up his observations of the Great Comet of 1811, and then turned to his observations of Mars, which had never been published. Miraculously, most of those records had escaped the fire, though a few of the drawings were damaged and had to be redrafted. Schroeter's engraver, Tischbein of Bremen, began to make copper plates of the drawings, but Schroeter's eyesight was failing, and the project was still unfinished when he died in August 1816. For some reason his heirs declined to carry it through to completion.15

Thereafter, Schroeter's manuscripts and drawings of Mars remained unknown until 1873, when François Terby, an enthusiastic student of the red planet who had a private observatory at Louvain, Belgium, and was diligently collecting drawings of Mars for his monograph Aréographie (1875),16 succeeded in tracking them down among the effects of one of Schroeter's nephews. Terby later deposited the material in the library of the University of Leyden, and Henricus Gerardus van de Sande Bakhuyzen, director of the Leyden Observatory, finally edited Schroeter's work on Mars and published it in 1881. Although the work appeared too late to have any real influence, Schroeter's drawings nevertheless remain valuable---indeed, as Bakhuyzen remarked, their worth is actually increased "because of Schroeter's erroneous view that the spots on Mars were mere cloud-forms, which sometimes changed very swiftly. For when Schroeter observed these spots, he was not predisposed to see the same details, but his different representations of them are as fully free of prejudice and independent of one another as possible."17

It remains surprising that Schroeter failed to recognize that the features he saw on the surface were the same. Many of his drawings show Syrtis Major, and other major features are also easily recognized; for instance, he made a vivid record of the round feature that later became known as Solis Lacus. The most interesting drawings are those that record the now-vanished curved hook, which, as mentioned earlier, Herschel first depicted in 1783, a marking Bakhuyzen referred to as "Spitze B" and Joseph Ashbrook called the "Arrowhead" (fig. 3).
Located at about longitude 240° W, it extended from Mare Cimmerium into the region known on later maps as Aethiopis and was one of the most visible features on Mars during the last two decades of the eighteenth century, rivaling, and sometimes even mistaken for, Syrtis Major itself. Subsequently the hook disappeared, in what Ashbrook called "the most striking change yet recorded on the surface of the red planet."18 The fact that its history is known at all is largely owing to the astronomer of Lilienthal and his candid records.

Before closing the chapter on Schroeter, there is one final fact to consider. We now know that from time to time much of the surface of Mars is obscured by dust storms, of which I will have much to say later. With his keen eye, large instruments, and long-sustained observations, Schroeter would have been in an ideal position to record dust storms had any occurred during the period of his watch. I have gone to the trouble of calculating the longitude of the central meridian for each of Schroeter's 231 drawings of Mars, and have concluded that there is no clear evidence of dust storm activity---a negative result which is itself of value, since it seems to indicate that there were no planet-encircling or global storms during the period covered by Schroeter's observations.

With the work of Herschel and Schroeter, the first phase of the physical study of Mars was under way. Its rotation period, axial inclination, seasons, polar caps, and atmosphere were now reasonably well known, and a tentative start had been made on the Martian geography. Whether the markings were permanent, as Herschel believed, or cloud formations, as supposed by Maraldi and Schroeter, remained the first order of business to be settled by the observers of the new century. But regardless of the outcome of those further investigations, the planet had acquired the distinction of being by all odds the most Earthlike, and thus had gained immeasurably in interest over the days when Fontenelle was able to dismiss it in a few lines.

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