|The Planet Mars: A History of Observation and Discovery|
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In 1609, after hearing reports from Holland of an invention by means of which "visible objects, though very distant from the eye of the observer, were seen distinctly as if nearby," Galileo Galilei, a professor of mathematics at the University of Padua in the Republic of Venice, was able to work out the principles of the invention for himself. He arranged two spectacle lenses in a lead tube to produce a crude telescope that magnified distant objects by a magnitude of 3x, and by August 1609 had managed to put together an instrument that magnified 8x or 10x, which he was showing off to the senators and other notables from atop the highest campaniles in Venice.
At first Galileo seems to have been interested mainly in the potential commercial applications of the new instrument. Not until November 1609, with a new telescope magnifying 20x, did he attempt a celestial application. On the evening of November 30, 1609, he turned this telescope toward the four-day-old Moon and saw that the lunar surface was rough and uneven, full of great cavities and mountains. By January 1610, he had made the first observations of the satellites of Jupiter, and also had shown that Venus could assume the crescent shape---something that was possible in the Copernican theory but not the Ptolemaic. He announced these discoveries in a lively little book, Sidereus Nuncius (Starry messenger), which he wrote at white heat and published in March 1610.1 Meanwhile, he had turned his telescope for the first time to Mars, which was then near the Sun and at almost its greatest distance from the Earth.
Ever since 1597, Galileo had been a confirmed Copernican. Copernicus had shown that Mars must move around the Sun outside the orbit of the Earth. That being the case, it could not show a crescent phase as Venus does, but it could have a gibbous phase---and indeed, the maximum phase, 47°, occurs when Earth is at its greatest angular distance from the Sun as seen from Mars. The planet then appears roughly like the Moon three or four days from full. Throughout 1610 Galileo examined Mars carefully with his small telescope, hoping to make out a phase. Though the disk was barely visible and the results remained inconclusive, he wrote tentatively to Father Benedetto Castelli, one of his former pupils, on December 30, 1610: "I ought not to claim that I can see the phases of Mars; however, unless I am deceiving myself, I believe I have already seen that it is not perfectly round."2
By then, Galileo had made yet another discovery, which he announced in anagram form to his correspondents, who included the Jesuit fathers at the Collegio Romano in Rome and Johannes Kepler in Prague. The anagram consisted of the following letters:
|s m a i s m r m i l m e p o e t a l e u m i b u n e n u g t t a u i r a s|
Earlier, Kepler had congratulated Galileo on his discovery of the Jovian satellites: "I am so far from disbelieving in the discovery . . . that I long for a telescope, to anticipate you, if possible, in discovering two around Mars (as the proportion seems to require), six or eight around Saturn, and perhaps one each round Mercury and Venus."3 He assumed that Galileo's latest discovery had to do with Mars and transposed the letters to read
|Salue umbistineum geminatum Martia proles.
Hail, twin companionship, children of Mars.4
But Kepler misconstrued the message; in fact, Galileo's anagram concerned Saturn and was correctly rearranged as
|Altissimum planetam tergeminum observavi.5
I have observed the most distant planet to have a triple form.
It was, in fact, the announcement of an astronomical enigma that would take half a century to solve. The triple form was actually Galileo's imperfect view of what was later shown to be the ring system of Saturn. Nevertheless, Kepler's two satellites of Mars would live on. His account may have inspired the later fancies of Jonathan Swift and Voltaire; and indeed, as we now know, the planet actually does have two satellites, though Kepler had no way of knowing this and merely made a lucky guess.
In the 20x instrument that Galileo used to make his first astronomical discoveries, Mars, even when closest to the Earth, would appear about the same size as a pea held at a distance of 8 feet (2.4 m). This and other early telescopes used a simple convex lens as the objective and a concave lens as the eyepiece, and had an inconveniently small field of view even at low power. Moreover, they suffered badly from spherical and chromatic aberrations. When light passes through a lens of spherical curvature, the rays near the periphery tend to focus at a point closer to the lens than those near the center---thus the image can never be brought into sharp focus. This is spherical aberration. Chromatic aberration occurs because light passing through the lens is broken into all the colors of the spectrum, and the different colors come to a focus at different points. This produces prismatic splendors around bright objects such as the Moon or Venus. Both types of aberrations are more noticeable in the light that passes through the outer parts of the lens. For this reason---and also because his lenses were by no means accurately figured---Galileo resorted to using cardboard rings in front of the object glasses so that the light would pass through only the central part of the lens, where the figure was most uniform.
Others attempted to improve the telescope and rid it of these faults. There was tremendous enthusiasm at the time, and many hoped to emulate Galileo's discoveries. As early as 1611, Kepler, in his Dioptrice, had proposed using a convex instead of a concave lens for the eyepiece. This is the basic idea of what became known as the astronomical telescope (as opposed to the Galilean, or Dutch, telescope). The field of view is much larger in the astronomical telescope, and although the image is inverted, this is of little importance in astronomical observations; in any case, that can be rectified by adding another lens. Unfortunately, Kepler did not actually attempt to build such a telescope, and his ideas remained little known. Apparently, the first person to actually use an astronomical telescope was a Jesuit astronomer named Christoph Scheiner, in 1617; other telescopes were built during the 1630s by Francesco Fontana, a Neapolitan lawyer and keen amateur astronomer.
With one of his telescopes, Fontana in 1636 produced a crude drawing of Mars. In this drawing the disk is shown as perfectly circular, and at the center is a dark spot which he described as looking like a "black pill." This black pill has sometimes been taken to represent one of the actual spots on the surface of the planet, but not so. Fontana later drew a similar black spot on Venus (he even used the same word, pill, to describe it), so there can be no doubt that the spot was the result of an optical defect in his telescope. Two years later, on August 24, 1638, Fontana made another drawing of Mars. The disk is shown as markedly gibbous in this drawing---much more so, in fact, than the planet can ever actually appear to be---and the black pill appears again, with a phase proportional to that of the disk.6
Although Fontana accomplished little of lasting significance, his drawings of Mars mark the beginning of what Camille Flammarion, in La Planète Mars, called the first period of the history of the planet.7 The drawings made during this first period, which lasted until 1830, are rudimentary and give no real idea of the physical constitution of the planet. We may marvel at the observers' slow progress, but we must also remember that Mars is a difficult object to observe. It is a small planet---its diameter is 4,238 miles (6,780 km), only one-half that of the Earth---and it is always more than 140 times farther away from Earth than the Moon. Except during the periods when it is very near the Earth, its disk is always small, and details are not easy to see.
Though Fontana cannot have made out any of the actual spots on Mars, others using better telescopes succeeded in doing so. A tantalizing glimpse was obtained by a Neapolitan Jesuit named Father Bartoli, who on December 24, 1644, described two patches on the lower part of the disk. More observations of patches were reported in 1651, 1653, and especially in July and August 1655, when Mars was near a perihelic opposition, by Giambattista Riccioli and Francesco Grimaldi, Jesuits of the Collegio Romano. But the true credit must go to Christiaan Huygens, the great Dutch astronomer (fig. 1).
Beginning in 1655, when he was twenty-six, Huygens began experimenting with new ways of figuring lenses for microscopes and telescopes. In the course of these experiments, he devised the first compound eyepiece, still known as the Huygenian. By March 1655 he had constructed a good 2-inch (5.1-cm) telescope of 10.5-foot (3.2-m) focal length and 50x magnification, with which he discovered Saturn's largest moon, now known as Titan. Soon afterward he was able to solve the enigma introduced by Galileo by showing that Saturn was surrounded by a ring.8 He also observed Mars, but though it had just passed a perihelic opposition in July 1655, he apparently did not get around to it until long after opposition---he could make out nothing on the disk except that it appeared to be "crossed by a sombre band."
Busy with other projects, including perfecting a pendulum clock and writing his book on the ring of Saturn, Huygens made no other observations of Mars until 1659. On November 28, at 7:00 P.M., he turned his telescope toward the planet, which was then near opposition and showing a disk 17.3" across. There were patches on the disk, and Huygens made a sketch showing a V-shaped marking, which, to anyone with the least knowledge of Mars, is immediately recognizable as Syrtis Major (for a long time known rather more descriptively as the Hourglass Sea). Huygens was able to detect a slight shift in the marking's position during the time he kept it under observation, and when he turned his telescope toward the planet again, on December 1, he found that it had returned to very nearly the same place on the disk. Thus he noted in his journal: "The rotation of Mars, like that of the Earth, seems to have a period of 24 hours."9
Huygens's main rival for telescopic glory at the time was Giovanni Domenico Cassini (fig. 2). Cassini was born in 1625 at Perinaldo, near Nice, though, according to Flammarion, he was "by temperament much more Italian than French."10 The French writer Bernard le Bovier de Fontenelle would later pay him the ultimate compliment by linking him with Galileo: "These two great men," he wrote, "made so many discoveries in the sky that they resemble Tiresias, who lost his sight for having seen some secrets of the gods"---an allusion to the fact that both Galileo and Cassini were blind in old age.
In 1648, Cassini was invited by Marquis Cornelio Malvasia, a wealthy amateur astronomer, to come to work at his private observatory at Panzano, near Bologna. This gave him access to telescopes with which he could begin to do useful research, and at the same time he completed his education under the Jesuits Riccioli and Grimaldi. Two years later, he succeeded Cavalieri in the chair of astronomy at Bologna University and became acquainted with the skillful Roman instrument maker Giuseppe Campani.
In 1664, Cassini turned a Campani telescope of 17-foot (5.2-m) focal length toward the planets, with remarkable results. On Jupiter he made out not only the dark belts but also various temporary spots, from which he worked out the rotation period at just under ten hours. He also paid attention to the Jovian satellites' eclipses and shadow transits, from which he drew up accurate tables of their motions. In 1666, he recorded spots on Venus and concluded---rather ambiguously---that it "completed its movement of either revolution or libration in less than one day, so that in twenty-three days, approximately, it shows the same aspect."11
During the same period, Cassini detected patches on the surface of Mars, despite the fact that the planet was not very favorably placed for observation---its opposition, on March 19, 1666, was an aphelic one. His drawings are primitive, and the spots, of which the most conspicuous are represented rather in the form of a dumbbell, are not easily related to those actually known to exist on the planet. Like Huygens, he noted their slow drift across the disk, and found that after a period of thirty-six or thirty-seven days they returned to the same positions at the same hour of the night, from which he was able to work out the rotation period at 24 hours, 40 minutes. Since Huygens's earlier results had never been published, Cassini's determination was completely independent, and it is very close to the actual value. This proves, if any proof were needed, that the markings he was following cannot have been illusory.12
The same opposition was observed by Robert Hooke, of the Royal Society of London, on the eve of the Great Fire that destroyed much of the city that year. In the 36-foot (11-m) telescope he was using, Hooke found that Mars was very nearly as large as the Moon viewed with the naked eye. Nevertheless, he wrote, he had a difficult time because of the unsteadiness of the air:
|Such had been the ill disposition of the Air for several nights, that from more than 20 observations of it, which I had made since its being Retrograde, I could find nothing of satisfaction, though I often imagin'd, I saw Spots, yet the Inflective veins of the Air (if I may so call those parts, which, being interspers'd up and down in it, have a greater or less Refractive power, than the Air next adjoyning, with which they are mixt) did make it confus'd and glaring, that I could not conclude upon any thing.13|
He persisted and eventually enjoyed a few nights of transparent and steady air, so that the disk of Mars became "so very well defined, and round, and distinct." Under these conditions he was able to make out some of the spots clearly, and his drawings, though primitive by any standard, do in a few cases show Syrtis Major and other Martian features in recognizable form.
Obviously, the telescopes of the late 1600s gave woefully inadequate definition. Moreover, another factor had entered into the discussion, which nowadays we refer to as the "seeing," the atmospheric conditions under which observations are made. Hooke had clearly described its role in planetary observation, and Huygens was also aware of it. He noted that the stars twinkled and that the edges of the Moon and planets trembled in the telescope, even when to all appearances the atmosphere appeared calm and serene. So frequent were the bad nights that Huygens warned against too hastily blaming the telescope for indifferent results.14
Wooed by Louis XIV, who wanted to add to the renown of the Académie des Sciences, Huygens left Holland for Paris in 1666 and took up quarters in the Bibliothèque de Roi. He would remain in France as the most prestigious member of the Académie for the next fifteen years. Three years later, Cassini was also drawn to France by the Sun King in order to lead the newly founded Paris Observatory.
On first arriving in Paris, Cassini found that detailed plans for the observatory had already been drawn up by Claude Perrault, the architect responsible for designing the new façade at the Louvre. Cassini, who had an astronomer's eye rather than an architect's, objected strongly to the plans. Eventually a meeting was arranged between Cassini and Perrault, with the king and his finance minister, Jean-Baptiste Colbert, also on hand. Cassini's great-grandson Jean Dominique Cassini IV later described the meeting:
|Perrault eloquently defended his plan and architectural style with beautiful sentences. My great-grandfather spoke French very poorly, and in defending the cause of astronomy he shocked the ears of the King, Colbert, and Perrault to such a point that Perrault in the zeal of his defense said to the King: "Sire, this windbag doesn't know what he is talking about." My great-grandfather kept silent and did well. The King agreed with Perrault and did badly. The result is that the observatory has no common sense.15|
Undaunted, Cassini set up the 17-foot (5.2-m) Campani telescope he had brought with him from Italy in the courtyard outside the observatory and set to work---with brilliant results. In 1671, with this telescope, he discovered a satellite of Saturn, Iapetus. A year later, with a 34-foot (10.4-m) Campani telescope, he added another, Rhea. Even these telescopes must have been difficult to handle; Cassini mounted the Campani lenses in light wooden tubes, which he suspended from a high mast on the observatory terrace. Later, he began using even longer telescopes, and in 1684 he discovered two more satellites of Saturn, Dione and Tethys, with Campani telescopes of 100- and 136-foot (30.4- and 41.5-m) focal lengths mounted atop an old wooden water tower which he had had transported to the observatory. The tower was equipped with a stairway and also had a balcony around the top to prevent his assistants from falling off on dark nights!
The two greatest observational astronomers of the age sometimes observed together, and both made important observations of Mars at the perihelic opposition of September 1672. Huygens made another drawing of the planet which shows Syrtis Major unmistakably, and also drew the first clear representation of the brilliant south polar cap. (It is often said that Cassini deserves credit for the discovery of the polar caps because one of his drawings from 1666 shows bright patches at the poles; however, I am not convinced, since the same drawing also shows bright patches of the same sort at both limbs.) Cassini concerned himself less with making physical observations of the planet than with measuring its position relative to the stars. By comparing his results with those obtained by another French astronomer, Jean Richer, who had traveled to Cayenne, French Guiana, Cassini worked out the parallax of Mars, which in 1672 was two and a half times greater than that of the Sun. This gave the distance to Mars, and from Kepler's harmonic law (which, as noted earlier, relates the distances of the planets to their periods of revolution) he was able to work out the value of the astronomical unit, the distance from the Earth to the Sun. In fact, there were large errors in the measurements, and Cassini was lucky that the distance he calculated, 87 million miles (140 million km), was as close to the actual value (92,955,800 miles, or 149,597,870 km) as it was.16
In the 1680s, Huygens returned to Holland to escape the persecution of Protestants that had arisen in an increasingly militant Catholic France. (Cassini, who remained a good Catholic, was never in danger; indeed, to the end of his life---he died in 1712---he remained a supporter of the Tychonic system rather than the Copernican, which had been condemned by the church.) In Holland, Huygens settled on a country estate at Hofwjick, near The Hague, and continued his efforts to improve the telescope.
Chromatic aberration had been the plague of observers from Galileo's time onward. Instrument makers naturally wanted to produce telescopes with larger apertures, but when they tried to do so, they found that the chromatic aberration became even worse. Eventually, it was found that by making the curvature of the lens shallower and its focal length longer, the effect of chromatic aberration could be reduced. Thus telescopes grew longer and longer. Huygens produced some of the first long telescopes, and his discoveries inspired the even longer telescopes of Johannes Hevelius, a brewer and city councillor of Danzig (now Gdansk), which reached lengths of 60, 70, and even 150 feet (18.2, 21.3, and 45.7 m); they were destroyed by the great fire at Danzig in 1679. Such telescopes were extremely unwieldy and difficult to use, and Huygens himself decided to turn his attention to tubeless, or "aerial," telescopes, in which the object glass was fixed to the top of a tall mast and the observer sighted along guy wires, which could be used to point the object glass in any desired direction. Bright objects such as planets could be found either by receiving them on a white pasteboard ring fixed around the eyepiece, which the observer held by hand, or, more conveniently, by receiving them on an oiled and translucent paper screen (fainter objects obviously posed much greater difficulties). The object glass was illuminated by a lantern, and the observer searched for the lantern's reflection in order to bring the lenses into alignment. In 1686, Huygens produced objectives with diameters of 7.5, 8, and 8.5 inches (19, 20, and 22 cm); their focal lengths were 123, 170, and 210 feet, respectively (37.5, 51.8, and 64 m). He used them to observe the perihelic opposition of Mars in 1686, though his sketches show no more detail than that recorded in his earlier sketches made with more modest instruments. His very last sketch of the planet was made on February 4, 1694, with Mars at an aphelic opposition. Huygens died in 1695.
With Huygens's passing, the great century of discovery came to a close. Astronomy had made tremendous strides, though relatively little had been learned about Mars. It was known to have various dark and light patches, and there were hints that these might change over time. As early as 1666, Cassini had suggested that, seen from a great distance, our globe would resemble the other planets. Following up a remark by Galileo, he had predicted that the seas would appear dark because they absorb sunlight, and the continents would appear bright. However, he stopped short of applying this explanation to the perceived differences in the Martian globe. In 1686, Bernard le Bovier de Fontenelle first published his charming Entretiens sur la pluralité des mondes (Conversations on the plurality of worlds), in which he speculated about the conditions of life and the nature of the inhabitants of the Moon, Mercury, and Venus. He gave cursory attention to Mars, however, saying only that "Mars has nothing curious that I know of; its days are not quite an hour longer than ours, and its years the value of two of ours. It's smaller than the Earth, it sees the Sun a little less large and bright than we see it; in sum, Mars isn't worth the trouble of stopping there."17 A far cry from the rich speculations that would gather around the planet in succeeding centuries! As yet, Mars, seen through the primitive telescopes of that era, had proved too unrewarding an object to pique much interest.
In the last years of his life, Huygens attempted to formulate his own ideas about extraterrestrial life, and "as they came into his head . . . clapt them down into common places."18 The result was his Kosmotheoros, which was finished by January 1695, though its author's death six months later delayed its publication until 1698. Huygens declared that the planets must have vegetation and animals, because without such life "we should sink them below the Earth in Beauty and Dignity; a thing that no Reason will permit."19 Though Mars, because of its greater distance from the Sun, would be much cooler than the Earth, Huygens felt that the inhabitants would be adapted to their conditions. Its rotation, he declared, was established without question from the movements of its spots, and proved to be similar to that of the Earth, while its axis seemed to be only slightly inclined to the plane of its orbit, so that there would not be much difference in the seasons for its inhabitants. Beyond this, there was little more to be said about the planet.
The situation, alas, was unlikely to change very rapidly. The aerial telescopes used by Huygens in the last years of the seventeenth century admitted little development beyond what they had achieved in their inventor's hands, and they were cumbersome and difficult to use. Huygens had bequeathed his aerial telescopes to the Royal Society of London, but they were scarcely ever used. The 123-foot (37.5-m) telescope was occasionally dusted off, but the results were not encouraging. "Those here that first tried to make use of this Glass," one of the members wrote in a note to the Philosophical Transactions of the Royal Society in 1718,
|finding for want of Practice, some difficulties in the Management thereof, were the occasion of its being laid aside for some time. Afterwards it was designed for making perpendicular Observations of the fixt Stars passing by our Zenith, to try if the Parallax of the Earths annual Orb might not be made sensible in so great a Radius, according to what Dr Hook had long since proposed: but in this we miscarried also, for want of a place of sufficient height and firmness, whereon to fix the Object Glass, so that it lay by neglected for many Years.20|
Clearly, aerial telescopes did not encourage protracted labor at the eyepiece, and it is little wonder that results were so meager over the next three quarters of a century---a period that deserves to be regarded as the "long night" of Martian studies.