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known, however, that parts of the Martian surface undergo seasonal changes in appearance and these are generally most marked at high latitudes; that is, near the poles. There is a possibility, therefore, that the changes Herschel reported may not have been caused by clouds.

Maps of Mars

Although several astronomers had made drawings of certain aspects of Mars, there was no complete map of the planet until the early 1830's when one was compiled by the German banker Wilhelm Beer in conjunction with the astronomer Johann H. von Madler. It was not a very good map and few of its features can be related to areas of Mars that are recognized today. An improved version, represented in figure 2.15, was produced in 1840. It shows the southern (left) and northern (right) hemispheres of Mars. The region indicated by the letter / is clearly Syr-tis Major and ae is the Sabaeus Sinus region.

FIGURE 2.15. Map of Mars made by Beer and von Màdler in 1840. (From C. Flammarion, "La Planète Mars," Vol. 1.)

Some, but by no means all, of the other dark areas can also be related to those which appear on modern maps.

The French astronomer Camille Flammarion, in his classical work entitled "La Planète Mars et ses Conditions d'Habitabilité," published in 1892, called attention to a surprising error in the 1840 map of Beer and Mâdler. The location of the northern tip / of Syrtis Major at the right of figure 2.15 is incorrect. It is shown between 90° and 100° (or so) of longitude, whereas it should have been between about 60° and 70°, as indicated in the map of the southern hemisphere.

An important aspect of the work of Beer and Mâdler is that they were the first to define Martian (areographic) latitude and longitude. The zero of latitude must, of course, lie on the equator and this, by definition, is at all points equally distant from the north and south poles of the planet. Because the poles, through which passes the axis of rotation, have fixed positions, the equator (and zero latitude) is determined automatically.

The selection of the zero of longitude, that is, the prime (or zero) meridian, however, is quite arbitrary. On Earth, the prime meridian is chosen so as to pass through Greenwich, a borough of London, England, where the famous Royal Observatory is located. On Mars, Beer and Màdler arbitrarily selected the small dark area marked a in figure 2.15 as the point through which the prime areographic meridian should pass. The zero of longitude on Mars currently used is essentially the same as that introduced by Beer and Mâdler. However, degrees of longitude are now numbered in the opposite direction around the planet. Thus, in a modern map, such as the one in figure 2.21, Syrtis Major is indicated between longitudes 280° and 295°. The corresponding longitudes in the system of Beer and Mâdler would be obtained by subtracting these numbers from 360°, yielding 80° and 65°.

After the middle of the 19th century, the sketches of Mars produced by various astronomers improved markedly, possibly because of the availability of better telescopes. Some good representations were drawn in 1858 by the papal astronomer Father Pierre Angelo Secchi in Rome. In 1863 he published the first known sketches of the planet in color, and these led him to suspect that the color of the Martian surface changed from time to time. This suspicion was confirmed 12 years later by Camille Flammarion.

Some excellent drawings of Mars were made in 1864 by the Reverend William R. Dawes in England. One of these is reproduced in figure 2.16, partly because of its interest in another connection, as will be explained shortly. The dark region of the figure shows clearly and with considerable accuracy Sabaeus Sinus, at the left, then the twin points of Meridianii Sinus, followed by Margaritifer Sinus and Aurorae Sinus in current terminology. The area at the bottom is presumably Mare Acidalium. Incidentally, Dawes

FIGURE 2.16. Drawing of Mars made by Dawes in 1864. (From C. Flammarion, '"La Planète Mars," Vol. 1.)

thought that, because of the clarity and constancy of the surface markings and the rarity of clouds, Mars had a "thin" atmosphere.

Naming the Features of Mars

The sketches of Mars made by several observers, including those of Secchi and Dawes, mentioned above, of N. Lockyer (1862), F. Kaiser ( 1862, 1864), and of others, were combined in 1867 by the well-known English astronomer, science writer, and lecturer Richard A. Proctor in the map shown in figure 2.17. In this map, Proctor made the first systematic attempt to name the characteristic features of the Martian surface as an aid to their identification. Most were named after various observers of the planet; for example, Tycho Brahe (Tycho Sea), Kepler (Kepler Land), Beer (Beer Sea), Mâdler (Mâdler Land), and Dawes (Dawes Ocean).

It may be noted that the zero of longitude, where the east and west hemispheres of Proc tor's map touch each other, corresponds as closely as possible to the prime meridian of Beer and Madler. The degrees of longitude are also numbered in the same manner; thus, Syrtis Major, called Kaiser Sea by Proctor, is seen between longitudes of approximately 70° and 90°.

An improved map of Mars, using Proctor's nomenclature, was published in England in 1877 by Nathaniel E. Green, but in general the proposed names were not received with any degree of enthusiasm. A possible reason may be found in the objections expressed by Flammarion. "Fault has been found," he wrote, "with Proctor for having given too much recognition to the astronomers of his own country, and for having repeated the same names." He pointed out, for example, that the name of Dawes was applied to no less than six features—ocean, continent, sea, strait, isle, and bay—and that this was a source of confusion. On the other hand, there were many eminent astronomers and other scientists whose names

FIGURE 2.17. Proctor's map of Mars with named features. (From C. Flammarion, "La Planète Mars," Vol. 1.)

might well have been used. In 1876, Flammarion drew a map of Mars and introduced his own nomenclature, in French, based on that of Proctor but avoiding duplications. This, however, was also destined to become of only historical interest.

For reasons that will be given in chapter III, there are certain times, at intervals of 15 or 17 years, when Earth and Mars are exceptionally close together. These occasions are of special interest to astronomers because Mars then exhibits its maximum apparent size and brightness. The conditions are consequently particularly favorable for observing the planet. Such a close approach occurred in 1877 and that year was marked by two events of historical significance in the study of Mars.

The first, which will be considered more fully in chapter IV, was the discovery by the American astronomer Asaph Hall that Mars has two moons or satellites. The report of this discovery was received without argument, and the moons of Mars soon became an accepted part of astronomy. The other event, described below, related to the so-called canals; it was to initiate a heated controversy that cannot be regarded as completely resolved more than 90 years later.

One observer, who took advantage of a moderately good telescope and the clear Italian skies during the favorable apparition of Mars in the summer of 1877, was Giovanni Virginio Schiaparelli, director of the Brera Observatory, Milan, Italy. In the first report of his studies of Mars, he recorded, with much greater accuracy than had previous observers, the latitude and longitude of 62 characteristic features on the surface of the planet. Furthermore, he disregarded Proctor's nomenclature, as well as that of Flammarion of which he may not have been aware, and introduced a completely new one.

The names used by Schiaparelli were in

Latin and were drawn from geography and mythology. The bright areas of the planet, called "lands" and "continents" by Proctor, were named after terrestrial countries, either real, such as Arabia, Hellas (Greece), Syria, etc., or mythical, such as Elysium, Amazonis, Utopia, etc. The dark areas, also following Proctor, were designated seas; e.g., Boreum Mare (North Sea), Tyrrhenum Mare (Tyrrhenian Sea), etc. Schiaparelli also identified a number of bays, as did Proctor, such as Sa-baeus Sinus (Sabian Bay) and Aurorae Sinus (Aurora Bay) ; some large bays he called gulfs, as for example, Golfo Sabeo. Several small dark areas were designated lakes; thus, Solis Lacus (Lake of the Sun) and Niliacup Lacus (Egyptian Lake). The system of Latin nomenclature introduced by Schiaparelli in 1877 received general approval and he and others added more names in later years. With some modifications, the system is still in common use (fig. 2.21).

In ascribing coordinates of latitude and longitude to the various regions on Mars, Schiaparelli used a zero point essentially identical with the ones employed by his predecessors, Beer and Madler and Proctor. This point, where the prime meridian intersects the equator, he called Fastigium Aryn, after the name of the mythical village shown on medieval maps as located on the cupola of Earth. In Schiaparelli's maps of Mars, the prime meridian passed through the center of Meridianii Sinus (Meridian Bay), and this was accepted until quite recently.

The "Canals" of Mars

Although Schiaparelli was responsible for some of the best contemporary maps of Mars and was the first to locate many of the features of the planet with considerable accuracy, he is probably best remembered in connection with his observations of canals. On Schiapa-

relli's maps there appear a number of dark, almost straight lines, several of them being many hundreds of miles in length. These are seen clearly in figure 2.18, which is a map of Mars prepared from observations made at five apparitions of Mars between 1877 and 1886. Schiaparelli emphasized, however, that only a relatively small number of the lines can be seen at any one instant (p. 25).

The remarkable linear features were named after rivers; for example, Indus, Ganges, Hiddekel (Tigris), etc. In his textual description, however, Schiaparelli referred to them as canali (plural of canale), the Italian word for channels. There is little doubt that he originally considered the canali to be wide, natural waterways connecting the various dark regions which he thought were bodies of water. The word "canale" was, however, translated as "canal" in most European languages, and this led to an erroneous impression concerning what Schiaparelli had intended the word to mean. In order to be visible from Earth, under the best conditions of telescopic viewing, the canals would have to be several miles wide. Even if they contained water, which they do not. they would hardly be called canals in the sense that the word is commonly used.

It is frequently stated that Schiaparelli not only discovered the canals, but was also responsible for introducing the term canale. But neither of these statements is strictly correct. The word canale was used about 1869 by Father Secchi, to whom reference has already been made. There is evidence that Secchi and Schiaparelli exchanged information about Mars, and Schiaparelli must have thought of canale as an accepted term because he introduced it in a casual manner in his report of the observations he made in 1877.

As far as the present writer has been able to determine, the first mention of canale by Schiaparelli is in his report entitled "Astro nomical and Physical Observations on the Axis of Rotation and on the Topography of the Planet Mars," published in Rome in 1878. On page 45 of his collected "Works," in the description of the second "horn" (or arm) of the Golfo Sabeo, he says: "This is formed by the inner tip [or extremity] of a gulf ending in a sharp point and is well defined. The canale which originates there at an opening is not easy to see, and some uncertainty may arise concerning it." Later, on page 167, he wrote: "Where the . . . peninsulas are connected to the two continental zones, they are flanked by the mouths of broad canali which cross these zones."

Moreover, Schiaparelli was not the first to identify linear features on Mars, as he himself pointed out. In a footnote to a report to the Society of Italian Spectroscopists, published in Palermo in 1878, but which may have been added later, Schiaparelli states: "Some people have been inclined to doubt the existence of the canali since they have not seen them. Yet many of these canali are not new and have already been seen by such excellent observers as Kaiser, Lockyer, Secchi, Green, etc." Schiaparelli might also have added the names of Dawes and Proctor, as the linear features are evident in the excellent drawings of the former, such as the one in figure 2.16, and in the rather crude map of the latter (fig. 2.17). Furthermore, the sketches made by the German astronomer Johannes Schroeter between 1785 and 1802, but which were not published until 1881, also showed similar lines.

Then, why is the discovery of the Martian canals so frequently attributed to Schiaparelli? The reason is twofold. First, he identified the canali by giving them specific names (of rivers) and, in so doing, called attention to their existence as a special aspect of the planet. Second, he recorded a much larger number of the linear features than had been

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FIGURE 2.18. Mercator projection map of Mars by Schiaparelli based on observations made at five oppositions in 1877 through 1966. (From C. Flammarion, "La Planète Mars," Vol. 1.)

io oo seen by any (or all) observers in the preceding 200 years. In the report to the Society of Spectroscopists mentioned above, Schiap-arelli wrote, in italics for emphasis: "There are no large continuous continental masses on Mars, as the entire surface of the planet is divided by many canali into an enormous number of islands." This is immediately apparent from the map in figure 2.18, where the white areas are assumed to be land, and the dark portions were evidently regarded by Schiaparelli as being seas, lakes, or rivers (canali).

During the apparition of Mars in 1879, Schiaparelli claimed he saw canals that had not been apparent to him at the preceding opportunity in 1877. Furthermore, he reported the first instance of gemination or twinning in which a canal appeared as two parallel lines instead of a single line.

The sketch in figure 2.19, made by Schiaparelli in 1888, shows several examples of the gemination of canals.

No other astronomers were able to see the numerous Martian canals until the 1886 opportunity when observers in the United States, England, and France claimed to have seen them. The delay in confirmation may possibly have been caused by the fact that the northern hemisphere of Mars, where Schiaparelli saw most of the canals (fig. 2.18), was more clearly visible from Earth during 1886 than it had been in the intervening apparitions of the planet.

Lowell and the Martian Canals

The problem of the canals on Mars would have been left to the scientists had it not been for the activities of Percival Lowell, a mem-

FIGURE 2.19. Drawing by Schiaparelli showing gemination of canals. (From C. Flammarion, "La Planète Mars," Vol. 1.)

FIGURE 2.20. Drawing of Mars by Lowell showing canal network. (From P. Lowell, "Mars as the Abode of Life," Macmillan.)

ber of a distinguished and wealthy family in Boston, and a writer on the Far East. Lowell had become interested in planetary astronomy, especially in Mars, and to pursue this interest, he founded the Lowell Observatory near Flagstaff, Ariz., in 1894, to take advantage of the high altitude and clear atmosphere. Within a short time, Lowell had observed the Martian canals and claimed to have confirmed the phenomenon of gemination. As the years went by he reported more and more canals, and his maps of Mars were essentially networks of canals. This is evident in one of Lowell's sketches reproduced in figure 2.20.

The black spots shown in the drawing, almost invariably at the junctions of canals, were first reported in 1892 by William H. Pickering at the Harvard Observatory at

Arequipa, Peru. He called them lakes, but Lowell, who identified almost 200 in all, referred to them as "oases," which he considered to be a more accurate description of their nature.

Altogether, Lowell identified over 500 canals. But out of fairness it should be mentioned that neither he nor Schiaparelli, who reported 113 canals, claimed that all the canals could be seen at any given time. For example, Schiaparelli said that the map in figure 2.18 "does not at all correspond to the appearance of Mars at any given period, because generally only a few [canals] are visible at one time." In his more flowery style, Lowell wrote: "Permanent the canals are in place, impermanent they prove in character. . . . Some will show when others remain hid and others will appear when the first have become

Name

Acidalium Mare. .

Aeolis

Aeria

Aetheria

Aethiopis

Amazonis

Amenthes

Aonius Sinus

Arabia

Araxes

Arcadia

Argyre

Arnon

Aurorae Sinus

Ausonia

Australe Mare

Baltia

Boreum Mare

Boreosyrtis

Candor

Casius

Cebrenia

Cecropia

Ceraunius

Cerberus

Chalce

Chersonesus

Chronium Mare...

Chryse

Chrysokeras

Cimmerium Mare.

,ong. deg.)

Lat. (deg.)

Name

Long, (deg.)

Lat. (deg.)

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