The Story of the Heavens

by Robert S. Ball

The close approaches of Mars to the earth afford us opportunities for making a careful telescopic scrutiny of his surface. It must not be expected that the details on Mars could be inspected with the same minuteness as those on the moon. Even under the most favourable circumstances, Mars is still more than a hundred times as far as the moon, and, therefore, the features of the planet have to be at least one hundred times as large if they are to be seen as distinctly as the features on the moon. Mars is much smaller than the earth. The diameter of the planet is 4,200 miles, but little more than half that of the earth. Fig. 50 shows the comparative sizes of the two bodies. We here reproduce two of the remarkable drawings[16] of Mars made by Professor William H. Pickering at the Lowell Observatory, Flagstaff A.T. Fig. 51 was taken on the 30th of July, 1894, and Fig. 52 on the 16th of August, 1894.

The southern polar cap on Mars, as seen by Professor William H. Pickering at Lowell Observatory on the 1st of July, 1894, is represented in Fig. 54.[17] The remarkable black mark intruding into the polar area will be noticed. In Fig. 53 are shown a series of unusually marked elevations and depressions upon the "terminator" of the planet, drawn as accurately as possible to scale by the same skilful hand on the 24th of August, 1894.

In making an examination of the planet it is to be observed that it does not, like the moon, always present the same face towards the observer. Mars rotates upon an axis in exactly the same manner as the earth. It is not a little remarkable that the period required by Mars for the completion of one rotation should be only about half an hour greater than the period of rotation of the earth. The exact period is 24 hours, 37 minutes, 22-3⁄4 seconds. It therefore follows that the aspect of the planet changes from hour to hour. The western side gradually sinks from view, the eastern side gradually assumes prominence. In twelve hours the aspect of the planet is completely changed. These changes, together with the inevitable effects of foreshortening, render it often difficult to correlate the objects on the planet with those on the maps. The latter, it must be confessed, fall short of the maps of the moon in definiteness and in certainty; yet there is no doubt that the main features of the planet are to be regarded as thoroughly established, and some astronomers have given names to all the prominent objects.

The markings on the surface of Mars are of two classes. Some of them are of an iron-grey hue verging on green, while the others are generally dark yellow or orange, occasionally verging on white. The former have usually been supposed to represent the tracts of ocean, the latter the continental masses on the ruddy planet. We possess a great number of drawings of Mars, the earliest being taken in the middle of the seventeenth century. Though these early sketches are very rough, and are not of much value for the solution of questions of topography, they have been found very useful in aiding us to fix the period of rotation of the planet on its axis by comparison with our modern drawings.

Early observers had already noticed that each of the poles of Mars is distinguished by a white spot. It is, however, to William Herschel that we owe the first systematic study of these remarkable polar caps. This illustrious astronomer was rewarded by a very interesting discovery. He found that these arctic tracts on Mars vary both in extent and distinctness with the seasons of the hemisphere on which they are situated. They attain a maximum development from three to six months after the winter solstice on that planet, and then diminish until they are smallest about three to six months after the summer solstice. The analogy with the behaviour of the masses of snow and ice which surround our own poles is complete, and there has until lately been hardly any doubt that the white polar spots of Mars are somewhat similarly constituted.

As the period of revolution of Mars around the sun is so much longer than our year, 687 days instead of 365, the seasons of the planet are, of course, also much longer than the terrestrial seasons. In the northern hemisphere of Mars the summer lasts for no fewer than 381 days, and the winter must be 306 days. In both hemispheres the white polar cap in the course of the long winter season increases until it reaches a diameter of 45° to 50°, while the long summer reduces it to a small area only 4° or 5° in diameter. It is remarkable that one of these white regions—that at the south pole—seems not to be concentric with the pole, but is placed so much to one side that the south pole of Mars appears to be quite free from ice or snow once a year.

Although many valuable observations of Mars were made in the course of the nineteenth century, it is only since the very favourable opposition of 1877 that the study of the surface of Mars has made that immense progress which is one of the most remarkable features of modern astronomy. Among the observers who produced valuable drawings of the planet in 1877 we may mention Mr. Green, whose exquisite pictures were published by the Royal Astronomical Society, and Professor Schiaparelli, of Milan, who almost revolutionised our knowledge of this planet. Schiaparelli had a refractor of only eight inches aperture at his disposal, but he was doubtless much favoured by the purity of the Italian sky, which enabled him to detect in the bright portions of the surface of Mars a considerable number of long, narrow lines. To these he gave the name of canals, inasmuch as they issued from the so-called oceans, and could be traced across the reputed continents for considerable distances, which sometimes reached thousands of miles.

The canals seemed to form a kind of network, which connected the various seas with each other. A few of the more conspicuous of these so-called canals appeared indeed on some of the drawings made by Dawes and others before Schiaparelli's time. It was, however, the illustrious Italian astronomer who detected that these narrow lines are present in such great numbers as to form a notable feature of the planet. Some of these remarkable features are shown in Figs. 51 and 52, which are copied from drawings made by Professor William H. Pickering at the Lowell Observatory in 1894.

Great as had been the surprise of astronomers when Schiaparelli first proclaimed the discovery of these numerous canals, it was, perhaps, surpassed by the astonishment with which his announcement was received in 1882 that most of the canals had become double. Between December, 1881, and February, 1882, thirty of these duplications appear to have taken place. Nineteen of these were cases of a well-traced parallel line being formed near a previously existing canal. The remaining canals were less certainly established, or were cases where the two lines did not seem to be quite parallel. A copy of the map of Mars which Schiaparelli formed from his observations of 1881–82 is given in Plate XVIII. It brings out clearly these strange double canals, so unlike any features that we know on any other globe.