Through the Telescope

by James Baikie

To the intending observer one important caution may be suggested. In observing and sketching the surface of Mars, do so independently. The chart which accompanies this chapter is given for the purpose of identifying markings which have been already seen, not for that of enabling the observer to see details which are beyond the power of his glass. No planet has been the cause of more illusion than Mars, and drawings of him are extant which resemble nothing so much as the photograph of an umbrella which has been turned inside out by a gust of wind. In such cases it may reasonably be concluded that there is something wrong, and that, unconsciously, 'the vision and the faculty divine' have been exercised at the expense of the more prosaic, but in this case more useful, quality of accuracy. By prolonged study of a modern chart of Mars, and a little gentle stretching of the imagination, the most unskilled observer with the smallest instrument will detect a multitude of canals upon the planet, to which there is but one objection, that they do not exist. There is enough genuine interest about Mars, even when viewed with a small glass, without the importation of anything spurious. In observation it will be noticed that as the rotation period of Mars nearly coincides with that of the earth, the change in the aspect presented from night to night will be comparatively small, the same object coming to the meridian thirty-seven minutes later each successive evening. Generally speaking, Mars is an easier object to define than either Venus or Jupiter, though perhaps scarcely bearing high powers so well as Saturn. There is no planet more certain to repay study and to maintain interest. He and Jupiter may be said to be at present the 'live' planets of the solar system in an astronomical sense.

[*] The opposition of a planet occurs 'whenever the sun, the earth, and the planet, as represented in their projected orbits, are in a straight line, with the earth in the middle.'

[†] That point in the orbit of a planet or comet which is nearest to the sun.

CHAPTER IX

THE ASTEROIDS

In the year 1772 Bode of Berlin published the statement of a curiously symmetrical relation existing among the planets of our system. The gist of this relation, known as Bode's law, though it was really discovered by Titius of Wittenberg, may be summed up briefly thus: 'The interval between the orbits of any two planets is about twice as great as the inferior interval, and only half the superior one.' Thus the distance between the orbits of the earth and Venus should, according to Bode's law, be half of that between the earth and Mars, which again should be half of that which separates Mars from the planet next beyond him. Since the discovery of Neptune, this so-called law has broken down, for Neptune is very far within the distance which it requires; but at the time of its promulgation it represented with considerable accuracy the actual relative positions of the planets, with one exception. Between Mars and Jupiter there was a blank which should, according to the law, have been filled by a planet, but to all appearance was not. Noticing this blank in the sequence, Bode ventured to predict that a planet would be found to fill it; and his foresight was not long in being vindicated.

Several continental astronomers formed a kind of planet-hunting society to look out for the missing orb; but their operations were anticipated by the discovery on January 1, 1801, of a small planet which occupied a place closely approximating to that indicated for the missing body by Bode's law. The news of this discovery, made by Piazzi of Palermo in the course of observations for his well-known catalogue of stars, did not reach Bode till March 20, and 'the delay just afforded time for the publication, by a young philosopher of Jena named Hegel, of a "Dissertation" showing, by the clearest light of reason, that the number of the planets could not exceed seven, and exposing the folly of certain devotees of induction who sought a new celestial body merely to fill a gap in a numerical series.'

The remarkable agreement of prediction and discovery roused a considerable amount of interest, though the planet actually found, and named Ceres after the patron-goddess of Sicily, seemed disappointingly small. But before very long Olbers, one of the members of the original planet-hunting society, surprised the astronomical world by the discovery of a second planet which also fulfilled the condition of Bode's law; and by the end of March, 1807, two other planets equally obedient to the required numerical standard were found, the first by Harding, the second by Olbers. Thus a system of four small planets, Ceres, Pallas, Juno, and Vesta, was found to fill that gap in the series which had originally suggested the search. To account for their existence Olbers proposed the theory that they were the fragments of a large planet which had been blown to pieces either by the disruptive action of internal forces or by collision with a comet; and this theory remained in favour for a number of years, though accumulating evidence against it has forced its abandonment.

It was not till 1845 that there was any addition to the number of the asteroids, as they had come to be named. In that year, however, Hencke of Driessen in Prussia, discovered a fifth, which has been named Astræa, and in 1847 repeated his success by the discovery of a sixth, Hebe. Since that time there has been a steady flow of discoveries, until at the present time the number known to exist is close upon 700, of which 569 have received permanent numbers as undoubtedly distinct members of the solar system; and this total is being steadily added to year by year, the average annual number of discoveries for the years 1902 to 1905 inclusive, being fifty-two. For a time the search for minor planets was a most laborious business. The planet-hunter had to construct careful maps of all the stars visible in a certain small zone of the ecliptic, and to compare these methodically with the actual face of the sky in the same zone, as revealed by his telescope. Any star seen in the telescope, and not found to be marked upon the chart, became forthwith an object of grave suspicion, and was watched until its motion, or lack of motion, relatively to the other stars either proved or disproved its planetary nature. At present this lengthy and wearisome process has been entirely superseded by the photographic method, in which a minor planet is detected by the fact that, being in motion relatively to the fixed stars, its image will appear upon the plate in the shape of a short line or trail, the images of the fixed stars being round dots. Of course the trail may be due to a planet which has already been discovered; but should there be no known minor planet in the position occupied by the trail, then a new member has been added to the system. Minor-planet hunting has always been a highly specialized branch of astronomy, and a few observers, such as Peters, Watson, Charlois and Palisa, and at present Wolf, have accounted for the great majority of the discoveries.