THE MOONS OF MARS.
IT is somewhat singular that during the last few years, in fact during the present century, many more planets than moons have been discovered within the Solar system. In the year 1777 it had become an article of faith among astronomers that as primary planets are more numerous than suns (judging from what we know of the solar system), so secondary planets are more numerous than planets. Our own moon, the four moons of Jupiter, and the five known moons of Saturn, brought the total number of satellites to ten, while the total number of known planets amounted only to six, including, of course, the Earth. Within four years, or on March 13, 1781, a new planet- Uranus—was discovered ; but only six years later, or on January 11, 1787, two satellites were found to attend this planet in its circuit round the sun. Two years later the number of known satellites of Saturn was Increased by two. So that now there were seven known planets and twice as many known satellites. We may pass over Sir W. Hersehers supposed discovery of four new satellites of Uranus, for beyond question, he was mistaken. Thus when the present century opened, the Sun was regarded as the centre of a scheme consisting of seven planets, fourteen moons, and the Saturnian ring-system. But from that time forth the discovery of planets took place at a far more rapid rate than that of moons or satellites. On the first day of the century the first member of that curious ring of planets circling between Mars and Jupiter, was discovered ; on March 29, 1807, the fourth. On September 23, Adams and Leverrier dis- covered Neptune. Two months later the fifth of the small planets was found, and a few days ago the 175th, making 176 planets discovered during the present century. In all that time, until the night of August 18th last, only four new satellites had been recog- nised,—viz., one attendant of Neptune, by Laesell, in 1846; two of Uranus, by the same observer, in 1847; and one of Saturn, by Bond, in America, and Lassen, independently, on September 19, 1848.
Astronomers had begun to think that no new moons would be discovered, unless, perhaps, some mighty telescope might detect another moon or two attending on the remote planet Neptune. It had been shown that four of the eight moons with which Uranus had been credited had no real existence, and one of the pair of moons assigned to Neptune had been in like manner dismissed to scientific limbo, along with Venus's satellite, the planet Vul- can, and the rings of Uranus. In vain had the mighty refractor of the Washington Observatory, the most perfect refracting tele- scope yet made, been turned on Uranus and Neptune to detect new satellites ; and if that telescope failed, none other in existence might be expected to succeed. The discovery of a new satellite was for the time being about the last thing of which astronomers hoped to hear, and certainly nothing would have been held to be more utterly unlikely (unless, perhaps, it were the detection of an attendant on Venus or Mercury) than the discovery of a moon attending on Mars. Strangely enough, of all the planets, Mars, though not really the only planet without a moon, bad alone come to be known as a moonless planet. "The snowy poles of moon- less Mars" were words which, though dismissed by the poet him- self who wrote them from the pages of the work in which they appeared, had somehow come to be as familiar to astronomers in connection with Mars as in the case of Venus the old line,— " Cynthis3 figures annulatur Mater Amorum,"
in which Galileo announced his detection of the varying phases of the Planet of Love.
Yet it is this small planet Mars, the least but one of all the primary planets of the solar system (excepting, of course, the members of that group of associated planets travelling between the orbits of Mars and Jupiter), and a planet studied under such favourable conditions that one would have thought no moon of his could have escaped detection, which is found to have not one moon only, but a pair. It is very easy to explain, without enter- ing into details unsuited to these columns, why astronomers were satisfied that if Mars has any moons at all they must be very small ones. Mara has been repeatedly examined with the most powerful telescopes under most favourable conditions. When nearest to us, he is about thirty-five millions of miles from the earth. Jupiter when nearest is eleven times farther from us. So that if Mars had a moon as large as the least of Jupiter's, or in other words, about as largo as our own moon, it would look 121 times larger than the least of Jupiter's moons. It would exceed that moon in brightness in a much greater degree, because Mars is much nearer to the Sun than Jupiter is, and there- fore receives much more light. In fact, Mars receives about nine times as much light as Jupiter. Thus a moon of his as large as ours would shine fully 1,000 times as brightly as the least of Jupiter's. But the least of Jupiter's moons can be seen in the smallest tele- scope ever used for astronomical purposes. The telescopes which have been turned on Mars witho at finding any satellite are fully 200 times more powerful as light-gatherers than the smallest, so that a moon showing a disc only 1-200,000th part of that which would be shown by a Martian moon as large as ours ought to be easily seen in the largest astronomical telescopes, From this it follows that if Mars has a moon whose diameter is greater than about the 450th part of our Moon's, that moon ought no more to escape observation with the powerful telescopes directed upon Mars than the moons of Jupiter escaped under the sellitiny of Galileo's telescopes. But that is as muelt as to say that Mars can have no moon more than five miles in diameter. Astonishing, though this may seem, it will be readily perceived to be just, when we consider that„ according to the, estimates of Mr. Stone, formerly &chief assistant at the Greenwich Observatory, the diameters of some of the smallest of the minor, planets are less than twenty miles ; and yet these, though much farther away than Mars, have been detected with telescopes by no means the most powerful of those employed by astronomers„ In fact, some of the tele- scopes turned upon MerfA are altogether too large and cumbrous, to be used in searching for sinall.planete..
We do not know how largethe two moons of Mars which have just been discovered actually are. As-yet we have only the. news of their discovery, and that they travel at distances of about 13,800 and.about 8,600, miles from the centre of Mars, in periods.of about 3.0 hours and about 15, hours respectively. But as a distance of 13,800 miles from the centre of Mare corresponds. to a. distance of more than 11,500 miles. from his surface, or fully two-and-a-half times his, diameter, there seem no reason why the glare of the planet itself should have coneealed the outer satellite,, at least, from view. We are compelled therefore to conclude that this, satellite (and probably the other also) must be very small. We shall be surprised if it has a diameter of more than, 10 miles, and as we have said, if it has a diameter of five miles it ought to have been discovered sooner.
The satellites of Mars thus manifestly belong to an order in the scale of creation quite different from that, to which our, own Moon and the moons of Jupiter and Saturn belong., One of. these Martian moons would be as inferior to our own as our Earth is to Saturn, or Mercury to Uranus ; and as astronomers justly regard. the Earth and Mercury as bodies quite different (in order) from, the giant planets Jupiter, Saturn, Uranus, and Neptune, the leapt of which exceeds manifold in mass all the four planets. Mars, Mercury, Venus, and the Earth, taken together, they must on the same principle relegate the two new satellites of Mars,to an order distinct from that to which belong the moons of, Jupiter and Saturn and our own Moon. Titan, the giant among the Saturnian moons, a body probably as large, at least,. as Mars, might have moons such as these newly discovered ones attending upon him, satellites of a satellite. So might the third and largest of. Jupiter's moons, which is little if at all inferior to Mercury in size. But we should. have but a small chance of detecting such. tertiary planets, even with, the mighty mirror of the a.rsonstown reflector.
The new bodies, small though they are, can tell us the mass of. Mars as truly as if they were moons exceeding, our own in size. A body no larger than a peppercorn, if we knew but the period in which it circled at known distance around 'a planet, would show the planet's mass as truly as a moon containing millions of billions of tons of matter. Weighed by the motions of. his moons (so far as the imperfect telegraphic information yet received can be used. for calculation), the mass of the planet Mars is not, as had hereto- fore been judged, about 118-thousandths of the Earth's, but only about 94-thousandths,---a reduction of mass equal to. about half the mass; of our own Moon.
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