Uranus has four satellites, and Neptune one. The remarkable thing about these satellites is that they revolve backward, or contrary to the direction in which all the other satellites belonging to the solar system revolve, and in which all the other planets rotate on their axis. In the case of Uranus, the plane in which the satellites revolve is not far from a position at right angles to the plane of the ecliptic; but in the case of Neptune, the plane of revolution of the satellites is tipped much farther backward. Since in every other case the satellites of a planet are situated nearly in the plane of the planet's equator, it may be assumed that the same rule holds with Uranus and Neptune; and, that being so, we must conclude that those planets rotate backward on their axes. This has an important bearing on the nebular hypothesis of the origin of the solar system, and at one time was thought to furnish a convincing argument against that hypothesis; but it has been shown that by a modification of Laplace's theory the peculiar behavior of Uranus and Neptune can be reconciled with it.
Very little is known of the surfaces of Uranus and Neptune. Indications of the existence of belts resembling those of Jupiter have been found in the case of both planets. There are similar belts on Saturn, and as they seem to be characteristic of large, rapidly rotating bodies of small density, it was to be expected that they would be found on Uranus and Neptune.
The very interesting opinion is entertained by some astronomers that there is at least one other great planet beyond Neptune. The orbits of certain comets are relied upon as furnishing evidence of the existence of such a body. Prof. George Forbes has estimated that this, as yet undiscovered, planet may be even greater than Jupiter in mass, and may be situated at a distance from the sun one hundred times as great as the earth's, where it revolves in an orbit a single circuit of which requires a thousand years.
Whether this planet, with a year a thousand of our years in length, will ever be seen with a telescope, or whether its existence will ever, in some other manner, be fully demonstrated, can not yet be told. It will be remembered that Neptune was discovered by means of computations based upon its disturbing attraction on Uranus before it had ever been recognized with the telescope. But when the astronomers in the observatories were told by their mathematical brethren where to look they found the planet within half an hour after the search began. So it is possible the suspected great planet beyond Neptune may be within the range of telescopic vision, but may not be detected until elaborate calculations have deduced its place in the heavens. As a populous city is said to furnish the best hiding-place for a man who would escape the attention of his fellow beings, so the star-sprinkled sky is able to conceal among its multitudes worlds both great and small until the most painstaking detective methods bring them to recognition.
CHAPTER VIII
THE MOON, CHILD OF THE EARTH AND THE SUN
Very naturally the moon has always been a great favorite with those who, either in a scientific or in a literary spirit, have speculated about the plurality of inhabited worlds. The reasons for the preference accorded to the moon in this regard are evident. Unless a comet should brush us—as a comet is suspected of having done already—no celestial body, of any pretensions to size, can ever approach as near to the earth as the moon is, at least while the solar system continues to obey the organic laws that now control it. It is only a step from the earth to the moon. What are 240,000 miles in comparison with the distances of the stars, or even with the distances of the planets? Jupiter, driving between the earth and the moon, would occupy more than one third of the intervening space with the chariot of his mighty globe; Saturn, with broad wings outspread, would span more than two thirds of the distance; and the sun, so far from being able to get through at all, would overlap the way more than 300,000 miles on each side.
In consequence, of course, of its nearness, the moon is the only member of the planetary system whose principal features are visible to the naked eye. In truth, the naked eye perceives the larger configurations of the lunar surface more clearly than the most powerful telescope shows the details on the disk of Mars. Long before the time of Galileo and the invention of the telescope, men had noticed that the face of the moon bears a resemblance to the appearance that the earth would present if viewed from afar off. In remote antiquity there were philosophers who thought that the moon was an inhabited world, and very early the romancers took up the theme. Lucian, the Voltaire of the second century of our era, mercilessly scourged the pretenders of the earth from an imaginary point of vantage on the moon, which enabled him to peer down into their secrets. Lucian's description of the appearance of the earth from the moon shows how clearly defined in his day had become the conception of our globe as only an atom in space.
"Especially did it occur to me to laugh at the men who were quarreling about the boundaries of their land, and at those who were proud because they cultivated the Sikyonian plain, or owned that part of Marathon around Œnoe, or held possession of a thousand acres at Acharnæ. Of the whole of Greece, as it then appeared to me from above, being about the size of four fingers, I think Attica was in proportion a mere speck. So that I wondered on what condition it was left to these rich men to be proud."[14]