This simple cosmogony, which merely translates what our eyes show us, has been that believed by every people in their infancy. Among the savage tribes of the two worlds, in America and in Africa, as in the ancient East, among the Romans as among the Egyptians and the ancient Greeks, this coarse simplicity and absolute ignorance of the constitution of the world prevailed. On this profoundly false basis all the ancient religions were founded. The social customs of modern peoples are based upon the same errors. Language has consecrated them; the earth is everywhere called the world, as the ancients called it (mundus, κόσμος); every one says the sun travels, or goes, from east to west, and that the stars rise and set.
Poetry has set its eternal seal on this vicious system, and has, so to speak, consecrated it, by clothing it with all the prestige of genius and imagination.
Modern astronomy has caused the false skies of antiquity to vanish away; it has dispersed the pretensions of the celestial vault, sown with brilliant spots, and substituted a simple mass of coloured air. It has revealed the true office of each of those stars which we see by day or by night. It has fixed, in an indisputable manner, the real place of the earth in the universe, and, to say the truth, that place is singularly small.
We know now, that the earth, far from being herself the world, is only an imperceptible point of the world. If we only compare it with the sun, we know that our globe is one million three hundred thousand times smaller than the sun. This takes us far away from the idea of the ancient Greeks, who thought they ventured much in asserting that the sun was as big as the Peloponnesus.
In addition, the earth has been dispossessed of all privileges. It was believed formerly to be unique and unrivalled, we now know that there are an infinity of other globes similar to the earth, so that she is no more than one individual in a group of other individuals who resemble her. We know that the earth figures among the planets, that she is only a planet of our system.
What, then, is a planet? the reader will ask. An attentive gaze directed to the stars of night will make him understand it. Let him examine, on any fine evening, the star which is pointed out to him as Mars or Jupiter, and to which a certain position is assigned at a given hour. Then, a few hours later, let him come and look once more for Mars or Jupiter, and he will perceive that the position of Mars or Jupiter, with respect to the other stars, is changed. Or he may do better still. Let him look at Mars or Jupiter through the telescope of an observatory, or the glass of one of those open-air astronomers who are to be found in the public ways in Paris and other great cities. Thus he may see Mars or Jupiter change his place under his own eyes. While the other stars remain motionless, Jupiter or Mars will pass away from the field of the glass.
There are, then, fixed stars and movable stars. The movable stars are the planets (πλανήτης, from πλάνος, wandering). The fixed stars are what we call stars. It is not difficult to distinguish the planets from the stars with the naked eye. The stars emit sparkling light, whence comes their name, from the Latin stellare, to shine, and their light twinkles. The planets, on the contrary, shine with a steady, mild, unvacillating light. The reason of this difference is, that the light shed by the stars is their own. The stars are so many suns resembling ours. They illumine worlds like our world, so prodigiously distant that we cannot even perceive them. The planets do not shine of themselves; they merely reflect, like gigantic mirrors, the light of the sun which illumines them, and renders them visible to us. Thus, the planets are stars which travel. They revolve around the sun. The earth, being a planet, is a travelling star, which revolves around the sun.
But the earth is not the only planet of our solar system. There are seven others, which do not differ essentially from the earth. The names of the eight planets which compose our solar system, are as follows, arranged according to their distance from the sun: Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Between Mars and Jupiter there is a collection of small bodies, which seem to be fragments of broken planets; they are called asteroïds. At present, in 1871, more than a hundred are known, and it is not yet fifty years since they were first sought for in the sky. These asteroïds may be collected together in our fancy, and formed into a separate group, which would be a ninth planet. Let us glance at the planets which compose our solar system.
Plates 4 and 5, which accompany these pages, will suffice to give an idea of the relative dimensions of the planets. In these two plates the planets are arranged according to the order of their distance from the sun. In plate 4, Mercury, Venus, the Earth, and Mars are represented; in plate 5, the asteroïds Jupiter, Saturn, Uranus, and Neptune. Mercury is the nearest planet to the sun, his distance from the central orb being only fourteen millions of leagues, which, in astronomy, is near neighbourhood. This planet revolves upon its axis with the same rapidity as the earth. The day, in Mercury, is only three minutes longer than ours (24h. 3ms.). Being closer to the sun than the earth is, Mercury turns more quickly round the sun, so that its year is only 88 days, whereas ours is 365 days.
We know that the sole cause of the inequality of the seasons, as well as of day and night in the planets, is the inclination of the star on its axis of rotation. If the planets, while revolving round the sun, retained the verticality of the axis which joins these north and south poles, there would be perfect equality in the distribution of the solar light and heat over the same latitudes; along each parallel there would be a complete regularity in the lighting and warming of the planet; the differences of heat and cold would not depend on anything but their greater or less distance from the sun. But this verticality only exists for two or three planets of our system. The others, and among them Mercury, Venus, the Earth, and Mars, are strongly inclined on their axis of rotation.