The Age of Reason/Part I/Chapter XIV
That part of the universe that is called the solar system (meaning the system of worlds to which our earth belongs, and of which Sol, or in English language, the Sun, is the center) consists, besides the Sun, of six distinct orbs, or planets, or worlds, besides the secondary bodies, called the satellites, or moons, of which our earth has one that attends her in her annual revolution round the Sun, in like manner as the other satellites or moons, attend the planets or worlds to which they severally belong, as may be seen by the assistance of the telescope.
The Sun is the center round which those six worlds or planets revolve at different distances therefrom, and in circles concentric to each other. Each world keeps constantly in nearly the same tract round the Sun, and continues at the same time turning round itself, in nearly an upright position, as a top turns round itself when it is spinning on the ground, and leans a little sideways.
It is this leaning of the earth (23½ degrees) that occasions summer and winter, and the different length of days and nights. If the earth turned round itself in a position perpendicular to the plane or level of the circle it moves in round the Sun, as a top turns round when it stands erect on the ground, the days and nights would be always of the same length, twelve hours day and twelve hours night, and the season would be uniformly the same throughout the year.
Every time that a planet (our earth for example) turns round itself, it makes what we call day and night; and every time it goes entirely round the Sun, it makes what we call a year, consequently our world turns three hundred and sixty-five times round itself, in going once round the Sun.
The names that the ancients gave to those six worlds, and which are still called by the same names, are Mercury, Venus, this world that we call ours, Mars, Jupiter, and Saturn. They appear larger to the eye than the stars, being many million miles nearer to our earth than any of the stars are. The planet Venus is that which is called the evening star, and sometimes the morning star, as she happens to set after, or rise before the Sun, which in either case is never more than three hours.
The Sun as before said being the center, the planet or world nearest the Sun is Mercury; his distance from the Sun is thirty-four million miles, and he moves round in a circle always at that distance from the Sun, as a top may be supposed to spin round in the tract in which a horse goes in a mill. The second world is Venus; she is fifty-seven million miles distant from the Sun, and consequently moves round in a circle much greater than that of Mercury. The third world is this that we inhabit, and which is eighty-eight million miles distant from the Sun, and consequently moves round in a circle greater than that of Venus. The fourth world is Mars; he is distant from the sun one hundred and thirty-four million miles, and consequently moves round in a circle greater than that of our earth. The fifth is Jupiter; he is distant from the Sun five hundred and fifty-seven million miles, and consequently moves round in a circle greater than that of Mars. The sixth world is Saturn; he is distant from the Sun seven hundred and sixty-three million miles, and consequently moves round in a circle that surrounds the circles or orbits of all the other worlds or planets.
The space, therefore, in the air, or in the immensity of space, that our solar system takes up for the several worlds to perform their revolutions in round the Sun, is of the extent in a strait line of the whole diameter of the orbit or circle in which Saturn moves round the Sun, which being double his distance from the Sun, is fifteen hundred and twenty-six million miles; and its circular extent is nearly five thousand million; and its globical content is almost three thousand five hundred million times three thousand five hundred million square miles.
But this, immense as it is, is only one system of worlds. Beyond this, at a vast distance into space, far beyond all power of calculation, are the stars called the fixed stars. They are called fixed, because they have no revolutionary motion, as the six worlds or planets have that I have been describing. Those fixed stars continue always at the same distance from each other, and always in the same place, as the Sun does in the center of our system. The probability, therefore, is that each of those fixed stars is also a Sun, round which another system of worlds or planets, though too remote for us to discover, performs its revolutions, as our system of worlds does round our central Sun.
By this easy progression of ideas, the immensity of space will appear to us to be filled with systems of worlds; and that no part of space lies at waste, any more than any part of our globe of earth and water is left unoccupied.
Having thus endeavoured to convey, in a familiar and easy manner, some idea of the structure of the universe, I return to explain what I before alluded to, namely, the great benefits arising to man in consequence of the Creator having made a plurality of worlds, such as our system is, consisting of a central Sun and six worlds, besides satellites, in preference to that of creating one world only of a vast extent.
- Those who supposed that the Sun went round the earth every 24 hours made the same mistake in idea that a cook would do in fact, that should make the fire go round the meat, instead of the meat turning round itself towards the fire.--Author.
- If it should be asked, how can man know these things? I have one plain answer to give, which is, that man knows how to calculate an eclipse, and also how to calculate to a minute of time when the planet Venus, in making her revolutions round the Sun, will come in a strait line between our earth and the Sun, and will appear to us about the size of a large pea passing across the face of the Sun. This happens but twice in about a hundred years, at the distance of about eight years from each other, and has happened twice in our time, both of which were foreknown by calculation. It can also be known when they will happen again for a thousand years to come, or to any other portion of time. As therefore, man could not be able to do these things if he did not understand the solar system, and the manner in which the revolutions of the several planets or worlds are performed, the fact of calculating an eclipse, or a transit of Venus, is a proof in point that the knowledge exists; and as to a few thousand, or even a few million miles, more or less, it makes scarcely any sensible difference in such immense distances.-—Author.