Philosophical Dictionary, by Voltaire


The laws of optics, which are founded upon the nature of things, have ordained that, from this small globe of earth on which we live, we shall always see the material heaven as if we were the centre of it, although we are far from being that centre; that we shall always see it as a vaulted roof, hanging over a plane, although there is no other vaulted roof than that of our atmosphere, which has no such plane; that our sun and moon will always appear one-third larger at the horizon than at their zenith, although they are nearer the spectator at the zenith than at the horizon.

Such are the laws of optics, such is the structure of your eyes, that, in the first place, the material heaven, the clouds, the moon, the sun, which is at so vast a distance from you; the planets, which in their apogee are still at a greater distance from it; all the stars placed at distances yet vastly greater, comets and meteors, everything, must appear to us in that vaulted roof as consisting of our atmosphere.

The sun appears to us, when in its zenith, smaller than when at fifteen degrees below; at thirty degrees below the zenith it will appear still larger than at fifteen; and finally, at the horizon, its size will seem larger yet; so that its dimensions in the lower heaven decrease in consequence of its elevations, in the following proportions:

At the horizon 100
At fifteen degrees above   68
At thirty degrees   50
At forty-five degrees   40

Its apparent magnitudes in the vaulted roof are as its apparent elevations; and it is the same with the moon, and with a comet.

It is not habit, it is not the intervention of tracts of land, it is not the refraction of the atmosphere which produces this effect. Malebranche and Régis have disputed with each other on this subject; but Robert Smith has calculated.

Observe the two stars, which, being at a prodigious distance from each other, and at very different depths, in the immensity of space, are here considered as placed in the circle which the sun appears to traverse. You perceive them distant from each other in the great circle, but approximating to each other in every circle smaller, or within that described by the path of the sun.

It is in this manner that you see the material heaven. It is by these invariable laws of optics that you perceive the planets sometimes retrograde and sometimes stationary; there is in fact nothing of the kind. Were you stationed in the sun, we should perceive all the planets and comets moving regularly round it in those elliptical orbits which God assigns. But we are upon the planet of the earth, in a corner of the universe, where it is impossible for us to enjoy the sight of everything.

Let us not then blame the errors of our senses, like Malebranche; the steady laws of nature originating in the immutable will of the Almighty, and adapted to the structure of our organs, cannot be errors.

We can see only the appearances of things, and not things themselves. We are no more deceived when the sun, the work of the divinity — that star a million times larger than our earth — appears to us quite flat and two feet in width, than when, in a convex mirror, which is the work of our own hands, we see a man only a few inches high.

If the Chaldæan magi were the first who employed the understanding which God bestowed upon them, to measure and arrange in their respective stations the heavenly bodies, other nations more gross and unintelligent made no advance towards imitating them.

These childish and savage populations imagined the earth to be flat, supported, I know not how, by its own weight in the air; the sun, moon, and stars to move continually upon a solid vaulted roof called a firmament; and this roof to sustain waters, and have flood-gates at regular distances, through which these waters issued to moisten and fertilize the earth.

But how did the sun, the moon, and all the stars reappear after their setting? Of this they know nothing at all. The heaven touched the flat earth: and there were no means by which the sun, moon, and stars could turn under the earth, and go to rise in the east after having set in the west. It is true that these children of ignorance were right by chance in not entertaining the idea that the sun and fixed stars moved round the earth. But they were far from conceiving that the sun was immovable, and the earth with its satellite revolving round him in space together with the other planets. Their fables were more distant from the true system of the world than darkness from light.

They thought that the sun and stars returned by certain unknown roads after having refreshed themselves for their course at some spot, not precisely ascertained, in the Mediterranean Sea. This was the amount of astronomy, even in the time of Homer, who is comparatively recent; for the Chaldæans kept their science to themselves, in order to obtain thereby, greater respect from other nations. Homer says, more than once, that the sun plunges into the ocean — and this ocean, be it observed, is nothing but the Nile — here, by the freshness of the waters, he repairs during the night the fatigue and exhaustion of the day, after which, he goes to the place of his regular rising by ways unknown to mortals. This idea is very like that of Baron Fœneste, who says, that the cause of our not seeing the sun when he goes back, is that he goes back by night.

As, at that time, the nations of Syria and the Greeks were somewhat acquainted with Asia and a small part of Europe, and had no notion of the countries which lie to the north of the Euxine Sea and to the south of the Nile, they laid it down as a certainty that the earth was a full third longer than it was wide; consequently the heaven, which touched the earth and embraced it, was also longer than it was wide. Hence came down to us degrees of longitude and latitude, names which we have always retained, although with far more correct ideas than those which originally suggested them.

The Book of Job, composed by an ancient Arab who possessed some knowledge of astronomy, since he speaks of the constellations, contains nevertheless the following passage: “Where wert thou, when I laid the foundation of the earth? Who hath taken the dimensions thereof? On what are its foundations fixed? Who hath laid the cornerstone thereof?”

The least informed schoolboy, at the present day, would tell him, in answer: “The earth has neither cornerstone nor foundation; and, as to its dimensions, we know them perfectly well, as from Magellan to Bougainville, various navigators have sailed round it.”

The same schoolboy would put to silence the pompous declaimer Lactantius, and all those who before and since his time have decided that the earth was fixed upon the water, and that there can be no heaven under the earth; and that, consequently, it is both ridiculous and impious to suppose the existence of antipodes.

It is curious to observe with what disdain, with what contemptuous pity, Lactantius looks down upon all the philosophers, who, from about four hundred years before his time, had begun to be acquainted with the apparent revolutions of the sun and planets, with the roundness of the earth, and the liquid and yielding nature of the heaven through which the planets revolved in their orbits, etc. He inquires, “by what degrees philosophers attained such excess of folly as to conceive the earth to be a globe, and to surround that globe with heaven.” These reasonings are upon a par with those he has adduced on the subject of the sibyls.

Our young scholar would address some such language as this to all these consequential doctors: “You are to learn that there are no such things as solid heavens placed one over another, as you have been told; that there are no real circles in which the stars move on a pretended firmament; that the sun is the centre of our planetary world; and that the earth and the planets move round it in space, in orbits not circular but elliptical. You must learn that there is, in fact, neither above nor below, but that the planets and the comets tend all towards the sun, their common centre, and that the sun tends towards them, according to an eternal law of gravitation.”

Lactantius and his gabbling associates would be perfectly astonished, were the true system of the world thus unfolded to them.

Last updated Sunday, March 27, 2016 at 12:01