Thoughts on Man, His Nature, Productions and Discoveries Part 24

(52) Ibid, p. 268.

(53) Phil. Transactions, Vol. XXIX, p. 457.

But the imperfectness of our instruments and means of observation have no small tendency to baffle the ambition of man in these curious investigations.

"The true quant.i.ty of the moon's parallax," says Bonnycastle, "cannot be accurately determined by the methods ordinarily resorted to, on account of the varying declination of the moon, and the inconstancy of the horizontal refractions, which are perpetually changing according to the state the atmosphere is in at the time. For the moon continues but for a short time in the equinoctial, and the refraction at a mean rate elevates her apparent place near the horizon, half as much as her parallax depresses it(54)."

(54) Astronomy, p. 265.

"It is well known that the parallax of the sun can never exceed nine seconds, or the four-hundredth part of a degree(55)." "Observations,"

says Halley, "made upon the vibrations of a pendulum, to determine these exceedingly small angles, are not sufficiently accurate to be depended upon; for by this method of ascertaining the parallax, it will sometimes come out to be nothing, or even negative; that is, the distance will either be infinite, or greater than infinite, which is absurd. And, to confess the truth, it is hardly possible for a person to distinguish seconds with certainty by any instruments, however skilfully they may be made; and therefore it is not to be wondered at, that the excessive nicety of this matter should have eluded the many ingenious endeavours of the most able opetators."(56).

(55) Ibid, p. 268.

(56) Phil. Transactions, Vol. XXIX, p. 456.

Such are the difficulties that beset the subject on every side. It is for the impartial and dispa.s.sionate observers who have mastered all the subtleties of the science, if such can be found, to determine whether the remedies that have been resorted to to obviate the above inaccuracies and their causes, have fulfilled their end, and are not exposed to similar errors. But it would be vain to expect the persons, who have "scorned delights, and lived laborious days" to possess themselves of the mysteries of astronomy, should be impartial and dispa.s.sionate, or be disposed to confess, even to their own minds, that their researches were useless, and their labours ended in nothing.

It is further worthy of our attention, that the instruments with which we measure the distance of the earth from the sun and the planets, are the very instruments which have been p.r.o.nounced upon as incompetent in measuring the heights of mountains(57). In the latter case therefore we have subst.i.tuted a different mode for arriving at the truth, which is supposed to be attended with greater precision: but we have no subst.i.tute to which we can resort, to correct the mistakes into which we may fall respecting the heavenly bodies.

(57) See above, Essay XXI.

The result of the uncertainty which adheres to all astronomical observations is such as might have been expected. Common readers are only informed of the latest adjustment of the question, and are therefore unavoidably led to believe that the distance of the sun from the earth, ever since astronomy became ent.i.tled to the name of a science, has by universal consent been recognised as ninety-five millions of miles, or, as near as may be, twenty-four thousand semi-diameters of the earth. But how does the case really stand?

Copernicus and Tycho Brahe held the distance to be twelve hundred semi-diameters; Kepler, who is received to have been perhaps the greatest astronomer that any age has produced, puts it down as three thousand five hundred semi-diameters; since his time, Riccioli as seven thousand; Hevelius as five thousand two hundred and fifty(58); some later astronomers, mentioned by Halley, as fourteen thousand; and Halley himself as sixteen thousand five hundred(59).

(58) They were about thirty and forty years younger than Kepler respectively.

(59) Halley, apud Philosophical Transactions, Vol. XXIX, p. 455.

The doctrine of fluxions is likewise called in by the astronomers in their attempts to ascertain the distance and magnitude of the different celestial bodies which compose the solar system; and in this way their conclusions become subject to all the difficulties which Berkeley has alleged against that doctrine.

Kepler has also supplied us with another mode of arriving at the distance and size of the sun and the planets: he has hazarded a conjecture, that the squares of the times of the revolution of the earth and the other planets are in proportion to the cubes of their distances from the sun, their common centre; and, as by observation we can arrive with tolerable certainty at a knowledge of the times of their revolutions, we may from hence proceed to the other matters we are desirous to ascertain. And that which Kepler seemed, as by a divine inspiration, to hazard in the way of conjecture, Newton professes to have demonstratively established. But the demonstration of Newton has not been considered as satisfactory by all men of science since his time.

Thus far however we proceed as we may, respecting our propositions on the subject of the solar system. But, beyond this, all science, real or pretended, deserts us. We have no method for measuring angles, which can be applied to the fixed stars; and we know nothing of any revolutions they perform. All here therefore seems gratuitous: we reason from certain alleged a.n.a.logies; and we can do no more.

Huygens endeavoured to ascertain something on the subject, by making the aperture of a telescope so small, that the sun should appear through it no larger than Sirius, which he found to be only in the proportion of 1 to 27,664 times his diameter, as seen by the naked eye. Hence, supposing Sirius to be a globe of the same magnitude as the sun, it must be 27,664 times as distant from us as the sun, in other words, at a distance so considerable as to equal 345 million diameters of the earth(60). Every one must feel on how slender a thread this conclusion is suspended.

(60) Encyclopaedia Londinensis, Vol. 11, p. 407.

And yet, from this small postulate, the astronomers proceed to deduce the most astounding conclusions. They tell us, that the distance of the nearest fixed star from the earth is at least 7,600,000,000,000 miles, and of another they name, not less than 38 millions of millions of miles. A cannon-ball therefore, proceeding at the rate of about twenty miles in a minute would be 760,000 years in pa.s.sing from us to the nearest fixed star, and 3,800,000 in pa.s.sing to the second star of which we speak. Huygens accordingly concluded, that it was not impossible, that there might be stars at such inconceivable distances from us, that their light has not yet reached the earth since its creation(61).

(61) Ibid, p. 408.

The received system of the universe, founded upon these so called discoveries, is that each of the stars is a sun, having planets and comets revolving round it, as our sun has the earth and other planets revolving round him. It has been found also by the successive observations of astronomers, that a star now and then is totally lost, and that a new star makes its appearance which had never been remarked before: and this they explain into the creation of a new system from time to time by the Almighty author of the universe, and the destruction of an old system worn out with age(62). We must also remember the power of attraction every where diffused through infinite s.p.a.ce, by means of which, as Herschel a.s.sures us, in great length of time a nebula, or cl.u.s.ter of stars, may be formed, while the projectile force they received in the beginning may prevent them from all coming together, at least for millions of ages. Some of these nebulae, he adds, cannot well be supposed to be at a less distance from us than six or eight thousand times the distance of Sirius(63). Kepler however denies that each star, of those which distinctly present themselves to our sight, can have its system of planets as our sun has, and considers them as all fixed in the same surface or sphere; since, if one of them were twice or thrice as remote as another, it would, supposing their real magnitudes to be equal, appear to be twice or thrice as small, whereas there is not in their apparent magnitudes the slightest difference(64).

(62) Encycl. Lond. Vol. II, p. 411.

(63) Ibid, p. 348.

(64) Ibid, p. 411.

Certainly the astronomers are a very fortunate and privileged race of men, who talk to us in this oracular way of "the unseen things of G.o.d from the creation of the world," hanging up their conclusions upon invisible hooks, while the rest of mankind sit listening gravely to their responses, and unreservedly "acknowledging that their science is the most sublime, the most interesting, and the most useful of all the sciences cultivated by man(65)."

(65) Ferguson, Astronomy, Section 1.

We have a sensation, which we call the sensation of distance. It comes to us from our sight and our other senses. It does not come immediately by the organ of sight. It has been proved, that the objects we see, previously to the comparison and correction of the reports of the organ of sight with those of the other senses, do not suggest to us the idea of distance, but that on the contrary whatever we see seems to touch the eye, even as the objects of the sense of feeling touch the skin.

But, in proportion as we compare the impressions made upon our organs of sight with the impressions made on the other senses, we come gradually to connect with the objects we see the idea of distance. I put out my hand, and find at first that an object of my sense of sight is not within the reach of my hand. I put out my hand farther, or by walking advance my body in the direction of the object, and I am enabled to reach it. From smaller experiments I proceed to greater. I walk towards a tree or a building, the figure of which presents itself to my eye, but which I find upon trial to have been far from me. I travel towards a place that I cannot see, but which I am told lies in a certain direction. I arrive at the place. It is thus, that by repeated experiments I acquire the idea of remote distances.

To confine ourselves however to the question of objects, which without change of place I can discover by the sense of sight. I can see a town, a tower, a mountain at a considerable distance. Let us suppose that the limit of my sight, so far as relates to objects on the earth, is one hundred miles. I can travel towards such an object, and thus ascertain by means of my other senses what is its real distance. I can also employ certain instruments, invented by man, to measure heights, suppose of a tower, and, by experiments made in ways independent of these instruments, verify or otherwise the report of these instruments.

The height of the Monument of London is something more than two hundred feet. Other elevations, the produce of human labour, are considerably higher. It is in the nature of the mind, that we conclude from the observation that we have verified, to the accuracy of another, bearing a striking a.n.a.logy to the former, that we have not verified. But a.n.a.logy has its limits. Is it of irresistible certainty, or is it in fact to be considered as approaching to certainty, because we have verified an observation extending to several hundred feet, that an observation extending to ninety-five millions of miles, or to the incredible distances of which Herschel so familiarly talks, is to be treated as a fact, or laid down as a principle in science? Is it reasonable to consider two propositions as a.n.a.logous, when the thing affirmed in the one is in dimension many million times as great as the thing affirmed in the other? The experience we have had as to the truth of the smaller, does it authorise us to consider the larger as unquestionable? That which I see with a bay of the sea or a wide river between, though it may appear very like something with which I am familiar at home, do I immediately affirm it to be of the same species and nature, or do I not regard it with a certain degree of scepticism, especially if, along with the resemblance in some points, it differs essentially, as for example in magnitude, in other points? We have a sensation, and we enquire into its cause. This is always a question of some uncertainty. Is its cause something of absolute and substantive existence without me, or is it not? Is its cause something of the very same nature, as the thing that gave me a similar sensation in a matter of comparatively a pigmy and diminutive extension?

All these questions an untrained and inquisitive mind will ask itself in the propositions of astronomy. We must believe or not, as we think proper or reasonable. We have no way of verifying the propositions by the trial of our senses. There they lie, to be received by us in the construction that first suggests itself to us, or not. They are something like an agreeable imagination or fiction: and a sober observer, in cold blood, will be disposed deliberately to weigh both sides of the question, and to judge whether the probability lies in favour of the actual affirmation of the millions of millions of miles, and the other incredible propositions of the travelling of light, and the rest, which even the most cautious and sceptical of the retainers of modern astronomy, find themselves compelled to receive.

But I shall be told, that the results of our observations of the distances of the heavenly bodies are unvaried. We have measured the distances and other phenomena of the sun, the moon, Mercury, Venus, Mars, Jupiter, Saturn, and their satellites, and they all fall into a grand system, so as to convey to every unprejudiced mind the conviction that this system is the truth itself. If we look at them day after day, and year after year, we see them for ever the same, and performing the same divine harmony. Successive astronomers in different ages and countries have observed the celestial orbs, and swept the heavens, and for ever bring us back the same story of the number, the dimensions, the distances, and the arrangement of the heavenly bodies which form the subject of astronomical science.

This we have seen indeed not to be exactly the case. But, if it were, it would go a very little way towards proving the point it was brought to prove. It would shew that, the sensations and results being similar, the causes of those results must be similar to each other, but it would not shew that the causes were similar to the sensations produced. Thus, in the sensations which belong to taste, smell, sound, colour, and to those of heat and cold, there is all the uniformity which would arise, when the real external causes bore the most exact similitude to the perceptions they generate; and yet it is now universally confessed that tastes, scents, sounds, colours, and heat and cold do not exist out of ourselves. All that we are ent.i.tled therefore to conclude as to the magnitudes and distances of the heavenly bodies, is, that the causes of our sensations and perceptions, whatever they are, are not less uniform than the sensations and perceptions themselves.

It is further alleged, that we calculate eclipses, and register the various phenomena of the heavenly bodies. Thales predicted an eclipse of the sun, which took place nearly six hundred years before the Christian era. The Babylonians, the Persians, the Hindoos, and the Chinese early turned their attention to astronomy. Many of their observations were accurately recorded; and their tables extend to a period of three thousand years before the birth of Christ. Does not all this strongly argue the solidity of the science to which they belong? Who, after this, will have the presumption to question, that the men who profess astronomy proceed on real grounds, and have a profound knowledge of these things, which at first sight might appear to be set at a distance so far removed from our ken?

The answer to this is easy. I believe in all the astronomy that was believed by Thales. I do not question the statements relative to the heavenly bodies that were delivered by the wise men of the East. But the supposed discoveries that were made in the eighteenth, and even in the latter part of the seventeenth century, purporting to ascertain the precise distance of the sun, the planets, and even of the fixed stars, are matters entirely distinct from this.

Among the earliest astronomers of Greece were Thales, Anaximander, Anaximenes and Anaxagoras. Thales, we are told, held that the earth is a sphere or globe, Anaximenes that it is like a round, flat table; Anaximander that the sun is like a chariot-wheel, and is twenty-eight times larger than the earth. Anaxagoras was put in prison for affirming that the sun was by many degrees larger than the whole Peloponnesus(66).

Kepler is of opinion that all the stars are at an equal distance from us, and are fixed in the same surface or sphere.