Part 26 (1/2)

By reason of the precession of the equinoxes, it is not always the saroups of stars, the same constellations, which are perceived in the heavens at the saes the constellations of winter will become those of summer and reciprocally

By reason of the precession of the equinoxes, the pole does not always occupy the saht star which is very justly named in the present day, the pole star, was far removed from the pole in the tiain appear renation of pole star has been, and will be, applied to stars very distant fro to explain natural pheno path, each precise observation throws him into new complications Seven spheres of crystal did not suffice for representing the phenomena as soon as the illustrious astronohth sphere was then wanted to account for a movement in which all the stars participated at the sa deprived the earth of its alleged iave a very simple explanation of the most minute circumstances of precession

He supposed that the axis of rotation does not remain exactly parallel to itself; that in the course of each complete revolution of the earth around the sun, the axis deviates from its position by a s the circumpolar stars to advance in a certain way towards the pole, he makes the pole advance towards the stars This hypothesis divested the reatest co had introduced into it A new Alphonse would have then wanted a pretext to address to his astronomical synod the profound remark, so erroneously interpreted, which history ascribes to the king of Castile

If the conception of Copernicus improved by Kepler had, as we have just seen, introduced a striking improvement into the mechanism of the heavens, it still re the position of the terrestrial axis during each successive year, would cause it to describe an entire circle of nearly 50 in diameter, in a period of about 26,000 years

Newton conjectured that this force arose from the action of the sun and moon upon the redundant ions of the earth: thus he made the precession of the equinoxes depend upon the spheroidal figure of the earth; he declared that upon a round planet no precession would exist

All this was quite true, but Newton did not succeed in establishi+ng it by a reat man had introduced into philosophy the severe and just rule: Consider as certain only what has been demonstrated The demonstration of the Newtonian conception of the precession of the equinoxes was, then, a great discovery, and it is to D'Aleeoeneral movement, in virtue of which the terrestrial axis returns to the same stars in a period of about 26,000 years He also connected with the theory of gravitation the perturbation of precession discovered by Bradley, that remarkable oscillation which the earth's axis experiences continually during itsto about eighteen years, is exactly equal to the time which the intersection of thethe 360 of the entire circumference

Geometers and astronoure and physical constitution which the earth ure and constitution

As soon as our countryman Richer discovered that a body, whatever be its nature, weighs less when it is transported nearer the equatorial regions, everybody perceived that the earth, if it was originally fluid, ought to bulge out at the equator Huyghens and Newton did reatest and least axes, the excess of the equatorial diameter over the line of the poles[28]

The calculation of Huyghens was founded upon hypothetic properties of the attractive force which holly inadht to have demonstrated; the theory of the latter was characterized by a defect of a still more serious nature: it supposed the density of the earth during the original state of fluidity, to be horeat problems we have recourse to such simplifications; when, in order to elude difficulties of calculation, we depart so widely from natural and physical conditions, the results relate to an ideal world, they are in reality nothing ination

In order to apply matheure of the earth it was necessary to abandon all idea of hoeneity, all constrained resemblance between the forms of the superposed and unequally dense strata; it was necessary also to exaenerality increased tenfold the difficulties of the problem; neither Clairaut nor D'Alembert was, however, arrested by theeometers, thanks to some essential developments due to their iendre, the theoretical deterure of the earth has attained all desirable perfection There now reigns the most satisfactory accordance between the results of calculation and those of direct inally fluid: analysis has enabled us to ascend to the earliest ages of our planet[30]

In the time of Alexander comets were supposed by the enerated in our ativing themselves much concern about the nature of those bodies, supposed theiomontanus and Tycho Brahe proved by their observations that they are situate beyond the moon; Hevelius, Dorfel, &c, made them revolve around the sun; Newton established that they move under the immediate influence of the attractive force of that body, that they do not describe right lines, that, in fact, they obey the laws of Kepler It was necessary, then, to prove that the orbits of comets are curves which return into themselves, or that the same comet has been seen on several distinct occasions This discovery was reserved for Halley By a ation of the circumstances connected with the apparitions of all the comets to be met with in the records of history, in ancient chronicles, and in astronomical annals, this eminent philosopher was enabled to prove that the comets of 1682, of 1607, and of 1531, were in reality so many successive apparitions of one and the same body

This identity involved a conclusion before which more than one astronomer shrunk It was necessary to admit that the tireat variation, a to as reat discordances due to the disturbing action of the planets?

The answer to this question would introduce coory of ordinary planets or would exclude them for ever The calculation was difficult: Clairaut discovered theit While success was still uncertain, the illustrious georeatest boldness, for in the course of the year 1758 he undertook to deter year when the conated the constellations, nay the stars, which it would encounter in its progress

This was not one of those reers and others formerly combined very skilfully with the tables oftheir lifetime: the event was close at hand The question at issue was nothing less than the creation of a new era in co of a reproach upon science, the consequences of which it would long continue to feel

Clairaut found by a long process of calculation, conducted with great skill, that the action of Jupiter and Saturn ought to have retarded the movement of the comet; that the ti, would be increased 518 days by the disturbing action of Jupiter, and 100 days by the action of Saturn, forht months

Never did a question of astronoitimate curiosity All classes of society awaited with equal interest the announced apparition A Saxon peasant, Palitzch, first perceived the comet Henceforward, from one extremity of Europe to the other, a thousand telescopes traced each night the path of the body through the constellations The route was always, within the limits of precision of the calculations, that which Clairaut had indicated beforehand The prediction of the illustrious geoard both to tireat and important triuraceful and inveterate prejudice As soon as it was established that the returns of coht be calculated beforehand, those bodies lost for ever their ancient prestige The most timid minds troubled themselves quite as little about them as about eclipses of the sun and moon, which are equally subject to calculation In fine, the labours of Clairaut had produced a deeper ienious, and acute reasoning of Bayle

The heavens offer to reflecting e than the equality which subsists between the movements of rotation and revolution of our satellite By reason of this perfect equality the moon always presents the same side to the earth The hemisphere which we see in the present day is precisely that which our ancestors saw in the enerations will perceive

The doctrine of final causes which certain philosophers have so abundantly reat number of natural phenomena was in this particular case totally inapplicable In fact, how could it be pretended thatincessantly the salimpse of the opposite hemisphere? On the other hand, the existence of a perfect,no necessary connection--such as the iven celestial body--was not less repugnant to all ideas of probability There were besides two other numerical coincidences quite as extraordinary; an identity of direction, relative to the stars, of the equator and orbit of the moon; exactly the saroup of singular phenomena, discovered by JD Cassini, constituted the mathematical code of what is called the _Libration of the Moon_

The libration of the moon fore ure of our satellite which was not observable from the earth, and thereby connected it coravitation

At the time when the moon was converted into a solid body, the action of the earth coure than if no attracting body had been situate in its vicinity The action of our globe rendered elliptical an equator which otherould have been circular This disturbing action did not prevent the lunar equator fro out in every direction, but the prominence of the equatorial diareater than that of the diameter which we see perpendicularly

The moon would appear then, to an observer situate in space and exaated towards the earth, to be a sort of pendulum without a point of suspension When a pendulus it back; when the principal axis of the moon recedes from its usual direction, the earth in like manner compels it to return

We have here, then, a coular pheno recourse to the existence of an almost miraculous equality between two movements of translation and rotation, entirely independent of each other Mankind will never see but one face of the moon Observation had informed us of this fact; noe know further that this is due to a physical cause which may be calculated, and which is visible only to the ation which the diameter of the moon experienced when it passed from the liquid to the solid state under the attractive influence of the earth