Great Astronomers - Robert Stawell Ball (the little red hen ebook .TXT) 📗

the sun, the moon, and the planets. These observations,

obtained with the greatest care, and purified as far as possible

from the errors by which they may be affected form, as it were,

the raw material on which the mathematician exercises his skill.

It is for him to elicit from the observed places the true laws

which govern the movements of the heavenly bodies. Here is indeed

a task in which the highest powers of the human

intellect may be worthily employed.

 

Among those who have laboured with the greatest success in the

interpretation of the observations made with instruments of

precision, Le Verrier holds a highly honoured place. To him it

has been given to provide a superb illustration of the success

with which the mind of man can penetrate the deep things of

Nature.

 

The illustrious Frenchman, Urban Jean Joseph Le Verrier, was

born on the 11th March, 1811, at St. Lo, in the department of

Manche. He received his education in that famous school for

education in the higher branches of science, the Ecole

Polytechnique, and acquired there considerable fame as a

mathematician. On leaving the school Le Verrier at first purposed

to devote himself to the public service, in the department of

civil engineering; and it is worthy of note that his earliest

scientific work was not in those mathematical researches in which

he was ultimately to become so famous. His duties in the

engineering department involved practical chemical research in the

laboratory. In this he seems to have become very expert, and

probably fame as a chemist would have been thus attained, had not

destiny led him into another direction. As it was, he did engage

in some original chemical research. His first contributions to

science were the fruits of his laboratory work; one of his papers

was on the combination of phosphorus and hydrogen, and another on

the combination of phosphorus and oxygen.

 

His mathematical labours at the Ecole Polytechnique had, however,

revealed to Le Verrier that he was endowed with the powers

requisite for dealing with the subtlest instruments

of mathematical analysis. When he was twenty-eight years old,

his first great astronomical investigation was brought forth.

It will be necessary to enter into some explanation as to the

nature of this, inasmuch as it was the commencement of the life-work which he was to pursue.

 

If but a single planet revolved around the sun, then the orbit

of that planet would be an ellipse, and the shape and size, as

well as the position of the ellipse, would never alter. One

revolution after another would be traced out, exactly in the same

manner, in compliance with the force continuously exerted by the

sun. Suppose, however, that a second planet be introduced into

the system. The sun will exert its attraction on this second

planet also, and it will likewise describe an orbit round the

central globe. We can, however, no longer assert that the orbit

in which either of the planets moves remains exactly an ellipse.

We may, indeed, assume that the mass of the sun is enormously

greater than that of either of the planets. In this case the

attraction of the sun is a force of such preponderating magnitude,

that the actual path of each planet remains nearly the same as if

the other planet were absent. But it is impossible for the orbit

of each planet not to be affected in some degree by the attraction

of the other planet. The general law of nature asserts that every

body in space attracts every other body. So long as there is only

a single planet, it is the single attraction between the sun and

that planet which is the sole controlling principle of the

movement, and in consequence of it the ellipse is described. But

when a second planet is introduced, each of the two bodies is not

only subject to the attraction of the sun, but each one of the

planets attracts the other. It is true that this mutual

attraction is but small, but, nevertheless, it produces some

effect. It “disturbs,” as the astronomer says, the elliptic

orbit which would otherwise have been pursued. Hence it follows

that in the actual planetary system where there are several

planets disturbing each other, it is not true to say that the

orbits are absolutely elliptic.

 

At the same time in any single revolution a planet may for most

practical purposes be said to be actually moving in an ellipse.

As, however, time goes on, the ellipse gradually varies. It

alters its shape, it alters its plane, and it alters its position

in that plane. If, therefore, we want to study the movements of

the planets, when great intervals of time are concerned, it is

necessary to have the means of learning the nature of the movement

of the orbit in consequence of the disturbances it has

experienced.

 

We may illustrate the matter by supposing the planet to be

running like a railway engine on a track which has been laid in

a long elliptic path. We may suppose that while the planet is

coursing along, the shape of the track is gradually altering.

But this alteration may be so slow, that it does not

appreciably affect the movement of the engine in a single

revolution. We can also suppose that the plane in which the

rails have been laid has a slow oscillation in level, and that

the whole orbit is with more or less uniformity moved slowly

about in the plane.

 

In short periods of time the changes in the shapes and positions

of the planetary orbits, in consequence of their mutual

attractions, are of no great consequence. When, however, we bring

thousands of years into consideration, then the displacements of

the planetary orbits attain considerable dimensions, and have, in

fact, produced a profound effect on the system.

 

It is of the utmost interest to investigate the extent to

which one planet can affect another in virtue of their mutual

attractions. Such investigations demand the exercise of the

highest mathematical gifts. But not alone is intellectual

ability necessary for success in such inquiries. It must be

united with a patient capacity for calculations of an arduous

type, protracted, as they frequently have to be, through many

years of labour. Le Verrier soon found in these profound

inquiries adequate scope for the exercise of his peculiar

gifts. His first important astronomical publication contained

an investigation of the changes which the orbits of several of

the planets, including the earth, have undergone in times past,

and which they will undergo in times to come.

 

As an illustration of these researches, we may take the case of

the planet in which we are, of course, especially

interested, namely, the earth, and we can investigate the

changes which, in the lapse of time, the earth’s orbit has

undergone, in consequence of the disturbance to which it has

been subjected by the other planets. In a century, or even in

a thousand years, there is but little recognisable difference

in the shape of the track pursued by the earth. Vast periods

of time are required for the development of the large

consequences of planetary perturbation. Le Verrier has,

however, given us the particulars of what the earth’s journey

through space has been at intervals of 20,000 years

back from the present date. His furthest calculation throws

our glance back to the state of the earth’s track 100,000

years ago, while, with a bound forward, he shows us what the

earth’s orbit is to be in the future, at successive intervals

of 20,000 years, till a date is reached which is 100,000

years in advance Of A.D. 1800.

 

The talent which these researches displayed brought Le Verrier

into notice. At that time the Paris Observatory was presided

over by Arago, a SAVANT who occupies a distinguished position in

French scientific annals. Arago at once perceived that

Le Verrier was just the man who possessed the qualifications

suitable for undertaking a problem of great importance and

difficulty that had begun to force itself on the attention of

astronomers. What this great problem was, and how astonishing

was the solution it received, must now be considered.

 

Ever since Herschel brought himself into fame by his superb

discovery of the great planet Uranus, the movements of this new

addition to the solar system were scrutinized with care and

attention. The position of Uranus was thus accurately

determined from time to time. At length, when sufficient

observations of this remote planet had been brought together,

the route which the newly-discovered body pursued through the

heavens was ascertained by those calculations with which

astronomers are familiar. It happens, however, that Uranus

possesses a superficial resemblance to a star. Indeed the

resemblance is so often deceptive that long ere its detection as

a planet by Herschel, it had been observed time after time by

skilful astronomers, who little thought that the star-like point

at which they looked was anything but a star. From these early

observations it was possible to determine the track of Uranus, and

it was found that the great planet takes a period of no less than

eighty-four years to accomplish a circuit. Calculations were made

of the shape of the orbit in which it revolved before its

discovery by Herschel, and these were compared with the orbit

which observations showed the same body to pursue in those later

years when its planetary character was known. It could not, of

course, be expected that the orbit should remain unaltered; the

fact that the great planets Jupiter and Saturn revolve in the

vicinity of Uranus must necessarily imply that the orbit of the

latter undergoes considerable changes. When, however, due

allowance has been made for whatever influence the attraction of

Jupiter and Saturn, and we may add of the earth and all the other

Planets, could possibly produce, the movements of Uranus were

still inexplicable. It was perfectly obvious that there must be

some other influence at work besides that which could be

attributed to the planets already known.

 

Astronomers could only recognise one solution of such a

difficulty. It was impossible to doubt that there must be some

other planet in addition to the bodies at that time known, and

that the perturbations of Uranus hitherto unaccounted for, were

due to the disturbances caused by the action of this unknown

planet. Arago urged Le Verrier to undertake the great problem of

searching for this body, whose theoretical existence seemed

demonstrated. But the conditions of the search were such that it

must needs be conducted on principles wholly different from any

search which had ever before been undertaken for a celestial

object. For this was not a case in which mere survey with a

telescope might be expected to lead to the discovery.

 

Certain facts might be immediately presumed with reference to

the unknown object. There could be no doubt that the unknown

disturber of Uranus must be a large body with a mass far exceeding

that of the earth. It was certain, however, that it must be so

distant that it could only appear from our point of view as a very

small object. Uranus itself lay beyond the range, or almost

beyond the range, of unassisted vision. It could be shown that

the planet by which the disturbance was produced revolved in an

orbit which must lie outside that of Uranus. It seemed thus

certain that the planet could not be a body visible to the unaided

eye. Indeed, had it been at all conspicuous its planetary

character would doubtless have been detected ages ago. The

unknown body must therefore be a planet which would have to be

sought for by telescopic aid.

 

There is, of course, a profound physical difference between a

planet and a star, for the star is a luminous sun, and the planet

is merely a dark body, rendered visible