Great Astronomers by Robert Stawell Ball (best ereader for students .txt) 📖

no doubt it must

ultimately have proved successful, was necessarily a very tedious

one, but to Professor Challis, unfortunately, no other method was

available. Thus it happened that, though Challis commenced his

search at Cambridge two months earlier than Galle at Berlin, yet,

as we have already explained, the possession of accurate star-maps

by Dr. Galle enabled him to discover the planet on the very first

night that he looked for it.

 

The rival claims of Adams and Le Verrier to the discovery of

Neptune, or rather, we should say, the claims put forward by their

respective champions, for neither of the illustrious investigators

themselves condescended to enter into the personal aspect of the

question, need not be further discussed here. The main points of

the controversy have been long since settled, and we cannot do

better than quote the words of Sir John Herschel when he

addressed the Royal Astronomical Society in 1848:—

 

“As genius and destiny have joined the names of Le Verrier and

Adams, I shall by no means put them asunder; nor will they ever be

pronounced apart so long as language shall celebrate the triumphs

Of science in her sublimest walks. On the great discovery of

Neptune, which may be said to have surpassed, by intelligible and

legitimate means, the wildest pretensions of clairvoyance, it

Would now be quite superfluous for me to dilate. That glorious

event and the steps which led to it, and the various lights

in which it has been placed, are already familiar to every one

having the least tincture of science. I will only add that as

there is not, nor henceforth ever can be, the slightest rivalry on

the subject between these two illustrious men—as they have met as

brothers, and as such will, I trust, ever regard each other—we

have made, we could make, no distinction between then, on this

occasion. May they both long adorn and augment our science, and

add to their own fame already so high and pure, by fresh

achievements.”

 

Adams was elected a Fellow of St. John’s College, Cambridge, in

1843; but as he did not take holy orders, his Fellowship, in

accordance with the rules then existing came to an end in 1852.

In the following year he was, however, elected to a Fellowship at

Pembroke College, which he retained until the end of his life. In

1858 he was appointed Professor of Mathematics in the University

of St. Andrews, but his residence in the north was only a brief

one, for in the same year he was recalled to Cambridge as Lowndean

Professor of Astronomy and Geometry, in succession to Peacock. In

1861 Challis retired from the Directorship of the Cambridge

Observatory, and Adams was appointed to succeed him.

 

The discovery of Neptune was a brilliant inauguration of the

astronomical career of Adams. He worked at, and wrote upon, the

theory of the motions of Biela’s comet; he made important

corrections to the theory of Saturn; he investigated the mass of

Uranus, a subject in which he was naturally interested from its

importance in the theory of Neptune; he also improved the methods

of computing the orbits of double stars. But all these must be

regarded as his minor labours, for next to the discovery of

Neptune the fame of Adams mainly rests on his researches upon

certain movements of the moon, and upon the November meteors.

 

The periodic time of the moon is the interval required for one

circuit of its orbit. This interval is known with accuracy at the

present day, and by means of the ancient eclipses the period of

the moon’s revolution two thousand years ago can be also

ascertained. It had been discovered by Halley that the period

which the moon requires to accomplish each of its revolutions

around the earth has been steadily, though no doubt slowly,

diminishing. The change thus produced is not appreciable when

only small intervals of time are considered, but it becomes

appreciable when we have to deal with intervals of thousands of

years. The actual effect which is produced by the lunar

acceleration, for so this phenomenon is called, may be thus

estimated. If we suppose that the moon had, throughout the ages,

revolved around the earth in precisely the same periodic time

which it has at present, and if from this assumption we calculate

back to find where the moon must have been about two thousand

years ago, we obtain a position which the ancient eclipses show to

be different from that in which the moon was actually situated.

The interval between the position in which the moon would have

been found two thousand years ago if there had been no

acceleration, and the position in which the moon was actually

placed, amounts to about a degree, that is to say, to an arc

on the heavens which is twice the moon’s apparent diameter.

 

If no other bodies save the earth and the moon were present in

the universe, it seems certain that the motion of the moon

would never have exhibited this acceleration. In such a simple

case as that which I have supposed the orbit of the moon would

have remained for ever absolutely unchanged. It is, however,

well known that the presence of the sun exerts a disturbing

influence upon the movements of the moon. In each revolution our

satellite is continually drawn aside by the action of the sun from

the place which it would otherwise have occupied. These

irregularities are known as the perturbations of the lunar

orbit, they have long been studied, and the majority of them

have been satisfactorily accounted for. It seems, however, to

those who first investigated the question that the phenomenon of

the lunar acceleration could not be explained as a consequence of

solar perturbation, and, as no other agent competent to produce

such effects was recognised by astronomers, the lunar acceleration

presented an unsolved enigma.

 

At the end of the last century the illustrious French

mathematician Laplace undertook a new investigation of the famous

problem, and was rewarded with a success which for a long time

appeared to be quite complete. Let us suppose that the moon lies

directly between the earth and the sun, then both earth and moon

are pulled towards the sun by the solar attraction; as, however,

the moon is the nearer of the two bodies to the attracting centre

it is pulled the more energetically, and consequently there is an

increase in the distance between the earth and the moon.

Similarly when the moon happens to lie on the other side of the

earth, so that the earth is interposed directly between the moon

and the sun, the solar attraction exerted upon the earth is more

powerful than the same influence upon the moon. Consequently in

this case, also, the distance of the moon from the earth is

increased by the solar disturbance. These instances will

illustrate the general truth, that, as one of the consequences of

the disturbing influence exerted by the sun upon the earth-moon

system, there is an increase in the dimensions of the average

orbit which the moon describes around the earth. As the time

required by the moon to accomplish a journey round the earth

depends upon its distance from the earth, it follows that among

the influences of the sun upon the moon there must be an

enlargement of the periodic time, from what it would have been

had there been no solar disturbing action.

 

This was known long before the time of Laplace, but it did not

directly convey any explanation of the lunar acceleration. It no

doubt amounted to the assertion that the moon’s periodic time was

slightly augmented by the disturbance, but it did not give any

grounds for suspecting that there was a continuous change in

progress. It was, however, apparent that the periodic time was

connected with the solar disturbance, so that, if there were any

alteration in the amount of the sun’s disturbing effect, there

must be a corresponding alteration in the moon’s periodic time.

Laplace, therefore, perceived that, if he could discover any

continuous change in the ability of the sun for disturbing the

moon, he would then have accounted for a continuous change in

the moon’s periodic time, and that thus an explanation of the

long-vexed question of the lunar acceleration might be

forthcoming.

 

The capability of the sun for disturbing the earth-moon system

is obviously connected with the distance of the earth from the

sun. If the earth moved in an orbit which underwent no change

whatever, then the efficiency of the sun as a disturbing agent

would not undergo any change of the kind which was sought for.

But if there were any alteration in the shape or size of the

earth’s orbit, then that might involve such changes in the

distance between the earth and the sun as would possibly afford

the desired agent for producing the observed lunar effect. It

is known that the earth revolves in an orbit which, though

nearly circular, is strictly an ellipse. If the earth were the

only planet revolving around the sun then that ellipse would

remain unaltered from age to age. The earth is, however, only one

of a large number of planets which circulate around the great

luminary, and are guided and controlled by his supreme attracting

power. These planets mutually attract each other, and in

consequence of their mutual attractions the orbits of the planets

are disturbed from the simple elliptic form which they would

otherwise possess. The movement of the earth, for instance, is

not, strictly speaking, performed in an elliptical orbit. We may,

however, regard it as revolving in an ellipse provided we admit

that the ellipse is itself in slow motion.

 

It is a remarkable characteristic of the disturbing effects of

the planets that the ellipse in which the earth is at any moment

moving always retains the same length; that is to say, its longest

diameter is invariable. In all other respects the ellipse is

continually changing. It alters its position, it changes its

plane, and, most important of all, it changes its eccentricity.

Thus, from age to age the shape of the track which the earth

describes may at one time be growing more nearly a circle, or at

another time may be departing more widely from a circle. These

alterations are very small in amount, and they take place with

extreme slowness, but they are in incessant progress, and their

amount admits of being accurately calculated. At the present

time, and for thousands of years past, as well as for thousands of

years to come, the eccentricity of the earth’s orbit is

diminishing, and consequently the orbit described by the earth

each year is becoming more nearly circular. We must, however,

remember that under all circumstances the length of the longest

axis of the ellipse is unaltered, and consequently the size of the

track which the earth describes around the sun is gradually

increasing. In other words, it may be said that during the

present ages the average distance between the earth and the sun is

waxing greater in consequence of the perturbations which the earth

experiences from the attraction of the other planets. We have,

however, already seen that the efficiency of the solar attraction

for disturbing the moon’s movement depends on the distance between

the earth and the sun. As therefore the average distance between

the earth and the sun is increasing, at all events during the

thousands of years over which our observations extend, it follows

that the ability of the sun for disturbing the moon must be

gradually diminishing.

 

[PLATE: CAMBRIDGE OBSERVATORY.]

 

It has been pointed out that, in consequence of the solar

disturbance, the orbit of the moon must be some what enlarged.

As it now appears that the solar disturbance is on the whole

declining, it follows that the orbit of the