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

by the sunlight which

falls upon it. Notwithstanding that a star is a sun thousands of

times larger than the planet and millions of times more remote,

yet it is a singular fact that telescopic planets possess an

illusory resemblance to the stars among which their course happens

to lie. So far as actual appearance goes, there is indeed only

one criterion by which a planet of this kind can be discriminated

from a star. If the planet be large enough the telescope will

show that it possesses a disc, and has a visible and measurable

circular outline. This feature a star does not exhibit. The

stars are indeed so remote that no matter how large they may be

intrinsically, they only exhibit radiant points of light, which

the utmost powers of the telescope fail to magnify into objects

with an appreciable diameter. The older and well-known planets,

such as Jupiter and Mars, possess discs, which, though not visible

to the unaided eye, were clearly enough discernible with the

slightest telescopic power. But a very remote planet like Uranus,

though it possessed a disc large enough to be quickly appreciated

by the consummate observing skill of Herschel, was nevertheless so

stellar in its appearance, that it had been observed no fewer than

seventeen times by experienced astronomers prior to Herschel. In

each case the planetary nature of the object had been overlooked,

and it had been taken for granted that it was a star. It

presented no difference which was sufficient to arrest attention.

 

As the unknown body by which Uranus was disturbed was certainly

much more remote than Uranus, it seemed to be certain that though

it might show a disc perceptible to very close inspection, yet

that the disc must be so minute as not to be detected except with

extreme care. In other words, it seemed probable that the body

which was to be sought for could not readily be discriminated from

a small star, to which class of object it bore a superficial

resemblance, though, as a matter of fact, there was the

profoundest difference between the two bodies.

 

There are on the heavens many hundreds of thousands of stars, and

the problem of identifying the planet, if indeed it should lie

among these stars, seemed a very complex matter. Of course it is

the abundant presence of the stars which causes the difficulty.

If the stars could have been got rid of, a sweep over the heavens

would at once disclose all the planets which are bright enough to

be visible with the telescopic power employed. It is the

fortuitous resemblance of the planet to the stars which enables it

to escape detection. To discriminate the planet among stars

everywhere in the sky would be almost impossible. If, however,

some method could be devised for localizing that precise region in

which the planet’s existence might be presumed, then the search

could be undertaken with some prospect of success.

 

To a certain extent the problem of localizing the region on the

sky in which the planet might be expected admitted of an immediate

limitation. It is known that all the planets, or perhaps I ought

rather to say, all the great planets, confine their movements to a

certain zone around the heavens. This zone extends some way on

either side of that line called the ecliptic in which the earth

pursues its journey around the sun. It was therefore to be

inferred that the new planet need not be sought for outside this

zone. It is obvious that this consideration at once reduces the

area to be scrutinized to a small fraction of the entire heavens.

But even within the zone thus defined there are many thousands of

stars. It would seem a hopeless task to detect the new planet

unless some further limitation to its position could be assigned.

 

It was accordingly suggested to Le Verrier that he should

endeavour to discover in what particular part of the strip of the

celestial sphere which we have indicated the search for the

unknown planet should be instituted. The materials available to

the mathematician for the solution of this problem were to be

derived solely from the discrepancies between the calculated

places in which Uranus should be found, taking into account the

known causes of disturbance, and the actual places in which

observation had shown the planet to exist. Here was indeed an

unprecedented problem, and one of extraordinary difficulty. Le

Verrier, however, faced it, and, to the astonishment of the world,

succeeded in carrying it through to a brilliant solution.

We cannot here attempt to enter into any account of the

mathematical investigations that were necessary. All that we can

do is to give a general indication of the method which had to be

adopted.

 

Let us suppose that a planet is revolving outside Uranus, at a

distance which is suggested by the several distances at which the

other planets are dispersed around the sun. Let us assume that

this outer planet has started on its course, in a prescribed path,

and that it has a certain mass. It will, of course, disturb the

motion of Uranus, and in consequence of that disturbance Uranus

will follow a path the nature of which can be determined by

calculation. It will, however, generally be found that the path

so ascertained does not tally with the actual path which

observations have indicated for Uranus. This demonstrates that

the assumed circumstances of the unknown planet must be in some

respects erroneous, and the astronomer commences afresh with an

amended orbit. At last after many trials, Le Verrier ascertained

that, by assuming a certain size, shape, and position for the

unknown Planet’s orbit, and a certain value for the mass of

the hypothetical body, it would be possible to account for the

observed disturbances of Uranus. Gradually it became clear to

the perception of this consummate mathematician, not only that

the difficulties in the movements of Uranus could be thus

explained, but that no other explanation need be sought for. It

accordingly appeared that a planet possessing the mass which

he had assigned, and moving in the orbit which his calculations

had indicated, must indeed exist, though no eye had ever beheld

any such body. Here was, indeed, an astonishing result. The

mathematician sitting at his desk, by studying the observations

which had been supplied to him of one planet, is able to

discover the existence of another planet, and even to assign the

very position which it must occupy, ere ever the telescope is

invoked for its discovery.

 

Thus it was that the calculations of Le Verrier narrowed greatly

the area to be scrutinised in the telescopic search

which was presently to be instituted. It was already known, as we

have just pointed out, that the planet must lie somewhere on

the ecliptic. The French mathematician had now further

indicated the spot on the ecliptic at which, according to his

calculations, the planet must actually be found. And now for an

episode in this history which will be celebrated so long as

science shall endure. It is nothing less than the telescopic

confirmation of the existence of this new planet, which had

previously been indicated only by mathematical calculation. Le

Verrier had not himself the instruments necessary for studying the

heavens, nor did he possess the skill of the practical astronomer.

He, therefore, wrote to Dr. Galle, of the Observatory at Berlin,

requesting him to undertake a telescopic search for the new planet

in the vicinity which the mathematical calculation had indicated

for the whereabouts of the planet at that particular time. Le

Verrier added that he thought the planet ought to admit of being

recognised by the possession of a disc sufficiently definite to

mark the distinction between it and the surrounding stars.

 

It was the 23rd September, 1846, when the request from Le Verrier

reached the Berlin Observatory, and the night was clear, so that

the memorable search was made on the same evening. The

investigation was facilitated by the circumstance that a diligent

observer had recently compiled elaborate star maps for certain

tracts of the heavens lying in a sufficiently wide zone on both

sides of the equator. These maps were as yet only partially

complete, but it happened that Hora. XXI., which included the very

spot which Le Verrier’s results referred to, had been just issued.

Dr. Galle had thus before his, eyes a chart of all the stars which

were visible in that part of the heavens at the time when the map

was made. The advantage of such an assistance to the search could

hardly be overestimated. It at once gave the astronomer another

method of recognising the planet besides that afforded by its

possible possession of a disc. For as the planet was a moving

body, it would not have been in the same place relatively to the

stars at the time when the map was constructed, as it occupied

some years later when the search was being made. If the body

should be situated in the spot which Le Verrier’s calculations

indicated in the autumn of 1846, then it might be regarded

as certain that it would not be found in that same place on a

map drawn some years previously.

 

The search to be undertaken consisted in a comparison made point

by point between the bodies shown on the map, and those stars in

the sky which Dr. Galle’s telescope revealed. In the course of

this comparison it presently appeared that a star-like object of

the eighth magnitude, which was quite a conspicuous body in the

telescope, was not represented in the map. This at once attracted

the earnest attention of the astronomer, and raised his hopes that

here was indeed the planet. Nor were these hopes destined to be

disappointed. It could not be supposed that a star of the eighth

magnitude would have been overlooked in the preparation of a chart

whereon stars of many lower degrees of brightness were set down.

One other supposition was of course conceivable. It might have

been that this suspicious object belonged to the class of

variables, for there are many such stars whose brightness

fluctuates, and if it had happened that the map was constructed at

a time when the star in question had but feeble brilliance, it

might have escaped notice. It is also well known that sometimes

new stars suddenly develop, so that the possibility that what

Dr. Galle saw should have been a variable star or should have been

a totally new star had to be provided against.

 

Fortunately a test was immediately available to decide whether

the new object was indeed the long sought for planet, or whether

it was a star of one of the two classes to which I have just

referred. A star remains fixed, but a planet is in motion.

No doubt when a planet lies at the distance at which this new

planet was believed to be situated, its apparent motion would be

so slow that it would not be easy to detect any change in the

course of a single night’s observation. Dr. Galle, however,

addressed himself with much skill to the examination of the place

of the new body. Even in the course of the night he thought he

detected slight movements, and he awaited with much anxiety the

renewal of his observations on the subsequent evenings. His

suspicions as to the movement of the body were then amply

confirmed, and the planetary nature of the new object was thus

unmistakably detected.

 

Great indeed was the admiration of the scientific world

at this superb triumph. Here was a mighty planet whose very

existence was revealed by the indications afforded by refined

mathematical calculation. At once the name of Le Verrier, already

known to those conversant with the more profound branches of

astronomy, became everywhere celebrated. It soon, however,

appeared, that the