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

of the heavenly bodies must necessarily be looked to for

the purpose of deciphering the celestial decrees regarding the

fate of man which the heavenly luminaries were designed to

announce.

 

Kepler threw himself with characteristic ardour into even this

fantastic phase of the labours of the astronomical professor; he

diligently studied the rules of astrology, which the fancies of

antiquity had compiled. Believing sincerely as he did in the

connection between the aspect of the stars and the state of human

affairs, he even thought that he perceived, in the events of

his own life, a corroboration of the doctrine which affirmed the

influence of the planets upon the fate of individuals.

 

[PLATE: KEPLER’S SYSTEM OF REGULAR SOLIDS.]

 

But quite independently of astrology there seem to have been many

other delusions current among the philosophers of Kepler’s time.

It is now almost incomprehensible how the ablest men of a few

centuries ago should have entertained such preposterous notions,

as they did, with respect to the system of the universe. As an

instance of what is here referred to, we may cite the

extraordinary notion which, under the designation of a discovery,

first brought Kepler into fame. Geometers had long known that

there were five, but no more than five, regular solid figures.

There is, for instance, the cube with six sides, which is, of

course, the most familiar of these solids. Besides the cube there

are other figures of four, eight, twelve, and twenty sides

respectively. It also happened that there were five planets, but

no more than five, known to the ancients, namely, Mercury, Venus,

Mars, Jupiter, and Saturn. To Kepler’s lively imaginations this

coincidence suggested the idea that the five regular solids

corresponded to the five planets, and a number of fancied

numerical relations were adduced on the subject. The absurdity of

this doctrine is obvious enough, especially when we observe that,

as is now well known, there are two large planets, and a host of

small planets, over and above the magical number of the regular

solids. In Kepler’s time, however, this doctrine was so far from

being regarded as absurd, that its announcement was hailed as a

great intellectual triumph. Kepler was at once regarded with

favour. It seems, indeed, to have been the circumstance which

brought him into correspondence with Tycho Brahe. By its means

also he became known to Galileo.

 

The career of a scientific professor in those early days appears

generally to have been marked by rather more striking vicissitudes

than usually befall a professor in a modern university. Kepler

was a Protestant, and as such he had been appointed to his

professorship at Gratz. A change, however, having taken place in

the religious belief entertained by the ruling powers of the

University, the Protestant professors were expelled. It seems

that special influence having been exerted in Kepler’s case on

account of his exceptional eminence, he was recalled to Gratz

and reinstated in the tenure of his chair. But his pupils had

vanished, so that the great astronomer was glad to accept a post

offered him by Tycho Brahe in the observatory which the latter had

recently established near Prague.

 

On Tycho’s death, which occurred soon after, an opening

presented itself which gave Kepler the opportunity his genius

demanded. He was appointed to succeed Tycho in the position of

imperial mathematician. But a far more important point, both for

Kepler and for science, was that to him was confided the use of

Tycho’s observations. It was, indeed, by the discussion of

Tycho’s results that Kepler was enabled to make the discoveries

which form such an important part of astronomical history.

 

Kepler must also be remembered as one of the first great

astronomers who ever had the privilege of viewing celestial

bodies through a telescope. It was in 1610 that he first held in

his hands one of those little instruments which had been so

recently applied to the heavens by Galileo. It should, however,

be borne in mind that the epoch-making achievements of Kepler did

not arise from any telescopic observations that he made, or,

indeed, that any one else made. They were all elaborately deduced

from Tycho’s measurements of the positions of the planets,

obtained with his great instruments, which were unprovided with

telescopic assistance.

 

To realise the tremendous advance which science received from

Kepler’s great work, it is to be understood that all the

astronomers who laboured before him at the difficult subject of

the celestial motions, took it for granted that the planets must

revolve in circles. If it did not appear that a planet moved in a

fixed circle, then the ready answer was provided by Ptolemy’s

theory that the circle in which the planet did move was itself in

motion, so that its centre described another circle.

 

When Kepler had before him that wonderful series of observations

of the planet, Mars, which had been accumulated by the

extraordinary skill of Tycho, he proved, after much labour, that

the movements of the planet refused to be represented in a

circular form. Nor would it do to suppose that Mars revolved in

one circle, the centre of which revolved in another circle. On no

such supposition could the movements of the planets be made to

tally with those which Tycho had actually observed. This led to

the astonishing discovery of the true form of a planet’s orbit.

For the first time in the history of astronomy the principle was

laid down that the movement of a planet could not be represented

by a circle, nor even by combinations of circles, but that it

could be represented by an elliptic path. In this path the sun is

situated at one of those two points in the ellipse which are known

as its foci.

 

[PLATE: KEPLER.]

 

Very simple apparatus is needed for the drawing of one those

ellipses which Kepler has shown to possess such astonishing

astronomical significance. Two pins are stuck through a sheet of

paper on a board, the point of a pencil is inserted in a loop of

string which passes over the pins, and as the pencil is moved

round in such a way as to keep the string stretched, that

beautiful curve known as the ellipse is delineated, while the

positions of the pins indicate the two foci of the curve. If the

length of the loop of string is unchanged then the nearer the pins

are together, the greater will be the resemblance between the

ellipse and the circle, whereas the more the pins are separated

the more elongated does the ellipse become. The orbit of a great

planet is, in general, one of those ellipses which approaches a

nearly circular form. It fortunately happens, however, that the

orbit of Mars makes a wider departure from the circular form than

any of the other important planets. It is, doubtless, to this

circumstance that we must attribute the astonishing success of

Kepler in detecting the true shape of a planetary orbit. Tycho’s

observations would not have been sufficiently accurate to have

exhibited the elliptic nature of a planetary orbit which, like

that of Venus, differed very little from a circle.

 

The more we ponder on this memorable achievement the more striking

will it appear. It must be remembered that in these days we know

of the physical necessity which requires that a planet shall

revolve in an ellipse and not in any other curve. But Kepler had

no such knowledge. Even to the last hour of his life he remained

in ignorance of the existence of any natural cause which ordained

that planets should follow those particular curves which geometers

know so well. Kepler’s assignment of the ellipse as the true form

of the planetary orbit is to be regarded as a brilliant guess, the

truth of which Tycho’s observations enabled him to verify. Kepler

also succeeded in pointing out the law according to which the

velocity of a planet at different points of its path could be

accurately specified. Here, again, we have to admire the sagacity

with which this marvellously acute astronomer guessed the deep

truth of nature. In this case also he was quite unprovided with

any reason for expecting from physical principles that such a law

as he discovered must be obeyed. It is quite true that Kepler had

some slight knowledge of the existence of what we now know as

gravitation. He had even enunciated the remarkable doctrine that

the ebb and flow of the tide must be attributed to the attraction

of the moon on the waters of the earth. He does not, however,

appear to have had any anticipation of those wonderful discoveries

which Newton was destined to make a little later, in which he

demonstrated that the laws detected by Kepler’s marvellous acumen

were necessary consequences of the principle of universal

gravitation.

 

[PLATE: SYMBOLICAL REPRESENTATION OF THE PLANETARY SYSTEM.]

 

To appreciate the relations of Kepler and Tycho it is necessary to

note the very different way in which these illustrious astronomers

viewed the system of the heavens. It should be observed that

Copernicus had already expounded the true system, which located

the sun at the centre of the planetary system. But in the days of

Tycho Brahe this doctrine had not as yet commanded universal

assent. In fact, the great observer himself did not accept the

new views of Copernicus. It appeared to Tycho that the earth not

only appeared to be the centre of things celestial, but that it

actually was the centre. It is, indeed, not a little remarkable

that a student of the heavens so accurate as Tycho should have

deliberately rejected the Copernican doctrine in favour of the

system which now seems so preposterous. Throughout his great

career, Tycho steadily observed the places of the sun, the moon,

and the planets, and as steadily maintained that all those bodies

revolved around the earth fixed in the centre. Kepler,

however, had the advantage of belonging to the new school. He

utilised the observations of Tycho in developing the great

Copernican theory whose teaching Tycho stoutly resisted.

 

Perhaps a chapter in modern science may illustrate the

intellectual relation of these great men. The

revolution produced by Copernicus in the doctrine of

the heavens has often been likened to the revolution

which the Darwinian theory produced in the views held by

biologists as to life on this earth. The Darwinian theory did not

at first command universal assent even among those naturalists

whose lives had been devoted with the greatest success to the

study of organisms. Take, for instance, that great naturalist,

Professor Owen, by whose labours vast extension has been given to

our knowledge of the fossil animals which dwelt on the earth in

past ages. Now, though Owens researches were intimately connected

with the great labours of Darwin, and afforded the latter material

for his epoch-making generalization, yet Owen deliberately refused

to accept the new doctrines. Like Tycho, he kept on rigidly

accumulating his facts under the influence of a set of ideas as to

the origin of living forms which are now universally admitted to

be erroneous. If, therefore, we liken Darwin to Copernicus, and

Owen to Tycho, we may liken the biologists of the present day to

Kepler, who interpreted the results of accurate observation upon

sound theoretical principles.

 

In reading the works of Kepler in the light of our modern

knowledge we are often struck by the extent to which his

perception of the sublimest truths in nature was associated with

the most extravagant errors and absurdities. But, of course, it

must be remembered that he wrote in an age in which even the

rudiments of science, as we now understand it, were almost

entirely unknown.

 

It may well be doubted whether any joy experienced by mortals is

more genuine than that which rewards the successful searcher after

natural truths. Every science-worker, be his efforts ever so

humble,