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

degree improbable that the stars should be so placed.

 

Doubtless, also, Copernicus felt a considerable difficulty as to

the nature of the materials from which Ptolemy’s wonderful sphere

was to be constructed. Nor could a philosopher of his penetration

have failed to observe that, unless that sphere were infinitely

large, there must have been space outside it, a consideration

which would open up other difficult questions. Whether infinite

or not, it was obvious that the celestial sphere must have a

diameter at least many thousands of times as great as that of the

earth. From these considerations Copernicus deduced the important

fact that the stars and the other celestial bodies must all be

vast objects. He was thus enabled to put the question in such a

form that it could hardly receive any answer but the correct one.

Which is it more rational to suppose, that the earth should turn

round on its axis once in twenty-four hours, or that thousands of

mighty stars should circle round the earth in the same time, many

of them having to describe circles many thousands of times greater

in circumference than the circuit of the earth at the equator?

The obvious answer pressed upon Copernicus with so much force that

he was compelled to reject Ptolemy’s theory of the stationary

earth, and to attribute the diurnal rotation of the heavens to the

revolution of the earth on its axis.

 

Once this tremendous step had been taken, the great difficulties

which beset the monstrous conception of the celestial sphere

vanished, for the stars need no longer be regarded as situated at

equal distances from the earth. Copernicus saw that they might

lie at the most varied degrees of remoteness, some being hundreds

or thousands of times farther away than others. The complicated

structure of the celestial sphere as a material object

disappeared altogether; it remained only as a geometrical

conception, whereon we find it convenient to indicate the places

of the stars. Once the Copernican doctrine had been fully set

forth, it was impossible for anyone, who had both the

inclination and the capacity to understand it, to withhold

acceptance of its truth. The doctrine of a stationary earth had

gone for ever.

 

Copernicus having established a theory of the celestial movements

which deliberately set aside the stability of the earth, it

seemed natural that he should inquire whether the doctrine of a

moving earth might not remove the difficulties presented in other

celestial phenomena. It had been universally admitted that the

earth lay unsupported in space. Copernicus had further shown

that it possessed a movement of rotation. Its want of stability

being thus recognised, it seemed reasonable to suppose that

the earth might also have some other kinds of movements as well.

In this, Copernicus essayed to solve a problem far more difficult

than that which had hitherto occupied his attention. It was a

comparatively easy task to show how the diurnal rising and

setting could be accounted for by the rotation of the earth.

It was a much more difficult undertaking to demonstrate that the

planetary movements, which Ptolemy had represented with so much

success, could be completely explained by the supposition that

each of those planets revolved uniformly round the sun, and that

the earth was also a planet, accomplishing a complete circuit of

the sun once in the course of a year.

 

[PLATE: EXPLANATION OF PLANETARY MOVEMENTS.]

 

It would be impossible in a sketch like the present to enter into

any detail as to the geometrical propositions on which this

beautiful investigation of Copernicus depended. We can only

mention a few of the leading principles. It may be laid down in

general that, if an observer is in movement, he will, if

unconscious of the fact, attribute to the fixed objects around

him a movement equal and opposite to that which he actually

possesses. A passenger on a canal-boat sees the objects on the

banks apparently moving backward with a speed equal to that by

which he is himself advancing forwards. By an application of this

principle, we can account for all the phenomena of the movements

of the planets, which Ptolemy had so ingeniously represented by

his circles. Let us take, for instance, the most characteristic

feature in the irregularities of the outer planets. We have

already remarked that Mars, though generally advancing from west

to east among the stars, occasionally pauses, retraces his steps

for awhile, again pauses, and then resumes his ordinary onward

progress. Copernicus showed clearly how this effect was produced

by the real motion of the earth, combined with the real motion of

Mars. In the adjoining figure we represent a portion of the

circular tracks in which the earth and Mars move in accordance

with the Copernican doctrine. I show particularly the case where

the earth comes directly between the planet and the sun, because

it is on such occasions that the retrograde movement (for so this

backward movement of Mars is termed) is at its highest. Mars is

then advancing in the direction shown by the arrowhead, and the

earth is also advancing in the same direction. We, on the earth,

however, being unconscious of our own motion, attribute, by the

principle I have already explained, an equal and opposite motion

to Mars. The visible effect upon the planet is, that Mars has two

movements, a real onward movement in one direction, and an

apparent movement in the opposite direction. If it so happened

that the earth was moving with the same speed as Mars, then the

apparent movement would exactly neutralise the real movement, and

Mars would seem to be at rest relatively to the surrounding stars.

Under the actual circumstances represented, however, the earth is

moving faster than Mars, and the consequence is, that the apparent

movement of the planet backwards exceeds the real movement

forwards, the net result being an apparent retrograde movement.

 

With consummate skill, Copernicus showed how the applications of

the same principles could account for the characteristic movements

of the planets. His reasoning in due time bore down all

opposition. The supreme importance of the earth in the system

vanished. It had now merely to take rank as one of the planets.

 

The same great astronomer now, for the first time, rendered

something like a rational account of the changes of the seasons.

Nor did certain of the more obscure astronomical phenomena escape

his attention.

 

He delayed publishing his wonderful discoveries to the world

until he was quite an old man. He had a well-founded apprehension

of the storm of opposition which they would arouse. However, he

yielded at last to the entreaties of his friends, and his book was

sent to the press. But ere it made its appearance to the world,

Copernicus was seized by mortal illness. A copy of the book was

brought to him on May 23, 1543. We are told that he was able to

see it and to touch it, but no more, and he died a few hours

afterwards. He was buried in that Cathedral of Frauenburg, with

which his life had been so closely associated.

 

TYCHO BRAHE.

 

The most picturesque figure in the history of astronomy is

undoubtedly that of the famous old Danish astronomer whose name

stands at the head of this chapter. Tycho Brahe was alike notable

for his astronomical genius and for the extraordinary vehemence of

a character which was by no means perfect. His romantic career as

a philosopher, and his taste for splendour as a Danish noble, his

ardent friendships and his furious quarrels, make him an ideal

subject for a biographer, while the magnificent astronomical work

which he accomplished, has given him imperishable fame.

 

The history of Tycho Brahe has been admirably told by Dr. Dreyer,

the accomplished astronomer who now directs the observatory at

Armagh, though himself a countryman of Tycho. Every student of

the career of the great Dane must necessarily look on Dr. Dreyer’s

work as the chief authority on the subject. Tycho sprang from an

illustrious stock. His family had flourished for centuries, both

in Sweden and in Denmark, where his descendants are to be met with

at the present day. The astronomer’s father was a privy

councillor, and having filled important positions in the Danish

government, he was ultimately promoted to be governor of

Helsingborg Castle, where he spent the last years of his life.

His illustrious son Tycho was born in 1546, and was the second

child and eldest boy in a family of ten.

 

It appears that Otto, the father of Tycho, had a brother named

George, who was childless. George, however, desired to adopt a

boy on whom he could lavish his affection and to whom he could

bequeath his wealth. A somewhat singular arrangement was

accordingly entered into by the brothers at the time when Otto was

married. It was agreed that the first son who might be born to

Otto should be forthwith handed over by the parents to George to

be reared and adopted by him. In due time little Tycho appeared,

and was immediately claimed by George in pursuance of the compact.

But it was not unnatural that the parental instinct, which had

been dormant when the agreement was made, should here interpose.

Tycho’s father and mother receded from the bargain, and refused to

part with their son. George thought he was badly treated.

However, he took no violent steps until a year later, when a

brother was born to Tycho. The uncle then felt no scruple in

asserting what he believed to be his rights by the simple process

of stealing the first-born nephew, which the original bargain had

promised him. After a little time it would seem that the parents

acquiesced in the loss, and thus it was in Uncle George’s home

that the future astronomer passed his childhood.

 

When we read that Tycho was no more than thirteen years old at the

time he entered the University of Copenhagen, it might be at

first supposed that even in his boyish years he must have

exhibited some of those remarkable talents with which he was

afterwards to astonish the world. Such an inference should not,

however, be drawn. The fact is that in those days it was

customary for students to enter the universities at a much earlier

age than is now the case. Not, indeed, that the boys of thirteen

knew more then than the boys of thirteen know now. But the

education imparted in the universities at that time was of a much

more rudimentary kind than that which we understand by university

education at present. In illustration of this Dr. Dreyer tells us

how, in the University of Wittenberg, one of the professors, in

his opening address, was accustomed to point out that even the

processes of multiplication and division in arithmetic might be

learned by any student who possessed the necessary diligence.

 

It was the wish and the intention of his uncle that Tycho’s

education should be specially directed to those branches of

rhetoric and philosophy which were then supposed to be a necessary

preparation for the career of a statesman. Tycho, however,

speedily made it plain to his teachers that though he was an

ardent student, yet the things which interested him were the

movements of the heavenly bodies and not the subtleties of

metaphysics.

 

[PLATE: TYCHO BRAHE.]

 

On the 21st October, 1560, an eclipse of the sun occurred, which

was partially visible at Copenhagen. Tycho, boy though he was,

took the utmost interest in this event. His ardour and

astonishment in connection with the circumstance were chiefly

excited by the fact that the time of the occurrence of the

phenomenon could be predicted with so much accuracy. Urged by his

desire to understand the matter thoroughly, Tycho sought to

procure some book which might explain what he so greatly wanted

to know. In those days books of