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

thousand years would necessarily elapse before a

complete circuit of the heavens was accomplished. Hipparchus

traced out this phenomenon, and established it on an impregnable

basis, so that all astronomers have ever since recognised the

precession of the equinoxes as one of the fundamental facts of

astronomy. Not until nearly two thousand years after Hipparchus

had made this splendid discovery was the explanation of its cause

given by Newton.

 

From the days of Hipparchus down to the present hour the science

of astronomy has steadily grown. One great observer after another

has appeared from time to time, to reveal some new phenomenon with

regard to the celestial bodies or their movements, while from time

to time one commanding intellect after another has arisen to

explain the true import of the facts of observations. The history

of astronomy thus becomes inseparable from the history of the

great men to whose labours its development is due.

 

In the ensuing chapters we have endeavoured to sketch the lives

and the work of the great philosophers, by whose labours the

science of astronomy has been created. We shall commence with

Ptolemy, who, after the foundations of the science had been laid

by Hipparchus, gave to astronomy the form in which it was taught

throughout the Middle Ages. We shall next see the mighty

revolution in our conceptions of the universe which are associated

with the name of Copernicus. We then pass to those periods

illumined by the genius of Galileo and Newton, and afterwards we

shall trace the careers of other more recent discoverers, by

whose industry and genius the boundaries of human knowledge have

been so greatly extended. Our history will be brought down late

enough to include some of the illustrious astronomers who laboured

in the generation which has just passed away.

 

PTOLEMY.

 

[PLATE: PTOLEMY.]

 

The career of the famous man whose name stands at the head of this

chapter is one of the most remarkable in the history of human

learning. There may have been other discoverers who have done

more for science than ever Ptolemy accomplished, but there never

has been any other discoverer whose authority on the subject of

the movements of the heavenly bodies has held sway over the minds

of men for so long a period as the fourteen centuries during which

his opinions reigned supreme. The doctrines he laid down in his

famous book, “The Almagest,” prevailed throughout those ages. No

substantial addition was made in all that time to the undoubted

truths which this work contained. No important correction was

made of the serious errors with which Ptolemy’s theories were

contaminated. The authority of Ptolemy as to all things in

the heavens, and as to a good many things on the earth (for the

same illustrious man was also a diligent geographer), was

invariably final.

 

Though every child may now know more of the actual truths of the

celestial motions than ever Ptolemy knew, yet the fact that his

work exercised such an astonishing effect on the human intellect

for some sixty generations, shows that it must have been an

extraordinary production. We must look into the career of this

wonderful man to discover wherein lay the secret of that

marvellous success which made him the unchallenged instructor of

the human race for such a protracted period.

 

Unfortunately, we know very little as to the personal history of

Ptolemy. He was a native of Egypt, and though it has been

sometimes conjectured that he belonged to the royal families of

the same name, yet there is nothing to support such a belief.

The name, Ptolemy, appears to have been a common one in Egypt in

those days. The time at which he lived is fixed by the fact that

his first recorded observation was made in 127 AD, and his last in

151 AD. When we add that he seems to have lived in or near

Alexandria, or to use his own words, “on the parallel of

Alexandria,” we have said everything that can be said so far as

his individuality is concerned.

 

Ptolemy is, without doubt, the greatest figure in ancient

astronomy. He gathered up the wisdom of the philosophers who had

preceded him. He incorporated this with the results of his

own observations, and illumined it with his theories. His

speculations, even when they were, as we now know, quite

erroneous, had such an astonishing verisimilitude to the actual

facts of nature that they commanded universal assent. Even in

these modern days we not unfrequently find lovers of paradox who

maintain that Ptolemy’s doctrines not only seem true, but actually

are true.

 

In the absence of any accurate knowledge of the science of

mechanics, philosophers in early times were forced to fall back

on certain principles of more or less validity, which they derived

from their imagination as to what the natural fitness of things

ought to be. There was no geometrical figure so simple and so

symmetrical as a circle, and as it was apparent that the heavenly

bodies pursued tracks which were not straight lines, the

conclusion obviously followed that their movements ought to be

circular. There was no argument in favour of this notion, other

than the merely imaginary reflection that circular movement, and

circular movement alone, was “perfect,” whatever “perfect” may

have meant. It was further believed to be impossible that the

heavenly bodies could have any other movements save those which

were perfect. Assuming this, it followed, in Ptolemy’s opinion,

and in that of those who came after him for fourteen centuries,

that all the tracks of the heavenly bodies were in some way or

other to be reduced to circles.

 

Ptolemy succeeded in devising a scheme by which the apparent

changes that take place in the heavens could, so far as he knew

them, be explained by certain combinations of circular movement.

This seemed to reconcile so completely the scheme of things

celestial with the geometrical instincts which pointed to the

circle as the type of perfect movement, that we can hardly wonder

Ptolemy’s theory met with the astonishing success that attended

it. We shall, therefore, set forth with sufficient detail the

various steps of this famous doctrine.

 

Ptolemy commences with laying down the undoubted truth that the

shape of the earth is globular. The proofs which he gives of this

fundamental fact are quite satisfactory; they are indeed the same

proofs as we give today. There is, first of all, the well-known

circumstance of which our books on geography remind us, that when

an object is viewed at a distance across the sea, the lower part

of the object appears cut off by the interposing curved mass of

water.

 

The sagacity of Ptolemy enabled him to adduce another argument,

which, though not quite so obvious as that just mentioned,

demonstrates the curvature of the earth in a very impressive

manner to anyone who will take the trouble to understand it.

Ptolemy mentions that travellers who went to the south reported,

that, as they did so, the appearance of the heavens at night

underwent a gradual change. Stars that they were familiar with in

the northern skies gradually sank lower in the heavens. The

constellation of the Great Bear, which in our skies never sets

during its revolution round the pole, did set and rise when a

sufficient southern latitude had been attained. On the other

hand, constellations new to the inhabitants of northern climes

were seen to rise above the southern horizon. These

circumstances would be quite incompatible with the supposition

that the earth was a flat surface. Had this been so, a little

reflection will show that no such changes in the apparent

movements of the stars would be the consequence of a voyage to the

south. Ptolemy set forth with much insight the significance of

this reasoning, and even now, with the resources of modern

discoveries to help us, we can hardly improve upon his arguments.

 

Ptolemy, like a true philosopher disclosing a new truth to the

world, illustrated and enforced his subject by a variety of happy

demonstrations. I must add one of them, not only on account of

its striking nature, but also because it exemplifies Ptolemy’s

acuteness. If the earth were flat, said this ingenious reasoner,

sunset must necessarily take place at the same instant, no matter

in what country the observer may happen to be placed. Ptolemy,

however, proved that the time of sunset did vary greatly as

the observer’s longitude was altered. To us, of course, this is

quite obvious; everybody knows that the hour of sunset may have

been reached in Great Britain while it is still noon on the

western coast of America. Ptolemy had, however, few of those

sources of knowledge which are now accessible. How was he to show

that the sun actually did set earlier at Alexandria than it would

in a city which lay a hundred miles to the west? There was no

telegraph wire by which astronomers at the two Places could

communicate. There was no chronometer or watch which could be

transported from place to place; there was not any other reliable

contrivance for the keeping of time. Ptolemy’s ingenuity,

however, pointed out a thoroughly satisfactory method by which the

times of sunset at two places could be compared. He was

acquainted with the fact, which must indeed have been known from

the very earliest times, that the illumination of the moon is

derived entirely from the sun. He knew that an eclipse of the

moon was due to the interposition of the earth which cuts off the

light of the sun. It was, therefore, plain that an eclipse of the

moon must be a phenomenon which would begin at the same instant

from whatever part of the earth the moon could be seen at the

time. Ptolemy, therefore, brought together from various quarters

the local times at which different observers had recorded the

beginning of a lunar eclipse. He found that the observers to the

west made the time earlier and earlier the further away their

stations were from Alexandria. On the other hand, the eastern

observers set down the hour as later than that at which the

phenomenon appeared at Alexandria. As these observers all

recorded something which indeed appeared to them simultaneously,

the only interpretation was, that the more easterly a place the

later its time. Suppose there were a number of observers along a

parallel of latitude, and each noted the hour of sunset to be

six o’clock, then, since the eastern times are earlier than

western times, 6 p.m. at one station A will correspond to 5 p.m.

at a station B sufficiently to the west. If, therefore, it is

sunset to the observer at A, the hour of sunset will not yet be

reached for the observer at B. This proves conclusively that the

time of sunset is not the same all over the earth. We have,

however, already seen that the apparent time of sunset would be

the same from all stations if the earth were flat. When Ptolemy,

therefore, demonstrated that the time of sunset was not the same

at various places, he showed conclusively that the earth was not

flat.

 

As the same arguments applied to all parts of the earth where

Ptolemy had either been himself, or from which he could gain the

necessary information, it followed that the earth, instead of

being the flat plain, girdled with an illimitable ocean, as was

generally supposed, must be in reality globular. This led at once

to a startling consequence. It was obvious that there could be no

supports of any kind by which this globe was sustained; it

therefore followed that the mighty object must be simply poised in

space. This is indeed an astonishing doctrine to anyone who

relies on what merely seems the evidence of the senses, without

giving to that evidence its due intellectual interpretation.