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

>According to our ordinary experience, the very idea of an object

poised without support in space, appears preposterous. Would it

not fall? we are immediately asked. Yes, doubtless it could not

remain poised in any way in which we try the experiment.

We must, however, observe that there are no such ideas as upwards

or downwards in relation to open space. To say that a body falls

downwards, merely means that it tries to fall as nearly as

possible towards the centre of the earth. There is no one

direction along which a body will tend to move in space, in

preference to any other. This may be illustrated by the fact that

a stone let fall at New Zealand will, in its approach towards the

earth’s centre, be actually moving upwards as far as any locality

in our hemisphere is concerned. Why, then, argued Ptolemy, may

not the earth remain poised in space, for as all directions are

equally upward or equally downward, there seems no reason why the

earth should require any support? By this reasoning he arrives at

the fundamental conclusion that the earth is a globular body

freely lying in space, and surrounded above, below, and on all

sides by the glittering stars of heaven.

 

The perception of this sublime truth marks a notable epoch in the

history of the gradual development of the human intellect. No

doubt, other philosophers, in groping after knowledge, may have

set forth certain assertions that are more or less equivalent to

this fundamental truth. It is to Ptolemy we must give credit,

however, not only for announcing this doctrine, but for

demonstrating it by clear and logical argument. We cannot easily

project our minds back to the conception of an intellectual state

in which this truth was unfamiliar. It may, however, be well

imagined that, to one who thought the earth was a flat plain of

indefinite extent, it would be nothing less than an intellectual

convulsion for him to be forced to believe that he stood upon a

spherical earth, forming merely a particle relatively to the

immense sphere of the heavens.

 

What Ptolemy saw in the movements of the stars led him to the

conclusion that they were bright points attached to the inside of

a tremendous globe. The movements of this globe which carried the

stars were only compatible with the supposition that the earth

occupied its centre. The imperceptible effect produced by a

change in the locality of the observer on the apparent brightness

of the stars made it plain that the dimensions of the terrestrial

globe must be quite insignificant in comparison with those of the

celestial sphere. The earth might, in fact, be regarded as a

grain of sand while the stars lay upon a globe many yards in

diameter.

 

So tremendous was the revolution in human knowledge implied by

this discovery, that we can well imagine how Ptolemy, dazzled as

it were by the fame which had so justly accrued to him, failed to

make one further step. Had he made that step, it would have

emancipated the human intellect from the bondage of fourteen

centuries of servitude to a wholly monstrous notion of this

earth’s importance in the scheme of the heavens. The obvious fact

that the sun, the moon, and the stars rose day by day, moved

across the sky in a glorious never-ending procession, and duly set

when their appointed courses had been run, demanded some

explanation. The circumstance that the fixed stars preserved

their mutual distances from year to year, and from age to age,

appeared to Ptolemy to prove that the sphere which contained those

stars, and on whose surface they were believed by him to be fixed,

revolved completely around the earth once every day. He would

thus account for all the phenomena of rising and setting

consistently with the supposition that our globe was stationary.

Probably this supposition must have appeared monstrous, even to

Ptolemy. He knew that the earth was a gigantic object, but, large

as it may have been, he knew that it was only a particle in

comparison with the celestial sphere, yet he apparently believed,

and certainly succeeded in persuading other men to believe, that

the celestial sphere did actually perform these movements.

 

Ptolemy was an excellent geometer. He knew that the rising and

the setting of the sun, the moon, and the myriad stars, could have

been accounted for in a different way. If the earth turned round

uniformly once a day while poised at the centre of the sphere of

the heavens, all the phenomena of rising and setting could be

completely explained. This is, indeed, obvious after a moment’s

reflection. Consider yourself to be standing on the earth at the

centre of the heavens. There are stars over your head, and half

the contents of the heavens are visible, while the other half are

below your horizon. As the earth turns round, the stars over your

head will change, and unless it should happen that you have taken

up your position at either of the poles, new stars will pass into

your view, and others will disappear, for at no time can you have

more than half of the whole sphere visible. The observer on the

earth would, therefore, say that some stars were rising, and that

some stars were setting. We have, therefore, two totally distinct

methods, each of which would completely explain all the observed

facts of the diurnal movement. One of these suppositions requires

that the celestial sphere, bearing with it the stars and other

celestial bodies, turns uniformly around an invisible axis, while

the earth remains stationary at the centre. The other supposition

would be, that it is the stupendous celestial sphere which remains

stationary, while the earth at the centre rotates about the same

axis as the celestial sphere did before, but in an opposite

direction, and with a uniform velocity which would enable it to

complete one turn in twenty-four hours. Ptolemy was mathematician

enough to know that either of these suppositions would suffice for

the explanation of the observed facts. Indeed, the phenomena of

the movements of the stars, so far as he could observe them, could

not be called upon to pronounce which of these views was true, and

which was false.

 

Ptolemy had, therefore, to resort for guidance to indirect lines

of reasoning. One of these suppositions must be true, and yet it

appeared that the adoption of either was accompanied by a great

difficulty. It is one of his chief merits to have demonstrated

that the celestial sphere was so stupendous that the earth itself

was absolutely insignificant in comparison therewith. If, then,

this stupendous sphere rotated once in twenty-four hours, the

speed with which the movement of some of the stars must be

executed would be so portentous as to seem well-nigh impossible.

It would, therefore, seem much simpler on this ground to adopt the

other alternative, and to suppose the diurnal movements were due

to the rotation of the earth. Here Ptolemy saw, or at all events

fancied he saw, objections of the weightiest description. The

evidence of the senses appeared directly to controvert the

supposition that this earth is anything but stationary. Ptolemy

might, perhaps, have dismissed this objection on the ground that

the testimony of the senses on such a matter should be entirely

subordinated to the interpretation which our intelligence would

place upon the facts to which the senses deposed. Another

objection, however, appeared to him to possess the gravest moment.

It was argued that if the earth were rotating, there is nothing to

make the air participate in this motion, mankind would therefore

be swept from the earth by the furious blasts which would arise

from the movement of the earth through an atmosphere at rest.

Even if we could imagine that the air were carried round with the

earth, the same would not apply, so thought Ptolemy, to any object

suspended in the air. So long as a bird was perched on a tree, he

might very well be carried onward by the moving earth, but the

moment he took wing, the ground would slip from under him at a

frightful pace, so that when he dropped down again he would find

himself at a distance perhaps ten times as great as that which a

carrier-pigeon or a swallow could have traversed in the same time.

Some vague delusion of this description seems even still to crop

up occasionally. I remember hearing of a proposition for balloon

travelling of a very remarkable kind. The voyager who wanted to

reach any other place in the same latitude was simply to ascend in

a balloon, and wait there till the rotation of the earth conveyed

the locality which happened to be his destination directly beneath

him, whereupon he was to let out the gas and drop down! Ptolemy

knew quite enough natural philosophy to be aware that such a

proposal for locomotion would be an utter absurdity; he knew that

there was no such relative shift between the air and the earth as

this motion would imply. It appeared to him to be necessary that

the air should lag behind, if the earth had been animated by a

movement of rotation. In this he was, as we know, entirely wrong.

There were, however, in his days no accurate notions on the

subject of the laws of motion.

 

Assiduous as Ptolemy may have been in the study of the heavenly

bodies, it seems evident that he cannot have devoted much thought

to the phenomena of motion of terrestrial objects. Simple,

indeed, are the experiments which might have convinced a

philosopher much less acute than Ptolemy, that, if the earth did

revolve, the air must necessarily accompany it. If a rider

galloping on horseback tosses a ball into the air, it drops again

into his hand, just as it would have done had he been remaining at

rest during the ball’s flight; the ball in fact participates in

the horizontal motion, so that though it really describes a curve

as any passer-by would observe, yet it appears to the rider

himself merely to move up and down in a straight line. This fact,

and many others similar to it, demonstrate clearly that if the

earth were endowed with a movement of rotation, the atmosphere

surrounding it must participate in that movement. Ptolemy did

not know this, and consequently he came to the conclusion

that the earth did not rotate, and that, therefore,

notwithstanding the tremendous improbability of so mighty an

object as the celestial sphere spinning round once in every

twenty-four hours, there was no course open except to believe that

this very improbable thing did really happen. Thus it came to

pass that Ptolemy adopted as the cardinal doctrine of his system a

stationary earth poised at the centre of the celestial sphere,

which stretched around on all sides at a distance so vast that the

diameter of the earth was an inappreciable point in comparison

therewith.

 

Ptolemy having thus deliberately rejected the doctrine of the

earth’s rotation, had to make certain other entirely erroneous

suppositions. It was easily seen that each star required exactly

the same period for the performance of a complete revolution

of the heavens. Ptolemy knew that the stars were at enormous

distances from the earth, though no doubt his notions on this

point came very far short of what we know to be the reality. If

the stars had been at very varied distances, then it would be so

wildly improbable that they should all accomplish their

revolutions in the same time, that Ptolemy came to the

conclusion that they must be all at the same distance, that is,

that they must be all on the surface of a sphere. This view,

however erroneous, was corroborated by the obvious fact that the

stars in the constellations preserved their relative places

unaltered for centuries.