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

his

income from the proceeds of a Welsh living, which, being a

sinecure, he was able to hold with his appointment at Bridstow.

It appears, however, that his clerical occupations were not very

exacting in their demands upon his time, for he was still able to

pay long and often-repeated visits to his uncle at Wandsworth,

who, being himself a clergyman, seems to have received occasional

assistance in his ministerial duties from his astronomical nephew.

 

The time, however, soon arrived when Bradley was able to make a

choice between continuing to exercise his profession as a divine,

or devoting himself to a scientific career. The Savilian

Professorship of Astronomy in the University of Oxford became

vacant by the death of Dr. John Keill. The statutes forbade that

the Savilian Professor should also hold a clerical appointment,

and Mr. Pound would certainly have been elected to the

professorship had he consented to surrender his preferments in the

Church. But Pound was unwilling to sacrifice his clerical

position, and though two or three other candidates appeared in the

field, yet the talents of Bradley were so conspicuous that he was

duly elected, his willingness to resign the clerical profession

having been first ascertained.

 

There can be no doubt that, with such influential friends as

Bradley possessed, he would have made great advances had he

adhered to his profession as a divine. Bishop Hoadly, indeed,

with other marks of favour, had already made the astronomer his

chaplain. The engrossing nature of Bradley’s interest in

astronomy decided him, however, to sacrifice all other prospects

in comparison with the opening afforded by the Savilian

Professorship. It was not that Bradley found himself devoid of

interest in clerical matters, but he felt that the true scope for

such abilities as he possessed would be better found in the

discharge of the scientific duties of the Oxford chair than in the

spiritual charge of a parish. On April the 26th, 1722, Bradley

read his inaugural lecture in that new position on which he was

destined to confer such lustre.

 

It must, of course, be remembered that in those early days the art

of constructing the astronomical telescope was very imperfectly

understood. The only known method for getting over the peculiar

difficulties presented in the construction of the refracting

telescope, was to have it of the most portentous length. In fact,

Bradley made several of his observations with an instrument of two

hundred and twelve feet focus. In such a case, no tube could be

used, and the object glass was merely fixed at the top of a high

pole. Notwithstanding the inconvenience and awkwardness of such

an instrument, Bradley by its means succeeded in making many

careful measurements. He observed, for example, the transit of

Mercury over the sun’s disc, on October 9th, 1723, he also

observed the dimensions of the planet Venus, while a comet which

Halley discovered on October the 9th, 1723, was assiduously

observed at Wanstead up to the middle of the ensuing month. The

first of Bradley’s remarkable contributions to the “Philosophical

Transactions” relates to this comet, and the extraordinary amount

of work that he went through in connection therewith may

be seen from an examination of his book of Calculations which is

still extant.

 

The time was now approaching when Bradley was to make the first of

those two great discoveries by which his name has acquired a

lustre that has placed him in the very foremost rank of

astronomical discoverers. As has been often the case in the

history of science, the first of these great successes was

attained while he was pursuing a research intended for a wholly

different purpose. It had long been recognised that as the earth

describes a vast orbit, nearly two hundred million miles in

diameter, in its annual journey round the sun, the apparent

places of the stars should alter, to some extent, in

correspondence with the changes in the earth’s position. The

nearer the star the greater the shift in its apparent place on the

heavens, which must arise from the fact that it was seen from

different positions in the earth’s orbit. It had been pointed out

that these apparent changes in the places of the stars, due to the

movement of the earth, would provide the means of measuring the

distances of the stars. As, however, these distances are

enormously great in comparison with the orbit which the earth

describes around the sun, the attempt to determine the distances

of the stars by the shift in their positions had hitherto proved

ineffectual. Bradley determined to enter on this research once

again; he thought that by using instruments of greater power, and

by making measurements of increased delicacy, he would be able to

perceive and to measure displacements which had proved so small as

to elude the skill of the other astronomers who had previously

made efforts in the same direction. In order to simplify the

investigation as much as possible, Bradley devoted his attention

to one particular star, Beta Draconis, which happened to pass near

his zenith. The object of choosing a star in this position was to

avoid the difficulties which would be introduced by refraction had

the star occupied any other place in the heavens than that

directly overhead.

 

We are still able to identify the very spot on which the telescope

stood which was used in this memorable research. It was erected

at the house then occupied by Molyneux, on the western extremity

of Kew Green. The focal length was 24 feet 3 inches, and the eye-glass was 3 and a half feet above the ground floor. The

instrument was first set up on November 26th, 1725. If there had

be any appreciable disturbance in the place of Beta Draconis in

consequence of the movement of the earth around the sun, the star

must appear to have the smallest latitude when in conjunction with

the sun, and the greatest when in opposition. The star passed the

meridian at noon in December, and its position was particularly

noticed by Molyneux on the third of that month. Any perceptible

displacement by parallax—for so the apparent change in position,

due to the earth’s motion, is called—would would have made the

star shift towards the north. Bradley, however, when observing it

on the 17th, was surprised to find that the apparent place of the

star, so far from shifting towards the north, as they had perhaps

hoped it would, was found to lie a little more to the south than

when it was observed before. He took extreme care to be sure that

there was no mistake in his observation, and, true astronomer as

he was, he scrutinized with the utmost minuteness all the

circumstances of the adjustment of his instruments. Still the

star went to the south, and it continued so advancing in the same

direction until the following March, by which time it had moved no

less than twenty seconds south from the place which it occupied

when the first observation was made. After a brief pause, in

which no apparent movement was perceptible, the star by the middle

of April appeared to be returning to the north. Early in June it

reached the same distance from the zenith which it had in

December. By September the star was as much as thirty-nine

seconds more to the north than it had been in March, then it

returned towards the south, regaining in December the same

situation which it had occupied twelve months before.

 

This movement of the star being directly opposite to the movements

which would have been the consequence of parallax, seemed to show

that even if the star had any parallax its effects upon the

apparent place were entirely masked by a much larger motion of a

totally different description. Various attempts were made to

account for the phenomenon, but they were not successful. Bradley

accordingly determined to investigate the whole subject in a more

thorough manner. One of his objects was to try whether the same

movements which he had observed in one star were in any similar

degree possessed by other stars. For this purpose he set up a new

instrument at Wanstead, and there he commenced a most diligent

scrutiny of the apparent places of several stars which passed at

different distances from the zenith. He found in the course of

this research that other stars exhibited movements of a similar

description to those which had already proved so perplexing. For

a long time the cause of these apparent movements seemed a

mystery. At last, however, the explanation of these remarkable

phenomena dawned upon him, and his great discovery was made.

 

One day when Bradley was out sailing he happened to remark that

every time the boat was laid on a different tack the vane at the

top of the boat’s mast shifted a little, as if there had been a

slight change in the direction of the wind. After he had noticed

this three or four times he made a remark to the sailors to the

effect that it was very strange the wind should always happen to

change just at the moment when the boat was going about. The

sailors, however, said there had been no change in the wind, but

that the alteration in the vane was due to the fact that the

boat’s course had been altered. In fact, the position of the

vane was determined both by the course of the boat and the

direction of the wind, and if either of these were altered there

would be a corresponding change in the direction of the vane.

This meant, of course, that the observer in the boat which was

moving along would feel the wind coming from a point different

from that in which the wind appeared to be blowing when the boat

was at rest, or when it was sailing in some different direction.

Bradley’s sagacity saw in this observation the clue to the

Difficulty which had so long troubled him.

 

It had been discovered before the time of Bradley that the passage

of light through space is not an instantaneous phenomenon. Light

requires time for its journey. Galileo surmised that the sun may

have reached the horizon before we see it there, and it was indeed

sufficiently obvious that a physical action, like the transmission

of light, could hardly take place without requiring some lapse of

time. The speed with which light actually travelled was, however,

so rapid that its determination eluded all the means of

experimenting which were available in those days. The penetration

of Roemer had previously detected irregularities in the observed

times of the eclipses of Jupiter’s satellites, which were

undoubtedly due to the interval which light required for

stretching across the interplanetary spaces. Bradley argued that

as light can only travel with a certain speed, it may in a

measure be regarded like the wind, which he noticed in the boat.

If the observer were at rest, that is to say, if the earth were a

stationary object, the direction in which the light actually does

come would be different from that in which it appears to come when

the earth is in motion. It is true that the earth travels but

eighteen miles a second, while the velocity with which light is

borne along attains to as much as 180,000 miles a second. The

velocity of light is thus ten thousand times greater than the

speed of the earth. But even though the wind blew ten

thousand times faster than the speed with which the boat was

sailing there would still be some change, though no doubt a very

small change, in the position of the vane when the boat was in

progress from the position it would have if the boat were at rest.

It therefore occurred to this most acute of astronomers that when

the telescope was pointed towards a star so as to