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

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early in 1758. But the attraction of the great planet would cause

delay, so that Halley assigned, for the date of its re-appearance,

either the end of 1758 or the beginning of 1759. Halley knew that

he could not himself live to witness the fulfilment of his

prediction, but he says: “If it should return, according to our

predictions, about the year 1758, impartial posterity will not

refuse to acknowledge that this was first discovered by an

Englishman.” This was, indeed, a remarkable prediction of an

event to occur fifty-three years after it had been uttered. The

way in which it was fulfilled forms one of the most striking

episodes in the history of astronomy. The comet was first seen on

Christmas Day, 1758, and passed through its nearest point to the

sun on March 13th, 1759. Halley had then been lying in his grave

for seventeen years, yet the verification of his prophecy reflects

a glory on his name which will cause it to live for ever in the

annals of astronomy. The comet paid a subsequent visit in 1835,

and its next appearance is due about 1910.

 

Halley next entered upon a labour which, if less striking to

the imagination than his discoveries with regard to comets, is

still of inestimable value in astronomy. He undertook a series of

investigations with the object of improving our knowledge of the

movements of the planets. This task was practically finished in

1719, though the results of it were not published until after his

death in 1749. In the course of it he was led to investigate

closely the motion of Venus, and thus he came to recognise for the

first time the peculiar importance which attaches to the

phenomenon of the transit of this planet across the sun. Halley

saw that the transit, which was to take place in the year 1761,

would afford a favourable opportunity for determining the distance

of the sun, and thus learning the scale of the solar system. He

predicted the circumstances of the phenomenon with an astonishing

degree of accuracy, considering his means of information, and it

is unquestionably to the exertions of Halley in urging the

importance of the matter upon astronomers that we owe the

unexampled degree of interest taken in the event, and the energy

which scientific men exhibited in observing it. The illustrious

astronomer had no hope of being himself a witness of the event,

for it could not happen till many years after his death. This did

not, however, diminish his anxiety to impress upon those who would

then be alive, the importance of the occurrence, nor did it lead

him to neglect anything which might contribute to the success of

the observations. As we now know, Halley rather overestimated

the value of the transit of Venus, as a means of determining the

solar distance. The fact is that the circumstances are such that

the observation of the time of contact between the edge of the

planet and the edge of the sun cannot be made with the accuracy

which he had expected.

 

In 1691, Halley became a candidate for the Savilian Professorship

of Astronomy at Oxford. He was not, however, successful, for his

candidature was opposed by Flamsteed, the Astronomer Royal of the

time, and another was appointed. He received some consolation for

this particular disappointment by the fact that, in 1696, owing to

Newton’s friendly influence, he was appointed deputy Controller of

the Mint at Chester, an office which he did not retain for long,

as it was abolished two years later. At last, in 1703, he

received what he had before vainly sought, and he was appointed to

the Savilian chair.

 

His observations of the eclipse of the sun, which occurred in

1715, added greatly to Halley’s reputation. This phenomenon

excited special attention, inasmuch as it was the first total

eclipse of the sun which had been visible in London since the

year 1140. Halley undertook the necessary calculations,

and predicted the various circumstances with a far higher degree

of precision than the official announcement. He himself observed

the phenomenon from the Royal Society’s rooms, and he minutely

describes the outer atmosphere of the sun, now known as the

corona; without, however, offering an opinion as to whether it

was a solar or a lunar appendage.

 

At last Halley was called to the dignified office which he of all

men was most competent to fill. On February 9th, 1720, he was

appointed Astronomer Royal in succession to Flamsteed. He found

things at the Royal Observatory in a most unsatisfactory state.

Indeed, there were no instruments, nor anything else that was

movable; for such things, being the property of Flamsteed,

had been removed by his widow, and though Halley attempted

to purchase from that lady some of the instruments which

his predecessor had employed, the unhappy personal differences

which had existed between him and Flamsteed, and which, as we have

already seen, prevented his election as Savilian Professor of

Astronomy, proved a bar to the negotiation. Greenwich Observatory

wore a very different appearance in those days, from that which

the modern visitor, who is fortunate enough to gain admission, may

now behold. Not only did Halley find it bereft of instruments, we

learn besides that he had no assistants, and was obliged to

transact the whole business of the establishment single-handed.

 

In 1721, however, he obtained a grant of 500 pounds from the Board

of Ordnance, and accordingly a transit instrument was erected in

the same year. Some time afterwards he procured an eight-foot

quadrant, and with these instruments, at the age of sixty-four, he

commenced a series of observations on the moon. He intended, if

his life was spared, to continue his observations for a period of

eighteen years, this being, as astronomers know, a very important

cycle in connection with lunar movements. The special object of

this vast undertaking was to improve the theory of the moon’s

motion, so that it might serve more accurately to determine

longitudes at sea. This self-imposed task Halley lived to carry

to a successful termination, and the tables deduced from his

observations, and published after his death, were adopted almost

universally by astronomers, those of the French nation being the

only exception.

 

Throughout his life Halley had been singularly free from illness

of every kind, but in 1737 he had a stroke of paralysis.

Notwithstanding this, however, he worked diligently at his

telescope till 1739, after which his health began rapidly to give

way. He died on January 14th, 1742, in the eighty-sixth year of

his age, retaining his mental faculties to the end. He was buried

in the cemetery of the church of Lee in Kent, in the same grave as

his wife, who had died five years previously. We are informed by

Admiral Smyth that Pond, a later Astronomer Royal, was afterwards

laid in the same tomb.

 

Halley’s disposition seems to have been generous and candid, and

wholly free from anything like jealousy or rancour. In person he

was rather above the middle height, and slight in build; his

complexion was fair, and he is said to have always spoken, as well

as acted, with uncommon sprightliness. In the eloge pronounced

upon him at the Paris Academie Des Sciences, of which Halley had

been made a member in 1719 it was said, “he possessed all the

qualifications which were necessary to please princes who were

desirous of instruction, with a great extent of knowledge and a

constant presence of mind; his answers were ready, and at the same

time pertinent, judicious, polite and sincere.”

 

[PLATE: GREENWICH OBSERVATORY IN HALLEY’S TIME.]

 

Thus we find that Peter the Great was one of his most ardent

admirers. He consulted the astronomer on matters connected with

shipbuilding, and invited him to his own table. But Halley

possessed nobler qualifications than the capacity of pleasing

Princes. He was able to excite and to retain the love and

admiration of his equals. This was due to the warmth of his

attachments, the unselfishness of his devotion to his friends,

and to a vein of gaiety and good-humour which pervaded all his

conversation.

 

BRADLEY.

 

James Bradley was descended from an ancient family in the county

of Durham. He was born in 1692 or 1693, at Sherbourne, in

Gloucestershire, and was educated in the Grammar School at

Northleach. From thence he proceeded in due course to Oxford,

where he was admitted a commoner at Balliol College, on March

15th, 1711. Much of his time, while an undergraduate, was

passed in Essex with his maternal uncle, the Rev. James Pound,

who was a well-known man of science and a diligent observer of the

stars. It was doubtless by intercourse with his uncle that young

Bradley became so expert in the use of astronomical instruments,

but the immortal discoveries he subsequently made show him to have

been a born astronomer.

 

The first exhibition of Bradley’s practical skill seems to be

contained in two observations which he made in 1717 and 1718.

They have been published by Halley, whose acuteness had led him to

perceive the extraordinary scientific talents of the young

astronomer. Another illustration of the sagacity which Bradley

manifested, even at the very commencement of his astronomical

career, is contained in a remark of Halley’s, who says: Dr. Pound

and his nephew, Mr. Bradley, did, myself being present, in the

last opposition of the sun and Mars this way demonstrate the

extreme minuteness of the sun’s parallax, and that it was not more

than twelve seconds nor less than nine seconds.” To make the

significance of this plain, it should be observed that the

determination of the sun’s parallax is equivalent to the

determination of the distance from the earth to the sun. At the

time of which we are now writing, this very important unit of

celestial measurement was only very imperfectly known, and the

observations of Pound and Bradley may be interpreted to mean that,

from their observations, they had come to the conclusion that the

distance from the earth to the sun must be more than 94 millions

of miles, and less than 125 millions. We now, of course, know

that they were not exactly right, for the true distance of the sun

is about 93 millions of miles. We cannot, however, but think

that it was a very remarkable approach for the veteran astronomer

and his brilliant nephew to make towards the determination of a

magnitude which did not become accurately known till fifty years

later.

 

Among the earliest parts of astronomical work to which Bradley’s

attention was directed, were the eclipses of Jupiter’s satellites.

These phenomena are specially attractive inasmuch as they can be

so readily observed, and Bradley found it extremely interesting to

calculate the times at which the eclipses should take place, and

then to compare his observations with the predicted times. From

the success that he met with in this work, and from his other

labours, Bradley’s reputation as an astronomer increased so

greatly that on November the 6th, 1718, he was elected a Fellow of

the Royal Society.

 

Up to this time the astronomical investigations of Bradley had

been more those of an amateur than of a professional astronomer,

and as it did not at first seem likely that scientific work would

lead to any permanent provision, it became necessary for the

youthful astronomer to choose a profession. It had been all

along intended that he should enter the Church, though for some

reason which is not told us, he did not take orders as soon as

his age would have entitled him to do so. In 1719, however, the

Bishop of Hereford offered Bradley the Vicarage of Bridstow, near

Ross, in Monmouthshire, and on July 25th, 1720, he having then

taken priest’s orders, was duly instituted in his vicarage. In

the beginning of the next year, Bradley had some addition to