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

place it

apparently in the centre of the field of view, yet it was not

generally the true position of the star. It was not, in fact,

the position in which the star would have been observed had the

earth been at rest. Provided with this suggestion, he explained

the apparent movements of the stars by the principle known as the

“aberration of light.” Every circumstance was accounted for as a

consequence of the relative movements of the earth and of the

light from the star. This beautiful discovery not only

established in the most forcible manner the nature of the movement

of light; not only did it illustrate the truth of the Copernican

theory which asserted that the earth revolved around the sun, but

it was also of the utmost importance in the improvement of

practical astronomy. Every observer now knows that, generally

speaking, the position which the star appears to have is not

exactly the position in which the star does actually lie. The

observer is, however, able, by the application of the principles

which Bradley so clearly laid down, to apply to an observation the

correction which is necessary to obtain from it the true place in

which the object is actually situated. This memorable achievement

at once conferred on Bradley the highest astronomical fame. He

tested his discovery in every way, but only to confirm its truth

in the most complete manner.

 

Halley, the Astronomer Royal, died on the 14th, January, 1742, and

Bradley was immediately pointed out as his successor. He was

accordingly appointed Astronomer Royal in February, 1742.

On first taking up his abode at Greenwich he was unable to conduct

his observations owing to the wretched condition in which he found

the instruments. He devoted himself, however, assiduously to

their repair, and his first transit observation is recorded on the

25th July, 1742. He worked with such energy that on one day it

appears that 255 transit observations were taken by himself alone,

and in September, 1747, he had completed the series of

observations which established his second great discovery, the

nutation of the earth’s axis. The way in which he was led to the

detection of the nutation is strikingly illustrative of the

extreme care with which Bradley conducted his observations. He

found that in the course of a twelvemonth, when the star had

completed the movement which was due to aberration, it did not

return exactly to the same position which it had previously

occupied. At first he thought this must be due to some

instrumental error, but after closer examination and

repeated study of the effect as manifested by many different

stars, he came to the conclusion that its origin must be sought in

some quite different source. The fact is that a certain change

takes place in the apparent position of the stars which is not due

to the movement of the star itself, but is rather to be attributed

to changes in the points from which the star’s positions are

measured.

 

We may explain the matter in this way. As the earth is not a

sphere, but has protuberant parts at the equator, the attraction

of the moon exercises on those protuberant parts a pulling effect

which continually changes the direction of the earth’s axis, and

consequently the position of the pole must be in a state of

incessant fluctuation. The pole to which the earth’s axis points

on the sky is, therefore, slowly changing. At present it happens

to lie near the Pole Star, but it will not always remain there.

It describes a circle around the pole of the Ecliptic, requiring

about 25,000 years for a complete circuit. In the course of its

progress the pole will gradually pass now near one star and now

near another, so that many stars will in the lapse of ages

discharge the various functions which the present Pole Star does

for us. In about 12,000 years, for instance, the pole will have

come near the bright star, Vega. This movement of the pole had

been known for ages. But what Bradley discovered was that the

pole, instead of describing an uniform movement as had been

previously supposed, followed a sinuous course now on one side and

now on the other of its mean place. This he traced to the

fluctuations of the moon’s orbit, which undergoes a continuous

change in a period of nineteen years. Thus the efficiency with

which the moon acts on the protuberant mass of the earth

varies, and thus the pole is caused to oscillate.

 

This subtle discovery, if perhaps in some ways less impressive

than Bradley’s earlier achievements of the detection of the

aberration of light, is regarded by astronomers as testifying even

in a higher degree to his astonishing care and skill as an

observer, and justly entitles him to a unique place among the

astronomers whose discoveries have been effected by consummate

practical skill in the use of astronomical instruments.

 

Of Bradley’s private or domestic life there is but little

to tell. In 1744, soon after he became Astronomer Royal,

he married a daughter of Samuel Peach, of Chalford, in

Gloucestershire. There was but one child, a daughter, who became

the wife of her cousin, Rev. Samuel Peach, rector of Compton,

Beauchamp, in Berkshire.

 

Bradley’s last two years of life were clouded by a melancholy

depression of spirits, due to an apprehension that he should

survive his rational faculties. It seems, however, that the ill

he dreaded never came upon him, for he retained his mental powers

to the close. He died on 13th July, 1762, aged seventy, and was

buried at Michinghamton.

 

WILLIAM HERSCHEL.

 

William Herschel, one of the greatest astronomers that has ever

lived, was born at Hanover, on the 15th November, 1738. His

father, Isaac Herschel, was a man evidently of considerable

ability, whose life was devoted to the study and practice of

music, by which he earned a somewhat precarious maintenance. He

had but few worldly goods to leave to his children, but he more

than compensated for this by bequeathing to them a splendid

inheritance of genius. Touches of genius were, indeed, liberally

scattered among the members of Isaac’s large family, and in the

case of his forth child, William, and of a sister several years

younger, it was united with that determined perseverance and rigid

adherence to principle which enabled genius to fulfil its perfect

work.

 

A faithful chronicler has given us an interesting account of

the way in which Isaac Herschel educated his sons; the narrative

is taken from the recollections of one who, at the time we are

speaking of, was an unnoticed little girl five or six years old.

She writes:—

 

“My brothers were often introduced as solo performers and

assistants in the orchestra at the Court, and I remember that I

was frequently prevented from going to sleep by the lively

criticisms on music on coming from a concert. Often I would keep

myself awake that I might listen to their animating remarks, for

it made me so happy to see them so happy. But generally their

conversation would branch out on philosophical subjects, when my

brother William and my father often argued with such warmth that

my mother’s interference became necessary, when the names—Euler,

Leibnitz, and Newton—sounded rather too loud for the repose of her

little ones, who had to be at school by seven in the morning.”

The child whose reminiscences are here given became afterwards the

famous Caroline Herschel. The narrative of her life, by Mrs. John

Herschel, is a most interesting book, not only for the account it

contains of the remarkable woman herself, but also because it

provides the best picture we have of the great astronomer to whom

Caroline devoted her life.

 

This modest family circle was, in a measure, dispersed at the

outbreak of the Seven Years’ War in 1756. The French proceeded to

invade Hanover, which, it will be remembered, belonged at this

time to the British dominions. Young William Herschel had already

obtained the position of a regular performer in the regimental

band of the Hanoverian Guards, and it was his fortune to obtain

some experience of actual warfare in the disastrous battle of

Hastenbeck. He was not wounded, but he had to spend the night

after the battle in a ditch, and his meditations on the occasion

convinced him that soldiering was not the profession exactly

adapted to his tastes. We need not attempt to conceal the fact

that he left his regiment by the very simple but somewhat risky

process of desertion. He had, it would seem, to adopt disguises

to effect his escape. At all events, by some means he succeeded

in eluding detection and reached England in safety. It is

interesting to have learned on good authority that many years

after this offence was committed it was solemnly forgiven. When

Herschel had become the famous astronomer, and as such visited

King George at Windsor, the King at their first meeting handed to

him his pardon for deserting from the army, written out in due

form by his Majesty himself.

 

It seems that the young musician must have had some difficulty in

providing for his maintenance during the first few years of his

abode in England. It was not until he had reached the age of

twenty-two that he succeeded in obtaining any regular appointment.

He was then made Instructor of Music to the Durham Militia.

Shortly afterwards, his talents being more widely recognised, he

was appointed as organist at the parish church at Halifax, and his

prospects in life now being fairly favourable, and the Seven

Years’ War being over, he ventured to pay a visit to Hanover to

see his father. We can imagine the delight with which old Isaac

Herschel welcomed his promising son, as well as his parental pride

when a concert was given at which some of William’s compositions

were performed. If the father was so intensely gratified on this

occasion, what would his feelings have been could he have lived to

witness his son’s future career? But this pleasure was not to be

his, for he died many years before William became an astronomer.

 

In 1766, about a couple of years after his return to England from

This visit to his old home, we find that Herschel had received a

further promotion to be organist in the Octagon Chapel, at Bath.

Bath was then, as now, a highly fashionable resort, and many

notable personages patronised the rising musician. Herschel had

other points in his favour besides his professional skill; his

appearance was good, his address was prepossessing, and even his

nationality was a distinct advantage, inasmuch as he was a

Hanoverian in the reign of King George the Third. On Sundays he

played the organ, to the great delight of the congregation, and on

week-days he was occupied by giving lessons to private pupils, and

in preparation for public performances. He thus came to be busily

employed, and seems to have been in the enjoyment of comfortable

means.

 

[PLATE: 7, NEW KING STREET, BATH, WHERE HERSCHEL LIVED.]

 

From his earliest youth Herschel had been endowed with that

invaluable characteristic, an eager curiosity for knowledge. He

was naturally desirous of perfecting himself in the theory of

music, and thus he was led to study mathematics. When he had once

tasted the charms of mathematics, he saw vast regions of knowledge

unfolded before him, and in this way he was induced to direct his

attention to astronomy. More and more this pursuit seems to have

engrossed his attention, until at last it had become an absorbing

passion. Herschel was, however, still obliged, by the exigency of

procuring a livelihood, to give up the best part of his time to

his profession as a musician; but his heart was eagerly fixed on

another science, and every spare moment was steadily devoted to

astronomy.