Great Astronomers - Robert Stawell Ball (the little red hen ebook .TXT) 📗
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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.
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