Astronomical Discovery

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appreciate their essential features properly without some biographical study. But the record of his life apart from his astronomical work is not in any way sensational; indeed it is singularly devoid of incident. He had not even a scientific quarrel. There was scarcely a man of science of that period who had not at least one violent quarrel with some one, save only Bradley, whose gentle nature seems to have kept him clear of them all. Judged by ordinary standards his life w

birth and

he Church, being ordained in 1719, and presented to the vicarage of Bridstow in Monmouthshire; but he only discharged the duties of vicar for a couple of years, for in 1721 he returned to Oxford as Professor of Astronomy, an appointment which involved the resignation of his livings; and so slight was this interruption to his career as an astronomer that we may almost disregard it, and consider him as an astronomer from the first.Brief clerical career. But to guard against a possible misconception, let me say that Bradley entered on a clerical career in a thoroughly earnest spirit; to do otherwis

ronomy not

h would be gratified. But Bradley got no astronomical teaching at Oxford either from Halley or others.but from his uncle, James Pound. The art of astronomical observation he learnt from his maternal uncle, the Rev. James Pound, Rector of Wansted, in Essex. He is the man to whom we owe Bradley's training and the great discoveries which came out of it. He was, I am glad to say, an Oxford man too; very much an Oxford man; for he seems to have spent some thirteen years there migrating from one Hall to another. His record indeed was such as good tutors of colleges frown upon; for it was seven years before he managed to take a degree at all; and he could not settle to anything. After ten years at Oxford he thought he would try medicine; after three years more he gave it up and went out in 1700 as chaplain to the East Indies. But he seems to have been a thoroughly lovable man, for news was brought of him four years later that he had a mind to come home, b

g their observations one after the other on the same paper; and it was to the pair of them together, rather than to the uncle alone, that Newton made his princely presents, and Halley wrote for help in his observations. There seems to be no doubt that the uncle and nephew were about this time the best astronomical observers in the world. There was no rivalry between them, and therefore there is no need to discuss whether the partnership was one of equal merit on both sides; but it is in

ne by Pound

-two pages of foolscap, and the other sixty; and it must be remembered that the calculations themselves were quite novel at that time. Of his skill in calculation, apart from his assiduity, we have a proof in a paper communicated to the Royal Society rather later (1726), where he determines the longitudes of Lisbon and New York from the eclipses of Jupiter's satellites, using observations which were not simultaneous, and had therefore to be corrected by an ingenious process which Bradley devised expressly for this purpose.Use of very long telescopes. And finally, his skill in the management of instruments is shown by his measuring the diameter of the planet Venus with a telescope actually 212? feet in length. It is difficult for us to realise in these days what this means; even the longest telescope of modern times does not exceed 100 feet in length, and it is mounted so conveniently with all the resources of modern engineering, in the shape of rising floors, &c., that the management of it is no more difficult than that of a 10-foot telescope. But Bradley had no engineering appliances beyond a pole to hold up one end of the telescope and his own clever fingers to work the other; and he managed to point the unwieldy weapon a

ppointed

by Sir Isaac Newton himself. He was accordingly elected on October 31, 1721, and forthwith resigned his livings. His resignation of the livings was necessitated by a definite statute of the University relating to the Professorship, and not by the existence of any very onerous duties attaching to it; indeed such duties seem to have been conspicuously absent,but continues to work at Wansted. and after Bradley's election he passed more time than eve

l Mol

eory that no such change could be detected. In the old days before the telescope it was perhaps easy to understand that the change might be too small to be noticed, but the telescope had made it possible to measure changes of position at least a hundred times as small as before, and still no "parallax," as the astronomical term goes, could be found for the stars. The observations of Galileo, and the measures of Tycho Brahé, as reduced to systematic laws by Kepler, and finally by the great Newton, made it clear that the Copernican theory was true: but no one had succeeded in proving its truth in this p

ts position is slowly changing owing to the precession of the equinoxes, but for two centuries it has been, and is still, under constant observation by London

a view of either confirming them or seeing what was wrong with them that Molyneux and Bradley started their search. They set up a much more delicate piece of apparatus than Hooke had employed.The instrument. It was a telescope 24 feet long pointed upwards to the star, and firmly attached to a large stack of brick chimneys within the house. The telescope was not absolutely fixed, for the lower end could be moved by a screw so as to make it point accurately to the star, and a plumb-line showed how far it was from the vertical when so

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her away the star is, the less will this inclination or "parallax" be; and the star is actually so far away that the inclination can only be detected with the utmost difficulty: the lines C K and A K are sensibly parallel to D B K. But Bradley did not know this; it was just this point which he was to examine, and he expected the greatest inclination in one direction to be in December. Accordingly when a few observations had been made on December 3, 5, 11, and 12 it was thought that the star had been caught at its most southerly apparent position, and might be expected thereafter to move northwards, if at all.Unexpected results. But when Bradley repeated the

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nt direction, were to be executing an oscillation, then all our plumb-lines would oscillate, and when the direction of a star like γ Draconis was compared with that of the plumb-line it would seem to vary, owing actually to the variation in the plumb-line. The earth might have a motion of this kind in two ways, which it will be necessary for us to distinguish, and the adopted names for

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g or following γ Draconis by a constant interval. Most of them would be too faint for observation with Bradley's telescope; but there was one bright enough to be observed, which also came within its limited range, and it was promptly put under surveillance when a nutation of the earth's axis was suspected. Careful watching showed that it was not affected in the same way as γ Draconis, and hence the movement could not be in the plumb-line. Was there, then, after all, some effect of the earth's atmosphere which had been overlooked? We have already remarked that

g.

is best seen by a diagram. First, it must be remarked that rays of light are o

oblique ray, D E, would be bent on entering the atmosphere at E along the path EF, and a star shining along D E would appear from F to be shining along the dotted line G E F. The atmosphere is not of the same density throughout, but thins out as we go upwards from the earth; and in cons

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e at A, and there would be no refraction. Similarly in "December" the light would fall at C on the stern, also vertically, and there would be no refraction. [The rays from the distant star in December are to be taken as sensibly parallel to those received in June, notwithstanding that the earth is on the opposite side of the sun, as

d not lie in this plane, nor did they lie in directions equally inclined to it. Making the proper allowance for thei

nding real

loss was becoming mellowed with time. His uncle's widow was only too glad to welcome back her nephew, though no longer to the old rectory, and she allowed him to set up a long telescope, even though he cut holes in her floor to pass it through. The object-glass end was out on the roof and the eye end down in the coal cellar; and accordingly in this coal cellar Bradley made the observations which led to his immortal discovery. He had a list of seventy stars to observe, fifty of which he observed pretty regularly. It may seem odd that he did not set up this new instrument at Oxford, but we find from an old memorandum that his professorship was not bringing him in quite ￡140 a year, and probably he was glad to accept his aunt's hospitality for reasons of economy. By watching these different stars he gradually got a clear conception of the laws of aberration. The real solution of the problem, according to a well-authenticated account, occurred to him almost accidentally.Finds the right clue. We all know the story of the apple falling and setting Newton to think about the causes of gravitation. It was a similarly trivial circumstance which suggested to Bradley the explanation which he had been seeking for two or three years in vain. In his own words, "at last, when he despaired of bein

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gy of

lt the umbrella. This analogy was stated by Lalande before the days of umbrellas in the following words: "Je suppose que, dans un temps calme, la pluie tombe perpendiculairement, et qu'on soit dans une voiture ouverte sur le devant; si la voiture est en repos, on ne re?oit pas la moindre goutte de pluie; si la voiture avanc

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opposite directions in the two cases. Hence the telescope must be tilted a little, in opposite directions, to catch the light; or, in other words, the star will appear to be farthest south in March, farthest north in September. And so at last the puzzle was solved, and the solution was found, as so often happens, to be of the simplest kind; so simple when once we know, and so terribly hard to imagine when

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are correct to a minute," he was wont to say, "and the fractions do not so much matter." With such a precept Bradley would never have made his discoveries. No quantity was too small in his eyes, and no sooner was the explanation of aberration satisfactorily established than he perceived that though it would account for the main facts, it would not explain all. There was something left. This is often the case in the history of science. A few years ago it was thought that we knew the constitution of our air completely-oxygen, nitrogen, water vapour, and carbonic acid gas; but a great physicist, Lord Rayleigh, found that after extracting all

thing to b

h would go through a cycle in about nineteen years, the period in which the moon's nodes revolve. He was not mathematician enough to work out the cause completely, but he saw clearly that to trace the whole effect he must continue the observations for nineteen years; and accordingly he entered on this long campaign without any hesitation. His instrument was still that in his aunt's house at Wansted, where he continued to live and make the observations for a few years, but in 1732 he removed to Oxford, as we shall see, and he must have made many journeys between Wansted and Oxford in the course of the remaining fifteen years during which he continued to trace out the effects of nutation. His aunt too left Wansted to accompany Bradley to Oxford, and the house passed into other hands.His nineteen years' campaign. It is to the lasting credit of the new occupant, Mrs. Elizabeth Williams, that the great astronomer

nce at

ying to find the solution: he had practically made up his mind about the solution, and the actual observations were to go on in a quiet methodical manner for nineteen years, so that he now had leisure to look about him for other employment. Dr. Keill, who had been Professor of Astronomy before Bradley, had attracted large classes to lectures, not on astronomy, but on experimental philosophy: but had sold his apparatus and goodwill to Mr. Whiteside, of Christ Church, one of the candidates who were disappointed by Bradley's election. In 1729 Bradley purchased the apparatus from Whiteside, and began to give lectures in experiment

Royal at

n.Letter from Earl of Macclesfield. "It is not only my friendship for Mr. Bradley that makes me so ardently wish to see him possessed of the position," wrote the Earl of Macclesfield to the Lord Chancellor; "it is my real concern for the honour of the nation with regard to science. For as our credit and reputation have hitherto not been inconsiderable amongst the astronomical part of the world, I should be extremely sorry we should forfeit it all at once by bestowing upon a man of inferior skill and abilities the most honourable, though not the most lucrative, post in the profession (a p

n entertain the least hope of success against him. But, my lord, we live in an age when most men how little soever their merit may be, seem to think themselves fit for whatever they can get, and oft

r Royal, though he did not resign his professorship at Oxford. Halley, Bradley, and Bliss, who were Astronomers Royal in succession, all held the appoin

ts very d

the screws were broken. There was no proper means of illuminating the cross-wires, and so on. With care and patience Bradley set all this right, and began observations. He had the good fortune to secure the help of his nephew, John Bradley, as assistant, and the companionship seems to have been as happy as that previous one of James Bradley and his uncle Pound. John Bradley was able to carry on the observations when his uncle was absent in Oxford, and the work the two got thro

n this respect. He managed to persuade the nation to furnish the funds for an equipment; but Halley, though a man of great ability in other ways, did not know a good instrument from a bad one; so that Bradley's first few years at the Observatory were wasted owing to the imperfection of the equipment.New instruments. When this was fully realised he asked for funds to buy new instruments, and such was the confidence felt in him that he got what he asked for without much difficulty. More than ￡1000, a large sum for those days,

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f what he accomplished, and then recall a particular incident, which shows how far ahead of his gene

of the movements of

ated directly from his own observation

on the length o

r knowledge of the refr

f the moon, and the promotion of the method

nstruments, and the use of these to form the basis of our

self a sufficient title to fame. Bradley accomplished them all

und variation

twenty years ago not to exist appreciably; but the work of Küstner and Chandler led to the discovery that it did exist, and its complexities have been unravelled, and will be considered in the sixth chapter. Now a century and a half ago Bradley was on the track of this "variation of latitude." His careful observations actually showed the motion of the pole, as Mr. Chandler has recently demonstrated; and, moreover, Bradley himself noticed that th

was created D.D. by diploma in 1742; and when his discovery of nutation was announced in 1748, and produced distinctions and honours of all kinds from over the world, we are told that "amidst all these distinctions, wide as the range of modern science, and permanent as its history, there was one which probably came nearer his heart, and was still more gratifying to his feeling than all. Lowth (afterwards Bishop of London), a popular man, an elegant scholar, and possessed of considerable eloquence, had in 1751 to make his last speech in the Sheldonian Theatre at Oxford as Professor of Poetry. In recording the benefits for which the University was

f "residual

once, since Bradley himself was puzzled for several years: that after finding one vera causa, and allowing for the effect of it, the observations may show traces of another which must again be patiently hunted, even though we spend nineteen years in the chase: and that again we may have to leave the complete rectification of the observations to posterity. But though we may admit the general helpfulness of these directions, and that this patient dealing with residual phenomena seems to be a meth