BRADLEY, JAMES. 



BRADLEY, JAMES. 



884 



astronomical observation. As early as 1716 there is a letter of Halley 

 to Found mentioning Bradley as an observer; and in 1718 and 1719, 

 we find some observations of double stars (Castor and y Virginia), 

 which have since been used by Sir J. Herschel in his determination of 

 the orbits which each of the pairs just mentioned describes round the 

 other (' Mem. R. Astron. Soc.,' vol. v., pp. 195, 202). At the same 

 time he turned his attention to the motions of Jupiter's satellites, and 

 detected, by observation, the greater part of the inequalities afterwards 

 discussed by Bailli. Tables of the satellites, from Bradley's observa- 

 tions, were published in Halley's collection, London, 1749, and in 

 ' Phil. Trans.' vol. xxx. 



Bradley was entered of Balliol College, Oxford, in 1710, and took 

 the degrees of B.A. and M.A. in 1714 and 1717. In 1718 he became a 

 fellow of the Royal Society. In 1719 he was ordained to the rectory 

 of Bridstow, in Monmouthshire. In 1720 he obtained another living, 

 but in 1721 resigned his preferments on obtaining the Savilian Profes- 

 sorship of Astronomy at Oxford, with the holding of which they are 

 incompatible. He also resigned the office of chaplain to Bishop Hoatily. 

 We find him now engaged in miscellaneous observation, particularly 

 with the long telescope introduced by HCYGHENS. With one of these 

 of 212 feet focal length, he measured the diameter of Venus in 1722. 

 Pound died in 1724, and in the next year Bradley began the observa- 

 tions which led to his great discover}-. 



The circumstances connected with the discovery of ABERRATION are 

 described under that title in the ARTS AND SCIENCES DIVISION of the 

 ENGLISH CYCLOPEDIA. The scene of the first observations was at the 

 house of Mr. Molyneux at Kew, which afterwards became the palace 

 of that name, but was some years back pulled down. The associated 

 observations of Bradley and Molyneux detected the motion of 7 Dra- 

 coni, and other stars, and established approximately the law of the 

 motion of the first. That the motion in declination depended in some 

 way or other on the latitude of the star was evident, and in this state 

 the matter stood, when Bradley in 1727 erected a zenith sector for 

 himself at Wanstead. The original entry of the first night's observa- 

 tion at Kew, which confirmed the fact of an unexplained motion in 7 

 Uraconis (Dec. 21, 1725), is preserved in Bradley's own handwriting. 

 The following, written on a torn bit of paper, is the earliest of the 

 observed phenomena which led to the greatest discovery of a man who 

 lias, more than any other, contributed to render a single observation of 

 a star correct enough for the purposes of astronomy : 



"Dec 21" Tuesday 5 h 40' sider. time 

 Adjusted y c mark to y e Plumb Line 

 & then y e Index stood at 8 

 5 h 48' 22" y e star entred 

 49 52J Star at y e Cross 

 51 24 Star went out 

 s could 



At soon as I let go y course 

 screw I perceived y e Star too 

 much to y" right hand & 

 so it continued till it passed 

 j' Cross thread and within a quarter 



was 

 of a minute after it had passed 



graduat 



I turned y" fine screw till I saw 

 y e light of y c star perfectly 



Dissected, and after y c obser 

 vation I found y" index 

 at llj. so that by this 

 observation y e 

 mark is about 3" :; 

 too much south, 

 but adjusting 



y e mark and plumbline 

 I found y e Index at 84." 



Bradley began his observations at Wanstead with a better instru- 

 ment than that at Kew, and capable of taking in a larger range of the 

 heavens. He soon confirmed the general fact which he had observed, 

 and it only remained to assign the cause. There is traditional evidence 

 to the following anecdote, first given by Dr. Thomson in his ' History 

 of the Royal Society ,' and adopted by Professor Rigaud ; but for which 

 the real authority is Dr. Kobison, who was old enough to have possibly 

 heard it from one of Bradley's contemporaries. Dr. Robison has given 

 the anecdote himself iu a part of his ' Mechanical Philosophy,' where 

 we should certainly not have gone to look for it, nor, we imagine, 

 would Professor Rigaud : namely, in the chapter on ' Seamanship,' 

 voL iv. p. 629 : " When he despaired of beiug able to account for the 

 phenomena which he had observed, a satisfactory explanation of it 

 occurred to him all at once when he was not in search of it. He 

 accompanied a pleasure party in a sail upon the river Thames. The 

 boat in which they were was provided with a mast which had a vane 

 upon the top of it. It blew a moderate wind, and the party sailed up 

 and down the river for a considerable time. Dr. Bradley remarked, 

 that every time the boat put about, the vane at the top of the boat's 

 mart shifted a little, as if there had been a slight change in the direc- 

 tion of the wind. He observed this three or four times without speak- 



ing : at last he mentioned it to the sailors, and expressed his surprise 

 that the wind should shift so regularly every time they put about. 

 The sailors told him that the wind had not shifted, but that the appa- 

 rent change was owing to the change in the direction of the boat, and 

 assured him that the same thing invariably happened in all cases." 

 By tracing this phenomenon to its cause, namely, the combined motion 

 of the boat and the wind, he was enabled to give the solution of the 

 star's motion, namely, a small change of place arising from the spec- 

 tator giving to the ray of light the effects of his own motion, as ex- 

 plained in the article Aberration, referred to above. 



Upon this discovery, several observations must be made, relative to 

 its importance in astronomy. It is the first positively direct and 

 unanswerable proof of the earth's motion. In the next place, the 

 explanation given was not purely an hypothetical one, or one which 

 would allow of any velocity being attributed to light which would 

 best answer to observed phenomena, but required that the velocity 

 already measured by Romer's observations of the retardation of the 

 eclipses of Jupiter's satellites should be the sufficient reason for the 

 annual oscillations of the fixed stars. A very simple geometrical 

 analysis of the problem shows that when the angle of aberration is 

 greatest, its sine must be the quotient of the earth's Telocity divided 

 by the velocity of light. Taking the first at 18 miles per second, 

 depending upon the correctness of the measurement of the earth's 

 orbit and of the length of the year, and the second at 200,000 miles 

 per second, which depends upon a third and distinct phenomenon, 

 namely, the observations of the time of eclipses of Jupiter's satellites 

 at different periods of the year, we find a priori, that the sine of the 

 greatest angle of aberration, if aberration there be, must be '00009, 

 which is the sine of 19 seconds nearly, and has been made iu round 

 numbers. The greatest aberration from the mean place observed by 

 Bradley was 20 seconds and two-tenths, in which the most correct 

 modern observations, in masses of thousands at a time, have not 

 shown an error of more than three-tenths of a second. This is one 

 of the reasons why we have said that, in the union of theoretical 

 sagacity with practical excellence, Bradley stands unrivalled. Newton, 

 Laplace, &c. were not observers. Flamsteed, Cassiui, &c. were not 

 great theorists. Halley, who of all the men of Bradley's time, united 

 the largest knowledge of both, was so far from being the equal of 

 Bradley in minuteness of observation, that he constantly declared his 

 suspicion of the impossibility of detecting a part of a second. Kepler 

 was skilful in the detection of the laws which phenomena follow, but 

 not in that of physical causes. In our opinion, Hipparchus is (dif- 

 ference of circumstances considered) the prototype of Bradley. The 

 time of the discovery of the cause of aberration was probably about 

 September 1728 ; it was communicated immediately to the Royal 

 Society ('Phil. Trans.,' No. 406, vol. xxxv., p. 637). 



In 1728 Bradley began lectures at Oxford, and in 1732 removed his 

 residence to that university. We pass over the various labours by 

 which he sustained the character of the " best astronomer in Europe," 

 given to him by Newton, and proceed to the year 1742, when he was 

 appointed astronomer royal. This was almost the last act of Sir 

 Robert Walpole's administration, who, as Professor Rigaud has well 

 observed, " appears to have determined that one of the first points he 

 would secure before his retirement was the nomination in question : 

 he declared his intention of resigning in the House of Commons on 

 the 2nd of February, and Bradley's appointment was dated tho 3rd." 

 From this time to 1747 he was engaged (among other things) in the 

 career of observation which led to his second great discovery of 

 nutation communicated iu that year ('Phil. Trans.' No. 485, voL xlv., 

 p. 1.) The phenomenon in its most simple state may be thus repre- 

 sented : the earth's axis, instead of describing a cone, describes a 

 fluted cone; or, the pole of the equator, iustead of moving uniformly 

 round the pole of the ecliptic in a small circle, describes a wavy or 

 undulating curve with a milled edge, if we may so speak, with about 

 1400 undulations in a complete revolution. The merit of Bradley 

 consists, firstly, in his determinatiou of so small a quantity, since tho 

 greatest effect of nutation is only half that of aberration, and dis- 

 tributed through nineteen years instead of one ; secondly, in his dis- 

 covery of the circumstance on which it depends, namely, the position 

 of the moon's orbit with respect to the equator. This orbit shifts the 

 position of its nodes gradually, making them complete a revolution in 

 about eighteen years and a half. This was also found to be tho 

 period in which the pole of the equator describes one of the waves 

 above mentioned, and subsequent investigation has confirmed tho 

 dependence of the greater part of tlie nutation on the motion of the 

 moon's node, by showing the former to be a consequence of the non- 

 sphericity of the earth, and of the moon's attraction on the protu- 

 berant parts. [PRECESSION AND NUTATION, iu ARTS AND SCIENCES 

 Div.] 



There is a third investigation of Bradley which stands out from the 

 rest, and displays considerable mathematical sagacity : we refer to his 

 empirical formula for the law of refraction. He was assisted in the 

 necessary computations by Maskelyne, who first appeared before the 

 world as the pupil of Bradley. In this very delicate research, Bradley 

 had again gone beyond his contemporaries in the evaluation of minute 

 quantities. His table is even yet very good for the first forty-five 

 degrees of zenith distance ; and his determination of the latitude of 

 Greenwich (an investigation depending for its accuracy upon that of 



