Of all men who have combined both astronomical theory and practice, Bradley is one of the most remarkable. In this respect, we must assign to him the first place in English history; and if we were disposed to add, in that of the world, we are convinced that no country would pretend to offer more than one candidate to dispute his claim.

James Bradley^[1] was born in March 1692–3, at Sherbourn in Gloucestershire. He was educated at the Grammar School of Northleach, and admitted of Baliol College, Oxford, in March 1710–11, where he proceeded to the degrees of B.A. and M.A. in the years 1714 and 1717 respectively. His mother’s brother was James Pound (deceased 1724), rector of Wanstead in Essex, and known as an observer, particularly by the observations which he furnished to Newton, as described in the Principia. With him Bradley spent much of his younger life, and was his assistant in his astronomical pursuits; and some observations of 1718–19 on double stars are in good accordance with the relative motions which have been since established in the case of those bodies. His tables of Jupiter’s satellites, on which he was employed at the same time, show that he had detected the greater part of the inequalities in their motions which have since been observed.

In 1718 he was elected fellow of the Royal Society; in 1719 he was ordained to the vicarage of Bridstow, in Monmouthshire; in the following year he received a sinecure preferment. But in 1721 he resigned these livings, on obtaining the Savilian professorship of Astronomy at Oxford, the holder of which, by the statutes, must not have any benefice. To finish what we may call the gazette of his life, he was engaged in observation (with what results we shall presently see) both at Kew and Wanstead till 1732, when he went to reside at Oxford, having since 1729 given yearly courses of lectures on Experimental Philosophy. In 1742 he was appointed to succeed Halley as Astronomer Royal, and he held this appointment for the remainder of his life. In the same year he obtained the degree of D.D. In 1752, having refused the living of Greenwich, because he thought the duty of a pastor to be incompatible with his other studies and necessary engagements, he was presented with a pension of 250l. The last observation made by him in the observatory is dated Sept. 1, 1761; and he died July 13, 1762, at Chalford in Gloucestershire, having been afflicted by various diseases for several years, and particularly by a depression of spirits, arising from the fear lest he should survive his faculties. He married in 1744, and left one daughter, who died at Greenwich in 1812.

There are now no lineal descendants of Bradley. Most of his writings, which were few in number, were published in the Philosophical Transactions. His personal merits are proved by the number of his friends, and the warmth with which they endeavoured to serve him when occasion arose, as well as by the strength of the testimonies which those who survived bore to his reputation as a man and a member of society.

We have much abridged the preceding account, in order to make room for a popular exposition of his two great discoveries—the aberration of light, and the nutation of the earth’s axis. If we were to blot these discoveries out of his life, there would remain an ample stock of useful labours, fully sufficient to justify us in stating that Bradley was unequalled as an observer, and of no mean character as a philosopher. But for the latter we must refer the reader to the excellent account from which our facts have been taken, or to any history of astronomy.

The parallax of the fixed stars had been long a subject of inquiry. If a body describe a circle, and a spectator on that body be unconscious of his own motion, all other bodies will appear to describe circles parallel to that of the spectator’s motion, and, absolutely speaking, equal to it; consequently, the greater the distance of the body from the spectator, the smaller will its apparent annual motion be; and it will not be circular, because the projection of the circle upon the apparent sphere of the heavens will foreshorten, and cause it to appear oval. If we suppose a star to describe an oval in the course of a year, the consequence will be that it will pass the spectator’s meridian sometimes before a star in the centre of the oval, sometimes after it; sometimes nearer to the pole of the heavens, and sometimes more distant; and the nature of the motion of this kind which would arise from parallax can be mathematically deduced. If the star be so distant that the oval is too small to be detected by measurement (which is hitherto the case with the fixed stars), then no alteration of place will be perceived on this account; but if an oval large enough to be observed be described in the course of a year, then the test of the phenomenon arising from the earth’s motion in its orbit is as follows:—Imagine a plane always passing through the centre of the sun, the centre of the earth, and the centre of the oval described by the star, then the place of the star in its oval must be in that plane; or draw the shortest distance on the globe from the centre of the oval to the sun, and the star will be on the point of the oval which lies in that distance.

In and before the time of Bradley, the refraction of light was not well determined, which would throw a doubt over any observations made to detect small quantities, unless the star which furnished them were situated in that part of the observer’s heaven in which there is no refraction, or next to none, that is, in or near his zenith. For the purpose of measuring annual parallax, therefore, stars had always been chosen which passed very nearly over the spot of observation, and instruments called zenith sectors (now almost out of use) were employed, which measured small angles of the meridian near the zenith, the latter point being ascertained by a plumb-line. Mr. Molyneux, a friend of Bradley, and a wealthy man, had caused the celebrated Graham to erect a large instrument of this kind at his house in Kew, afterwards the palace. Bradley and Molyneux observed with this instrument the star ? in the Dragon, which passed nearly through the zenith of that place, in December 1725. The star was found to pass the meridian more and more to the south of the zenith, until the following March, when it was about twenty seconds (about the sixty-five thousandth part of the whole circuit of the heavens) lower than at first. It was afterwards traced back again to its first position in the following December, allowing for the precession of the equinoxes. Other stars were examined in the same way, and the result was, that all stars were found to describe small^[2] ovals in the course of the year. But on comparing the situations of the stars in their small orbits with the corresponding places of the sun, it was evident that the cause of the phenomenon could not be the change of place arising from the orbital motion of the earth. Various hypotheses proposed by Bradley were found insufficient. In 1727 he erected a zenith sector for himself at Wanstead; and by further observations, and using different stars, he came at length to this fact, that instead of the star being in the place which annual parallax would give it, it was always in the position which it should have had a quarter of a year later: or that if the observer could measure the oval with sufficient exactness, and were to find the time of the year from the star, on the supposition of annual parallax being the cause of the star’s orbit, he would suppose himself in March instead of December, and so on.

That the phenomenon then had a regular connexion with the place of the earth was evident; but it was not that sort of connexion arising from the mere change of place of the earth. It is related^[3] that he was led to the true explanation by observing that the vane at the top of a boat’s mast changed its direction a little whenever the boat was put about, and made to go in a contrary direction; and that on his remarking that it was curious the wind should shift every time the boat was put about, he was assured by the boatmen that the same thing always happened. Be this as it may, he proposed to the Royal Society, in 1728, his beautiful explanation of the annual motion which he had observed in the stars; namely, that it is caused by the alteration in the apparent direction of the rays of light, arising from the earth being in motion. Suppose a stream of bullets fired into a carriage in motion, in a line perpendicular to its side, and so directed as to hit the middle of the first window, but not with sufficient velocity to reach any part of the second window. It is plain that they will strike the hinder pannel, which the motion of the carriage brings forward, and that to passengers in the inside the direction of the stream will appear to be from the middle of the window at which it enters to the opposite hinder pannel: whereas, had the carriage been at rest, it would have appeared to pass through the centre of both windows. And to make the stream really pass through both windows it must, if the carriage be in motion, be directed through the nearer window towards the foremost pannel on the other side. A ray of light is in the same situation with regard to the spectator, both as to the diurnal and the annual motion of the earth. The former gives an insensible aberration only; the latter, one which though small is sensible. The smallness of the latter aberration arises from the velocity of light being more than ten thousand times that of the earth in its orbit. And it must be remembered that the motion of light was not an hypothesis, invented to form the basis of Bradley’s explanation, but was ascertained before his time, by RÖmer, from a phenomenon of an entirely different nature; namely, the retardation observed in the eclipses of Jupiter’s satellites, as the planet moved from the earth. The absolute deduction of the laws of aberration was completed by Bradley.

The other great discovery of Bradley, namely, the nutation, or oscillatory motion of the earth’s axis, was completed in 1747. In his Wanstead observations he had observed some minute discrepancies, which at that time might be attributed to errors of observation; but after he was able to clear the apparent place of a star from the effects of aberration, the field became open to consider and assign the laws of smaller variations. By continual observation, he found a small irregularity in the places of the stars, depending upon the position of the moon’s node. Newton had already shown it to be a consequence of gravitation, that the sun must produce a small oscillation in the earth’s axis: Bradley showed that a larger oscillation must arise from the moon, and be completed in the course of a revolution, not of the moon, but of the point where her orbit cuts the ecliptic. This discovery is therefore not of so original a character as the last, since astronomers had for some time been in the habit of trying to reconcile every discrepancy which they observed by supposing a nutation; but to Bradley belongs the merit of discovering that small irregularity which really can be reconciled to such a supposition, and its physical causes. The easiest way of conceiving the effect of nutation is as follows:—The precession of the equinoxes, discovered by Hipparchus, has this effect, that the fixed stars, so called, appear to move round the pole of the ecliptic, at the rate of a revolution in about 26,000 years. Instead of a star, let a small oval describe the same course, and let the star in the mean while move round that oval in the course of nineteen years. The motion thus obtained will represent the combined effect of precession and nutation.

To these discoveries of Bradley we owe, as Delambre observes, the accuracy of modern astronomy. It must be remarked, that no individual, whose previous labours have caused public opinion to point him out as most fit for the part of Astronomer Royal, has ever been passed over when occasion occurred, from the time of Flamsteed to that at which we write. It is the fair reward of such a course, that the reputation which each successive occupant brought to that position should be considered as appertaining to him in the public capacity which it gained for him; and this being granted, it may be truly said that there is no institution in the world which has, upon the whole, done so much towards the advancement of correct astronomy as the Observatory of Greenwich.