Halley's work is represented at the Observatory by two of his instruments which are still preserved there, and which hang on the west wall of the present transit room: the Iron Quadrant afterwards made famous by the observations of Bradley, and 'Halley's Transit,' the first of the great series of instruments upon which the fame of Greenwich chiefly rests. This transit instrument seems to have been set up in a small room at the west end of what is now known as the North Terrace. His quadrant was mounted on the pier which is now the base of the pier of the astrographic telescope. This pier was the first extension which the Observatory received from the original building.
On the breakdown of his health Halley nominated as his successor, James Bradley; indeed, it is stated that he offered to resign in his favour. He had known him then for over twenty years, and that keen and generous appreciation of merit in others which was characteristic of Halley had led him very early to recognize Bradley's singular ability.
James Bradley was born in 1692 or 1693, of an old North of England family. His birthplace was Sherbourne, in Gloucestershire, and he was educated at North Leach Grammar School and at Baliol College, Oxford. During the years of his undergraduateship he resided much with his uncle, the Rev. James Pound, Rector of Wanstead, Essex, an ardent amateur astronomer, a frequent visitor at the Observatory in Flamsteed's time, and one of the most accurate observers in the country. From him, no doubt, he derived his love of the science, and possibly some of his skill in observation.
Bradley's earliest observations seem to have been devoted to the phenomena of Jupiter's satellites and to the measures of double stars. The accuracy with which he followed up the first drew the attention of Halley, and so began a friendship which lasted through life. His observations of double stars, particularly of Castor, only just failed to show him the orbital movement of the pair, because his attention was drawn to other subjects before it had become sufficiently obvious.
In 1719 Bradley and his uncle made an attempt to determine the distance of the sun through observations of Mars when in opposition, observations which were so accurate that they sufficed to show that the distance of the sun could not be greater than 125 millions of miles, nor less than about 94 millions. The lower limit which they thus found has proved to be almost exactly correct, our best modern determinations giving it as 93 millions. The instrument with which the observations were made was a novel one, being 'moved by a machine that made it to keep pace with the stars;' in other words, it was the first, or nearly the first, example of what we should now call a clock-driven equatorial.
That same year he was offered the Vicarage of Bridstow, near Ross, in Monmouthshire, where, having by that time taken priest's orders, he was duly installed, July, 1720. To this was added the sinecure Rectory of Llandewi-Velgry; but he held both livings only a very short time. In 1721 the death of Dr. John Keill rendered vacant the Savilian Professorship of Astronomy at Oxford, for which Bradley became a candidate, and was duly elected, and resigned his livings in consequence.
It was whilst he was Savilian Professor that Bradley made that great discovery which will always be associated with his name. Though professor at Oxford, he had continued to assist his uncle, Mr. Pound, at his observations at Wanstead, and after the death of the latter he still lived there as much as possible, and continued his astronomical work. But in 1725 he was invited by Mr. Samuel Molyneux, who had set up a twenty-four-foot telescope made by Graham as a zenith tube at his house on Kew Green, to verify some observations which he was making. These were of the star Gamma Draconis, a star which passes through the zenith of London, and which, therefore, had been much observed both by Flamsteed and Hooke, inasmuch as by fixing a telescope in an absolutely vertical position—a position which could be easily verified—it was easy to ascertain if there was any minute change in the apparent position of the star. Dr. Hooke had declared that there was such a change, a change due to the motion of the earth in its orbit, which would prove that the star was not an infinite distance from the earth, the seeming change of its place in the sky corresponding to the change in the place of the earth from which the observer was viewing it.
Bradley found at once that there was such a change—a marked one. It amounted to as much as 1´´ of arc in three days; but it was not in the direction in which the parallax of the star would have moved it, but in the opposite. Whether, therefore, the star was near enough to show any parallax or not, some other cause was giving rise to an apparent displacement of the star, which entirely masked and overcame the effect of parallax.
So far, Bradley had but come to the same point which Flamsteed had reached. Flamsteed had detected precisely the same apparent displacement of stars, and, like Hooke, had ascribed it to parallax. Cassini had shown that this could not be the case, as the displacement was in the wrong direction; and there the matter had rested. Bradley now set to follow the question up. Other stars beside Gamma Draconis were found to show a displacement of the same general character, but the amount varied with their distance from the plane of the ecliptic, the earth's orbit. The first explanation suggested was that the axis of the earth, which moves very nearly parallel to itself as the earth moves round the sun, underwent a slight regular 'wobble' in the course of a year. To check this, a star was observed on the opposite side of the pole from Gamma Draconis; then Bradley investigated as to whether refraction might explain the difficulty, but again without success. He now was most keenly interested in the problem, and he purchased a zenith telescope of his own, made, like that of Molyneux, by Graham, and mounted it in his aunt's house at Wanstead, and observed continuously with it. The solution of the problem came at last to him as he was boating on the Thames. Watching a vane at the top of the mast, he saw with surprise that it shifted its direction every time that the boat was put about. Remarking to the boatmen that it was very odd that the wind should change just at the same moment that there was a shift in the boat's course, they replied that there was no change in the wind at all, and that the apparent change of the vane was simply due to the change of direction of the motion of the boat.