December 19 1962 - The Aberration of Starlight

This particular programme was devised to mark the anniversary of the death of James Bradley, the third Astronomer Royal, one of whose major contribu­tions to astronomy was the discovery of aberration. Taking part with me was Colin Ronan; we were able to show films which we took in the Octagon Room at the original Greenwich Observatory, where one of Bradley's zenith telescopes still remains. Incidentally, the old Observatory is open to the public, and is well worth visiting.

James Bradley, the third Astronomer Royal, died in 1762. Now­adays, 200 years after his death, he is still remembered as a particularly accurate observer who carried out an immense amount of valuable work; yet, strangely enough, his most famous discovery was purely accidental.

In Bradley's time, our knowledge of the universe was compara­tively limited. The scale of the Solar System was reasonably well known, but little had been found out about the stars. For example, the distances of the stars were completely unknown; all that could be said with certainty was that they were very remote indeed. This was the problem which Bradley set out to attack; and though he was unsuccessful, his methods were perfectly sound.

Greenwich Observatory had been founded during the previous century mainly to provide British seamen with a reliable star- catalogue. In those days all sea navigation was uncertain; finding one's latitude was easy enough, but to determine longitude was extremely difficult, since no reliable timekeepers for use at sea had been developed. Astronomical observations appeared to be the only solution, and Charles II (who had many good points, as well as his famous weaknesses!) was sympathetic. In due course John Flamsteed, the first Astronomer Royal, produced a star catalogue which was by far the best of its time, though it is true that the invention of the chronometer made it unnecessary for the purpose originally intended.

Flamsteed's work was continued by his successor, Edmond Halley, who is best remembered for his work dealing with the comet which now bears his name. Upon Halley's death, in 1742, the Rev. James Bradley was appointed Astronomer Royal. The choice was an excellent one. Bradley, then professor of astronomy at Oxford, had already made his name as a skilful observer, and he proved to be equally good as an organizer. The observatory buildings, originally designed by Wren, were extended, and new instruments were purchased. In 1749 King George II made a grant of £ 1,000 for this purpose; among the instruments so acquired was a brass quadrant, made by John Bird at a cost of £300, which is still at Flamsteed House - the old Greenwich Observatory. Before his death, Bradley had completed a catalogue of 60,000 stars which was of remarkable accuracy, and was of permanent value. Bradley had in fact begun his main work before he became Astronomer Royal; but before describing his results, something must be said about his methods.

Most people are familiar with the principle of parallax; sur­veyors make use of it to measure the distances of objects which are inaccessible for some reason or other. If such an object is observed against a background, it will appear to shift according to the direction from which it is viewed. Provided that the length of the base-line is known, and also the angle of shift, the distance of the object may be calculated. The diagram overleaf will make this clear. Object A is observed first from point X and then from point Y; knowledge of the length XY and also the angle XAY will lead to a determination of the distance of A.

To measure star-distances, Bradley needed an immensely long base-line, and the only one available was the orbit of the Earth. He knew the approximate distance between the Earth and the Sun (93,000,000 miles); therefore the orbit must provide a base-line of twice this amount (186,000,000 miles) if observations are made at intervals of a half-year. In the diagram, X now becomes the Earth in January and Y the Earth in July, point S marking the position of the Sun. A will represent a relatively near star, seen against a background of more distant stars. All that Bradley had to do was to measure the shift, which would be a measure of the star's parallax.

This was simple enough in theory, but extremely troublesome in practice, because the parallax shift would be very small. There was also the difficulty of selecting a star which was comparatively close — no easy matter when all star distances were so completely un­known. We now know that even Bradley's observations, accurate though they were, were not good enough to allow him to solve the problem.

The star selected for observation was Gamma Draconis, which is of the second magnitude, and which lies not far from the brilliant Vega in Lyra. In fact, it is now known that Gamma Draconis is not one of our nearest stellar neighbours, since it lies 104 light-years away, so that it was a bad choice; but Bradley concentrated upon it because Gamma Draconis passes over the zenith at Greenwich, and could therefore be studied with a special instrument known as a zenith telescope or zenith sector. This is a telescope provided with a plumb line to ensure that it is vertical. Stars very close to the zenith are observed, and the telescope is moved on to the star by a screw which is mounted conveniently close to the eyepiece. The instrument is adjusted until the star is exactly in the centre of the field; the graduations on the screw give the exact position of the telescope, and hence the exact position of the star.

Bradley's zenith sector had been made in 1727 by George Graham, one of the most celebrated instrument-makers of the time. Originally it had been erected at Wanstead, at the house of Bradley's aunt, widow of the astronomer James Pound, and it was actually at Wanstead that Bradley made the observations which led him on to his greatest discovery. The instrument was shifted to Greenwich in 1749, and is still to be seen there, in the Octagon Room at Flamsteed House. The tube is 12J feet long, and the aperture of the object-glass is 2 85 inches.

Late in 1728 Bradley began work upon Gamma Draconis, hoping to find an annual shift which could be explained as being due to parallax. He found movement indeed, but before long it became clear that parallax was not the cause. The phenomenon was entirely different, and for some time Bradley was at a loss. There is a famous story that he realized the truth one day when he was out sailing on the Thames. He noticed that when the boat changed its course, the pennant at the masthead pointed in a different direction; when he mentioned it to the waterman, he was told that this was because the pennant flew in a direction which depended upon the wind as well as on the boat's direction. The story may or may not be true, but in any case Bradley saw that the puzzling shift of Gamma Draconis was due to a combination of two movements. One was the motion of the Earth; the other, the motion of light down the long telescope tube.

If you walk along in a rainstorm, holding up an umbrella, you must tilt the umbrella forward if you wish to avoid being drenched. If you alter your direction, you must also alter the tilt of your umbrella. What Bradley observed in Gamma Draconis was an effect similar in principle. Light travels at a finite speed, and the Earth itself is in motion, so that the telescope tube must also be 'tilted' according to the changing direction of the Earth in its yearly orbit round the Sun.

The velocity of light had been measured during the previous century by the Danish astronomer Ole Romer. Bradley had re­determined it, and his value was remarkably close to the true figure of approximately 186,000 miles per second. Consequently, his measures of the shifts of Gamma Draconis - the aberration of the star's light - enabled him to Work out that the velocity of light is 10,000 times faster than the velocity of the Earth in its orbit. It had long been known that our world moved round the Sun, but it may be said that Bradley was the first to provide direct observational proof of the Earth's motion. When he died, after a regime at Greenwich lasting for twenty years, the original problem of star distances was still unsolved, but it is worth noting that Bradley made a considerable contribution to it. Needless to say, all stars show aberration effects, and these must be disentangled from the genuine parallax shifts, so that until the aberration is known it is quite impossible to determine the amount of parallax. And though his discovery of aberration was in the nature of an accident, it is none the less creditable for that. It could never have been made without Bradley's superb skill as an observer; the fact that he was unable to measure parallax was due solely to the inadequacy of his instruments, even though they were probably the best in existence at that time.

Oddly enough, the same problem led to another accidental discovery many years later. William Herschel, who discovered the planet Uranus in 1781 but whose most important contributions were in the field of stellar astronomy, returned to the whole matter, and concentrated upon observing double stars. He believed, naturally enough, that a double star was due to nothing more than a line of sight effect, with one component lying more or less in front of the other as seen from the Earth; the nearer component should, then, show a parallax shift relative to the more distant. Like Bradley, he failed - but for a different reason. It became obvious that in most cases, at least, the two stars of a double were genuinely associated, forming what is termed a binary system in revolution round a common centre of gravity. Since the components were, therefore, equally distant from us, no relative parallax could be expected.

Success came finally in the late eighteen-thirties, when three parallax measures were made: by F. W. Bessel (61 Cygni, a faint star in the Swan); Henderson, at the Cape of Good Hope (Alpha Centauri); and Struve (Vega in Lyra). The main credit is justly given to Bessel, who was the first to announce his results, though in point of fact Henderson's measures were made slightly earlier. All these stars are much closer than Gamma Draconis. Alpha Centauri, indeed, lies at a distance of not much more than four light-years, so that its annual parallax is much greater than that of Gamma Draconis. Proxima, a faint member of the Alpha Centauri, remains the closest known star with the obvious exception of our own Sun.

The method of trigonometrical parallax has its limitations, but hundreds of stars have had their distances determined in this way, and the whole principle has been of the greatest importance in modern astrophysics. So in these days of vast telescopes and space- rockets, it is only fair to pause for a moment and remember James Bradley, that great astronomer who died 200 years ago.