But it was the first kind of change, the nutation, which Bradley suspected; and very early in the series of observations he had already begun to test this hypothesis. If it was not the star, but the earth and the plumb-line, which were in motion, then other stars ought to be affected. The telescope had been deliberately restricted in its position to suit γ Draconis; but since the stars circle round the Pole, if we draw a narrow belt in the heavens with the Pole as centre, and including γ Draconis, the other stars included would make the same circuit, preceding 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 since the star passes directly overhead there should be practically no refraction; and this assumption was made by Molyneux and Bradley in choosing this particular star for observation. It follows at once, if we assume that the atmosphere surrounds the earth in spherical layers.Anomalous refraction. But perhaps this was not so? Perhaps, on the contrary, the atmosphere was deformed by the motion of the earth, streaming out behind her like the smoke of a moving engine? No possibility must be overlooked if the explanation of this puzzling fact was to be got at.
Fig. 3.
The way in which a deformation of the atmosphere might explain the phenomenon is best seen by a diagram. First, it must be remarked that rays of light are only bent by the earth’s atmosphere, or “refracted,” if they enter it obliquely.
If the atmosphere were of the same density throughout, like a piece of glass, then a vertical ray of light, A B (see [Fig. 3]), entering the atmosphere at B would suffer no bending or refraction, and a star shining from the direction A B would be seen truly in that direction from C. But an 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 consequence there is no clear-cut surface, B E, and no sudden bending of the rays as at E: they are gradually bent at an infinite succession of imaginary surfaces. But it still remains true that there is no bending at all for vertical rays; and of oblique rays those most oblique are most bent.
Fig. 4.
Now, suppose the atmosphere of the earth took up, owing to its revolution round the sun, an elongated shape like that indicated in diagram 4, and suppose the star to be at a great distance away to the right of the diagram. When the earth is in the position labelled “June,” the light would fall vertically on the nose of the atmosphere 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 was remarked on p. 98.] But in March and September the rays would strike obliquely on the sides of the supposed figure, and thus be bent in opposite directions, as indicated by the dotted lines; and the extreme positions would thus occur in March and September, as had been observed. The explanation thus far seems satisfactory enough.