“If the proper motion of the stars be admitted,” said Herschel, “who can deny that of our Sun?” Of course it was plain that the motion of the Sun could only be detected through the resulting apparent motion of the stars. Thus, if the Sun is moving in a certain direction, the stars in front will appear to open out, while those behind will close up. But the problem is by no means so easy as this. The stars are also in motion, and, before the solar motion can be discovered, the proper motions of the stars—themselves very minute—have to be decomposed into two parts, the real motion of the star, and the apparent motion, resulting from the movement of the Solar System. To any astronomer but Herschel the problem would have been insoluble. Only sixty years had elapsed since Halley had announced the proper motions of the brighter stars which had been previously supposed to be immovable—hence the name of “fixed stars.” Herschel did not deal with the motions of many stars. Only a few proper motions were known with accuracy when he attacked the problem in 1783. Making use of the proper motions of seven stars, and separating the real from the apparent motion, he found that the Solar System was moving towards a point in the constellation Hercules, the “apex” being marked by the star λ Herculis. The rate of the solar motion, Herschel thought, was “certainly not less than that which the Earth has in her annual orbit.” This extraordinary discovery was one of Herschel’s greatest works. “Its directness and apparent artlessness,” Miss Clerke remarks, “strike us dumb with wonder.” In 1805 Herschel again attacked the subject, utilising the proper motions of thirty-six stars. His second inquiry, on the whole, confirmed his previous result, the “apex” being again situated in Hercules; but the determination of 1783 was probably the more accurate of the two. Herschel was far in advance of his time regarding the solar motion. The two greatest astronomers of the next generation, Bessel and Sir John Herschel, rejected the results reached by Sir William Herschel. But in 1837 Argelander, after a profound mathematical discussion, confirmed Herschel’s views, and proved the solar motion to be a reality. Since that date the problem has been attacked by various methods by Otto Struve, Gauss, Mädler, Airy, Dunkin, Ludwig Struve, Newcomb, Kapteyn, Campbell, and others, with the result that the reality of the solar motion and of the direction fixed by Herschel has been proved beyond a doubt. As Sir Robert Ball well remarks, mathematicians have exhausted every refinement, “but only to confirm the truth of that splendid theory which seems to have been one of the flashes of Herschel’s genius.”