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 CHAPTER II HISTORICAL SURVEY

6. The phenomenon of the astronomical aberration of light was discovered by Bradley as the outcome of an effort, conducted with unusual care, to detect traces of annual parallax in certain stars which passed near his zenith, and so were amenable to accurate measurement. His observations exhibited displacements in the position of each star, which had the expected period of a year; but instead of being towards the Sun they were towards a perpendicular direction, that of the Earth's motion in its orbit, while they followed the same law of the sine of the inclination as parallax would do. After many attempts to coordinate his observations, the clue to the aberrational method of representing them was suggested to Bradley, it is said, by casual observation of a flag floating at the masthead of a ship; when the ship changed its course, the flag flew in a different direction. The analogy is rather more direct when it is the drift of the clouds that is the object of remark; the apparent direction from which they come is different from me real direction when the observer is himself in motion relative to the air that carries them. So, the observer of the star being in motion along with the Earth, the apparent direction in which the light from the star appears to him to come may be expected to be different from its real direction; thus leading to the usual elementary representation of the