ABERRATION OF LIGHT. An expression used to describe the phenomena that arise from the fact that light requires appreciable time for its transmission through space. The motion of light traveling from a star or a planet toward the earth, combined with the earth's own motion, causes an apparent displacement of the stars on the sky: they all appear to occupy positions a little different from their true ones. In explaining this phenomenon, we often use the analogy of a man running in a rain-storm. Though the raindrops may be fall ing straight down, they will seem to the running man to descend on his face slantingly. Light, too. may be coming down, as it were, vertically, but as the earth, with the observer on it, is hurrying through space, there will be produced a similar apparent slant of the light, and we shall see the stars displaced on the sky in the direction of the terrestrial motion. But since the motion of our planet takes place in a closed, oval curve, the apparent displacement of the stars is now in one direction. and now in another, corre
sponding to the earth's position in one or the other half of its oval path. The result is that the stars themselves seem to move each year through a small curve; and this is a sort of miniature reproduction of the earth's orbit around the sun. the celestial body under observation is itself in motion with respect to our earth, as is the case with the other planets of the solar system, a further somewhat analo gous displacement is produced. Astronomers therefore need to correct all their observations by a process of calculation, so as to reduce them to what they would be if no such thing as aber ration existed. Aberration was discovered by James Bradley, and was announced to the Royal Society of England in 1729.