Thursday, July 16, 2009


In physics, the apparent change of direction of a light source that results from motion of the observer across the line of sight. To a person on earth, the stars appear to exhibit a small annual variation in position. The apparent motion is produced by the orbital motion of the earth. This aberration is not to be confused with the annual parallax, caused by the shift in the location of the earth during the year. Aberration is analogous to the sidewise drift of a boat crossing a river. Because of the current, the actual path of the boat deviates from direction in which it is headed. Similarly, light from a star appears to come from a direction determined by the combined velocities of the earth and of light. For a star in the direction of the earth’s motion, the effect is zero. For a star viewed at right angles to the motion, the aberration angle attains its maximum value of 20.5 inch, or .0001 radian, which is the ratio of the orbital speed of the earth to that of light.

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