Aberration of light

Aberration of light (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon defined as an apparent motion of the heavenly bodies; stars describing more or less elliptic annual orbits, according to the latitude of the star; consequently at any moment the star appears to be displaced from its true position. This apparent motion is due to the finite speed of light relative to the velocity of an observer on the Earth. It has three components: diurnal aberration, annual aberration, and secular aberration. Secular aberration is due to the motion of the Sun through space, whose rectilinear motion when combined with a star's rectilinear motion cannot be distinguished from the deflection due to light-time and is thus ignored. Diurnal aberration is due to Earth's rotation on its axis and does not exceed 0.32" (it is most pronounced for equatorial observers). The rest of this article concerns annual aberration due to the velocity of the Earth in its annual orbit about the Sun.

Generalization

To generalize this, let S (fig. 3) be the Sun, ABCD the Earth's orbit, and s the true position of a star. When the Earth is at A, in consequence of aberration, the star is displaced to a point a, its displacement sa being parallel to the Earth's motion at A; when the Earth is at B, the star appears at b; and so on throughout an orbital revolution of the Earth. Every star, therefore, describes an apparent orbit, which, if the line joining the Sun and the star be perpendicular to the plane ABCD, will be exactly similar to that of the Earth, i.e. almost a circle. As the star decreases in latitude, this circle will be viewed more and more obliquely, becoming a flatter and flatter ellipse until, with zero latitude, it degenerates into a straight line (fig. 4).

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The major axis of any such aberrational ellipse is always parallel to AC, i.e. the ecliptic, and since it is equal to the ratio of the velocity of light to the velocity of the Earth, it is necessarily constant. This constant length subtends an angle of about 40" at the Earth; the constant of aberration is half this angle. The J2000.0 value is 20.49552". The minor axis, on the other hand, is not constant, but, as we have already seen, depends on the latitude, being the product of the major axis into the sine of the latitude.

Related Topics:
Major axis - J2000.0

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Assured that his explanation was true, Bradley corrected his observations for aberration, but he found that there still remained a residuum which was evidently not a parallax, for it did not exhibit an annual cycle. He reverted to his early idea of a nutation of the Earth's axis, and was rewarded by the discovery that the Earth did possess such an oscillation. Bradley recognized the fact that the experimental determination of the aberration constant gave the ratio of the velocities of light and of the Earth; hence, if the velocity of the Earth be known, the velocity of light is determined.

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