Apparent displacement of a star or other celestial body resulting from Earth's orbital motion around the Sun. The maximum displacement is about 20.49 seconds of arc. It depends on the ratio of Earth's orbital speed to the speed of light and Earth's direction of motion and thus provides confirmation that Earth orbits the Sun rather than the reverse.
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Deviation of light rays by lenses or curved mirrors which causes the images to be blurred. Spherical aberration occurs because curvature in a lens or mirror causes rays falling on the outer edges to be brought to a focus at a different point than those falling on the middle. This makes the images formed appear blurred. Chromatic aberration, which occurs in lenses but not mirrors, is the failure of a lens to focus all colours (wavelengths) of light in the same plane; the image appears blurred and shows rainbow-coloured fringes around the edges. Seealso astigmatism.
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