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Brightness changes of Uranus and Neptune at 440 nanometers were highly correlated from 1956 to 1966. Recent observations of Saturn's satellite Titan and of Uranus and Neptune show a steady brightening at 551 and 472 nanometers since 1972. Either the solar constant is slightly variable or solar activity causes correlated changes in the albedos of planetary bodies.
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G. W. Lockwood (1975) studied this question.
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