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URANUS' CHANGING COLOR: 1993-2025

Abstract

The writer and other small-telescope operators have carried out brightness measurements of Uranus since 1993 using a filter transformed to the Johnson R system. Uranus’ South and North polar hoods faced us in 1993 and 2025 but its equator faced us in 2007. Essentially, Uranus was 30-40 % brighter in red light in 1993 and 2025 than in 2007. The normalized brightness for that planet, based on measurements made between 1993 and late 2007, was R(1, a) = –6.645 – 0.0061 SL where SL is the absolute value of the sub latitude of Uranus in degrees. The R(1, a) value is the normalized R-filter brightness of that planet when it is 1.0 au from the Earth and Sun. The sub-latitude is the mean of sub-Earth and sub-solar latitudes. During the early 1990s, the South Polar Hood faced the Earth and Sun. After 2007, the North Polar Hood began tilting towards the Earth. Between 2007 and 2025, the R(1, a) value changed as: R(1, a) = –6.705 – 0.0061 SL. These results are also consistent with the North Polar Hood being about 0.06 magnitudes brighter than the South Polar Hood. The V-filter brightness increases by about 0.1 magnitudes because of the polar hoods tilting to the Earth (Schmude, 2008), (Schmude 2025), whereas the corresponding R-filter increases is 0.3 to 0.4 magnitudes. Therefore, the polar hoods cause Uranus to be less blue when facing us than when its equator faces us.

Acknowledgements

The writer is grateful to those who submitted R-filter brightness measurements of Uranus and to those who assisted him in making R-filter brightness measurements of Uranus.

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