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Cassini gave the rotation as twenty-three hours, by observing a bright spot on her surface. Shroter made it 23h. 21m. 19s. This value was supported by others. In 1890 Schiaparelli announced that Venus rotates, like our moon, once in one of her revolutions, and always directs the same face to the sun.
In 1675 Cassini found the ring to be double, the concentric rings being separated by a black band a fact which was placed beyond dispute by Herschel, who also found that the thickness of the ring subtends an angle less than 0".3. Shroter estimated its thickness at 500 miles. Many speculations have been advanced to explain the origin and constitution of the ring.
This was called libration, and the explanation was clear when it was understood that in showing always the same face to us she makes one revolution a month on her axis uniformly, and that her revolution round the earth is not uniform. Galileo said that the mountains on the moon showed greater differences of level than those on the earth. Shroter supported this opinion. W. Herschel opposed it.
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