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tic. Dusky spots were also noticed as tran-
sient. Hooke[5] measured the motion of one
in 1664. In 1665 Cassini, with a fine tele-
scope, 35-feet focal length, observed many
spots moving from east to west, whence he
concluded that Jupiter rotates on an axis
like the earth. He watched an unusually
permanent spot during twenty-nine rotations,
and fixed the period at 9h. 56m. Later he
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