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stars. Olaus Romer was born at Copen-
hagen in 1644. In 1675, by careful study
of the times of eclipses of Jupiter’s satel-
lites, he discovered that light took time to
traverse space. Its velocity is 186,000 miles
per second. In 1681 he took up his duties as
astronomer at Copenhagen, and built the
first transit circle on a window-sill of his
house. The iron axis was five feet long and
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