Page 582 - 89 history of astronomy_opt

the nearest known fixed star; and its light
takes 4ˆa years to reach us. The light year
is taken as the unit of measurement in the
starry heavens, as the earth’s mean distance
is ”the astronomical unit” for the solar system.[3]
The proper motions and parallaxes com-
bined tell us the velocity of the motion of
ˆ
these stars across the line of sight: I l Cen-
tauri 14.4 miles a second=4.2 astronomical
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