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earth; if to the red end, it means recession.
The displacement of lines does not tell us
whether the motion is in the star, the earth,
or both. The displacement of the lines be-
ing measured, we can calculate the rate of
approach or recession in miles per second.
In 1868 Huggins[6] succeeded in thus mea-
suring the velocities of stars in the direction
of the line of sight.
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