Page 425 - 89 history of astronomy_opt

of a luminous body, like the pitch or note
of a sounding body, must be changed by
velocity of approach or recession. Everyone
has noticed on a railway that, on meeting
a locomotive whistling, the note is lowered
after the engine has passed. The pitch of
a sound or the colour of a light depends on
the number of waves striking the ear or eye
in a second. This number is increased by
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