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Suppose that in one second, a gun fires three pellets. The pellets leave the
gun a fraction of an inch apart, and that's their spacing as they hit the target.
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But if the gun is moving away from the target, this changes. The second pellet
is fired from a greater distance than the first, and the third is fired from
still farther. So the group of pellets could be several inches apart when it
hits the target.
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Galaxy stationary with respect to earth
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Galaxy moving away from earth
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The same thing happens to light from a galaxy that's moving away from the earth.
The crests of the lightwave are like the pellets. If the galaxy emits three crests
in a particular amount of time, each subsequent crest has to travel farther than the
one before it. So the light's wavelength - the distance between its crests - is longer
when it reaches the earth than it was when it left the galaxy.
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