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How Redshifting Works
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.
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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