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Of Time and Space and Other ThingsAt least they are evenly distributed for as far out as we can see, and we can see pretty far. Assumption 3, that galaxies (not stars) are of uniform average brightness throughout space, is harder to handle. However, we have no reason to suspect,-6at distant galaxies are consistently larger or smaller than nearby ones, and if the galaxies come to some uniform average size and star-content, then it certainly seems reasonable to suppose they are uniformly bright as well. Well, then, why is the night sky black? We're back to that. Let's try another tack. Astronomers can determine whether a distant luminous object is approaching us- or receding from us by studying its spectrum (that is, its lijzht as spread out in a rainbow of wavelengths from short wavelength violet to long-wavelength red). The spectrum is crossed by dark lines which are in a fixed position if the object is motionless with respect to us. If the object is approaching us, the lines shift toward the violet. If the object is receding from us, the lines shift toward the red ...» |
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