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Download Figure 10-6 The same star field shown in Figure
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.' '*'..,~ Figure 10-6 The same star field shown in Figure 2-10 indicated. (Courtesy of M. Walker, Lick Observatory) with apparent .' •. ><'••••• magnitudes Hipparchus misjudged the magnitudes of some of the brighter stars, however. When the magnitude scale was extended and expressed by a mathematical formula, it developed that the brighter stars are brighter than those of the first magnitude; indeed they are even brighter than those of zero magnitude. The only way to express these hitherto unsuspected magnitudes and yet retain the old scale (thereby avoiding a wholesale revision of stellar catalogs) was to adopt a few negative magnitudes. As a result, Sirius, the brightest star in the sky aside from the sun, has an apparent magnitude of -1.4 and the star 61 Cygni A has an apparent magnitude of +5.4 and is just visible to the naked eye on a clear moonless night when viewed from a place far from the lights, smoke, and smog of our modern cities. The upshot is that a magnitude scale set up some 2000 years ago is still in use, although stretched somewhat on a mathematical rack. After the revisions, a zero-magnitude star is exactly 100 times brighter than a fifth-magnitude star, which is fairly close to Hipparchus' scale. Thus for a magnitude difference of 5 there is a change in brightness by a factor of 100.