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Orders of Magnitude

7 Jun 2015, 00:00 UTC
Orders of Magnitude
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¡SkyCaramba! Weekly astronomy blog for the week ending June 13, 2015
How would you organize all the stars in our sky? By color? By distance? By size? Many astronomers organize them by brightness. Hipparchus is the first known to do that. About 129 B.C., he considered the brightest to be the biggest. He called them stars of first magnitude. Naturally, he called the second brightest stars second magnitude. He kept going until he reached the sixth brightest stars. Stars of sixth magnitude were the dimmest.
Hipparchus’s rankings are convenient, but not perfect. About A.D. 140, it was clear to Claudius Ptolemy that some stars are brighter than others within each magnitude class. But rather than redo Hipparchus’s work, Ptolemy simply included notes about that. For European astronomers, that was a good enough way of doing things for nearly 15 centuries.
The first big revision came when Galileo Galilei pointed a telescope at the sky and discovered there’s more to it than meets the naked eye. Seeing stars that could be seen only with a telescope, he proposed in 1610 that they regarded as being of the seventh magnitude. Then more powerful telescopes with better lenses revealed even more stars. As ...

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