If you've followed my blogging, you know I love books. On Friday, November 2, I visited Linda Hall Library in Kansas City with Astronomy Editor David J. Eicher and contributing editor Raymond Shubinski. Linda Hall ranks as one of the world's finest science libraries, and it has a terrific collection of rare astronomy books. During the visit, we sat at a table and paged through a first edition of Isaac Newton's Principia — the book in which...

In this blog and my next, I'm going to try to demystify the magnitude system — the scale astronomers use to measure the brightness of a celestial object. Let me start with a little history. The first known observer to describe and catalog differences in star brightnesses was Greek astronomer Hipparchus (ca. 190–120 B.C.). He divided his catalog of roughly 850 visible stars into six brightness ranges, or magnitudes. He called the brightest 1st...