Star magnitudes run backwards because they began as a ranking of size
In astronomy, smaller numbers mean brighter stars, and the brightest go negative: Sirius sits at minus 1.46, the Sun at minus 27. The oddity dates back two thousand years, to when the most prominent stars were called first-class and the faintest sixth. For centuries astronomers believed those classes measured how big the stars were.
Hipparchus catalogued stars by apparent brightness in the second century BCE, and Ptolemy, working in Alexandria three centuries later, sorted them into six grades and gave us the word magnitude. To the naked eye a brilliant star like Arcturus genuinely looks larger than a faint one. As late as 1736, the mathematician John Keill explained the classes as ranks of bigness set by distance, while admitting there were nearly as many grades as stars, and that Tycho and Ptolemy disagreed over where some belonged.
Tycho Brahe even tried measuring the sizes, concluding that first-magnitude stars spanned about 2 arcminutes, a fifteenth of the full Moon's width. Telescopes shrank those figures but did not kill the idea, because early instruments produced false discs that looked larger for brighter stars. Astronomers from Galileo to Jacques Cassini took the discs for real. Only in the nineteenth century, once parallax measurements revealed how remote stars are and optics explained diffraction, did everyone accept that stars appear as points of light.
Early photometry, matching real stars against an artificial one projected into the eyepiece, showed first-magnitude stars were about 100 times brighter than sixth. So in 1856 Norman Pogson of Oxford fixed the scale: five steps equal exactly a factor of 100, making each step about 2.512 times. That logarithmic rule let the scale stretch past its ancient limits in both directions, to Venus at minus 5 and to objects far too faint for the eye. Astronomers now resolve differences of a hundredth of a magnitude.
Two versions coexist. Apparent magnitude is how bright something looks from here; absolute magnitude is how bright it would look from a standard 10 parsecs. That exposes distance tricks: Betelgeuse looks slightly dimmer than Alpha Centauri A yet emits thousands of times more light. Stargazers also use magnitude to rate skies; at a dark site, stars of sixth magnitude or fainter are visible.
Source: Magnitude (astronomy)
