At least according to HistoryoftheUniverse.com

From the article;
"Around 1850 astronomers wanted to be able to give a definite magnitude to each star. N. R. Pogson proposed that a 6th magnitude star should be 1/100 of the brightness of a star of the 1st magnitude. To keep the steps between magnitudes equal, this meant that stars of the 2nd magnitude are about 2/5 as bright as a star of the 1st magnitude. Now the question arose, which star has the first magnitude? One of the stars near the north pole of the sky was chosen, along with a group of about 100 others covering a wide range of brightness, to be used as standards for the astronomical magnitude scale. They were chosen so they can be seen for comparison purposes at any time of the year from northern telescopes. Unfortunately this introduced yet another complication! To include very bright objects, the scale had to be extended to zero, and then to negative numbers! So a star of magitude 0 is 2.5 times as bright as a star of magitude 1, and a star of magnitude -1 is 2.5 times as bright again."

Regards,
SteveS