Hipparchus (160?-125? BC) devised the magnitude
scale that we use to specify how bright a star or other celestial
object is. What he said was that the brightest stars were to be
called 1st magnitude stars. Stars that were just perceptibly
dimmer were to be called 2nd magnitude stars, and so
one, down to 6th magnitude which was considered to
be the naked eye limit.
When instrumentation for more precise measurements
of brightness became available, consideration was given to change
the magnitude scale system and to devise a new system. The decision
was made to keep the old system.
It was found out that 5 magnitude steps equal
a factor of 100 in brightness. At the same time, other research
showed that the eye (as well as the ear) responds the ratio of
a change, rather that the amount of change. With this information,
the size of one step in magnitude was calculated as 2.512 (2.512
times itself 5 times equals 100).
The then known value of the brightness of the star Polaris was used to define a 2nd magnitude star. When this was done, it was discovered that using this system, there are stars brighter than 1st magnitude. This factor of 2.512 is then used, leading to 0 magnitude, -1 magnitude, and so on.