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Magnitudes
Magnitudes of Selected Objects
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The method we use today to compare the apparent brightness (magnitude)
of stars began with Hipparchus, a Greek astronomer who lived in the
second century BC. Hipparchus called the brightest star in each
constellation "first magnitude." Ptolemy, in 140 A.D., refined
Hipparchus' system and used a 1 to 6 scale to compare star brightness,
with 1 being the brightest and 6 the faintest. This is similar to
the system used in ranking tennis players, etc. First rank is better
than second, etc. Unfortunately, Ptolemy did not use the brightest
star, Sirius, to set the scale, so it has a negative magnitude.
(Imagine being ranked -1.5 in the tennis rankings!)
Astronomers in the mid-1800's quantified these numbers and modified
the old Greek system. Measurements demonstrated that 1st magnitude
stars were 100 times brighter than 6th magnitude stars. It has also
been calculated that the human eye perceives a one-magnitude change
as being 2 and ½ times brighter, so a change in 5 magnitudes
would seem to be 2.5 to the fifth power (or approximately 100) times
brighter. Therefore a difference of 5 magnitudes has been defined
as being equal to a factor of exactly 100 in apparent brightness.
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