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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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