•All the systems of
photometry that exist today are based on Ptolemy’s original system.
–Remarkably,
with just a few exceptions, Ptolemy’s magnitudes agree
well with modern photometry (although to low precision).
–We now define
the magnitude scale mathematically such that a difference of 5 magnitudes is
a factor of 100 in brightness.
–Or, a difference
of 1 magnitude is a factor of 100.4 in the
“irradiance of the detector”.
