  We study the absorption spectra of the yellow excitons in Cu$_2$O in high
magnetic fields using polarization-resolved optical absorption measurements
with a high frequency resolution. We show that the symmetry of the yellow
exciton results in unusual selection rules for the optical absorption of
polarized light and that the mixing of ortho- and para- excitons in magnetic
field is important. Our calculation of the energies of the yellow exciton
series in an arbitrary magnetic field gives an excellent fit to experimental
data and allows us to understand the complex structure of excitonic levels and
their magnetic field dependence, which resolves the old-standing disagreement
between the results of optical absorption and cyclotron resonance measurements.
