  Acoustic modes can be used to study the physics of the interior of a cavity,
and this is especially useful when the inside region is inaccessible. Many
astrophysicists use such sound waves as an essential tool in their research.
Here we focus on two separate sub-fields in which oscillations on the surface
of a sphere are studied -- Helioseismology and CMBology -- the surface being
either the solar or cosmic photosphere. Both research areas use the language of
spherical harmonics, as well as sharing many close similarities in the
underlying physics. However, there are also many fundamental differences, which
we explain in this pedagogical article.
