A competing idea is one that is
central to the astronomy,
called the principle of mediocrity.
This is the idea that the Earth is
not particularly special, nor is the
Sun, nor our galaxy, nor our part of the
Universe. Things everywhere are pretty
much the same.
This is crucial to astronomy because we
must assume in astronomy that by studying
any particular star, we are more or less
studying all stars like that star.
You should think about how these
two competing principles underlie
various arguments and lines of evidence
that we will study in this class.