Throughout
recorded
history
,
several
____________________
and
cosmogonies
have
been
proposed
to
account
for
observations
of
the
____________________
.
The
earliest
quantitative
geocentric
models
were
developed
by
the
ancient
____________________
philosophers
.
Over
the
centuries
,
more
precise
observations
and
improved
____________________
of
gravity
led
to
Copernicus's
heliocentric
____________________
and
the
Newtonian
model
of
the
____________________
System
,
respectively
.
Further
improvements
in
____________________
led
to
the
realization
that
the
Solar
System
is
embedded
in
a
____________________
composed
of
billions
of
____________________
,
the
____________________
Way
,
and
that
other
____________________
exist
outside
it
,
as
far
as
astronomical
instruments
can
reach
.
Careful
studies
of
the
distribution
of
these
galaxies
and
their
spectral
lines
have
led
to
much
of
modern
cosmology
.
Discovery
of
the
red
shift
and
cosmic
microwave
background
radiation
suggested
that
the
____________________
is
expanding
and
had
a
beginning
.
