In the astronomy of the solar system, it is natural to measure time in days
or years, lengths in units of the mean earth-sun
distance, 
(AU stands for astronomical
unit; it is the counterpart of the Bohr radius), and masses in units of the
mass of the sun (or of the earth). Generally, the earth is taken as the
standard planet, and the sun as the standard star. In stellar astronomy, the
sun's mass and its radius are used as units of mass and length. In galactic
astronomy and cosmology one often sets the speed of light equal to 1, which
is the same as using the light-second (or the
light-year) as the unit of length. An alternative unit
of length, which arises from the method of measuring distances to nearby
stars, is the parsec (parallax-second), which is the
distance at which 
subtends a second of arc. The correspondence
with SI units is that 
and 
.