Describe the range of lengths and the relationships between various units of length.

The sizes of objects in the universe vary over a very wide range. The dimension of an atomic nucleus is approximately $10^{-14} \, m$, while the length of the observable universe is about $10^{26} \, m$.
We use special units for short and large lengths:
$1 \, fm$ (fermi) $= 10^{-15} \, m$
Size of nucleus $\approx 10^{-14} \, m$
$1 \, \mathring{A}$ ($\mathring{A}$) $= 10^{-10} \, m$
$1 \, nm$ (nanometer) $= 10^{-9} \, m$
$1 \, \mu m$ (micrometer) $= 10^{-6} \, m$
$1 \, mm$ (millimeter) $= 10^{-3} \, m$
$1 \, km$ (kilometer) $= 10^{3} \, m$
$1 \, Mm$ (megameter) $= 10^{6} \, m$
$1 \, AU$ (astronomical unit) $= 1.496 \times 10^{11} \, m$
$1 \, ly$ (light year) $= 9.46 \times 10^{15} \, m$
$1 \, pc$ (parsec) $= 3.08 \times 10^{16} \, m$
Size of galaxy $\approx 10^{21} \, m$