In order to obtain quantitative properties of galaxies, we must know their DISTANCE. Accurate DISTANCES in astronomy are absolutely crucial.
The precision of distances measurements DROPS with distance.
We may be confident to one part in 100 nearby, but our
confidence drops to one part in 3 or 4 for external galaxies.
Distances obtained nearby serve as a calibration
of distances further away. The step wise inter-dependency is
frequently referred to as:
The "Steps" (rungs) in the Cosmic Distance Ladder:
1) For < 1 light year (Solar System): Radar off rocky solar system objects
2) Up to few 100 light years (local stars): Stellar Parallax
3) Up to few 100,000 light years (scale of galaxy): HR Diagram (Spec. Parallax)
4) Up to few 10-million light years (scale of Local Galaxies): Variable Stars
5) Up to a billion light years (scale of Local Universe): Type I Supernova
6) Up to ten billion light years (the most distant galaxies/quasars): Hubble's Law