The Galactocentric Perspective
The Milky Way:
Herschel 1800--The slab universe
Kapteyn 1900--The red blood cell universe
Harlow Shapley 1917--We are not at the center of the disk.
   Globular clusters orbit galactic center, sun 2/3rds way out.
   He used proper motions of Cepheids-->Distance (11 stars).
   Shapley-Curtis debate 1920--Nebulae are within our island             
   universe (Shapley). Nebulae may be other galaxies (Curtis).
   Edwin Hubble 1923--distance to Andromeda galaxy found from
   Cepheid Variable. 2.25 Mill. vs 100,000 LY--Milky way size.
Spiral arms: 21 cm radio radiation allows us to see through             
     dust.  Its Doppler shift tells us how arms are moving.
     Galaxy rotates once every 225 million years (sun-240 km/s).
     Differential rotation--spiral arms should wind up in 50 turns.
Galactic rotation curves don't obey Newton's Laws for the
      visible matter-->dark matter (about 10 x the visible                    
      mass–much of it in halo).