section_path: Physical diameters/Angular diameter
section_name: Angular diameter
paragraph_text: Angular diameter
  
  As early as the time of Edwin Hubble in 1936, there have been attempts to characterize the diameters of galaxies. The earliest efforts were based on the observed angle subtended by the galaxy and its estimated distance, leading to an angular diameter (also called "metric diameter"). This type of measurement is subject to two significant issues, namely that the estimated distance to the galaxy must be corrected for the redshift-related space expansion and that collections of angular-diameter data are subject to selection bias as more distant observations preferentially select the most luminous objects.