code: def add_source(self, source):
          
          if not type(source)==Source:
              raise TypeError('Can only add Source object.')
  
          if len(self.sources)==0:
              self.sources.append(source)
          else:
              ind = 0
              for s in self.sources:
                  # Keep sorted order of separation
                  if source.separation < s.separation:
                      break
                  ind += 1
  
              self.sources.insert(ind, source)
docs: Adds source to observation, keeping sorted order (in separation)