def _fk4_B_matrix(obstime):
 T = ((obstime.jyear - 1950.0) / 100.0)
 if getattr(T, u'shape', ()):
  T.shape += (1, 1)
 return (_B1950_TO_J2000_M + (_FK4_CORR * T))

def _fk4_B_matrix ( obstime ) : T = ( ( obstime . jyear - 1950.0 ) / 100.0 ) if getattr ( T , u'shape' , ( ) ) : T . shape += ( 1 , 1 ) return ( _B1950_TO_J2000_M + ( _FK4_CORR * T ) )
this is a correction term in the fk4 transformations because fk4 is a rotating system - see murray 89 eqn 29 .