code: def _arc(self, prom, sig):
          
          arcm = arc(prom['ra'], prom['decl'], 
                     sig['ra'], sig['decl'], 
                     self.mcRA, self.lat)
          arcz = arc(prom['raZ'], prom['declZ'], 
                     sig['raZ'], sig['declZ'], 
                     self.mcRA, self.lat)
          return {
              'arcm': arcm,
              'arcz': arcz
          }
docs: Computes the in-zodiaco and in-mundo arcs 
          between a promissor and a significator.