def compute_delta(lambda, beta, eps)
      a = Math.sin(beta) * Math.cos(eps) \
        + Math.cos(beta) * Math.sin(lambda) * Math.sin(eps)
      return Math.asin(a)
    rescue => e
      raise
    end
def compute_delta(lambda, beta, eps)
      a = Math.sin(beta) * Math.cos(eps) \
        + Math.cos(beta) * Math.sin(lambda) * Math.sin(eps)
      return Math.asin(a)
    rescue => e
      raise
    end
=========================================================================
 δ の計算

 * δ = arcsisn((sinβ cosε + cosβ sinλ sinε )

 @param:  lambda  (unit: rad)
 @param:  beta    (unit: rad)
 @param:  eps     (unit: rad)
 @return: delta   (unit: rad)
=========================================================================