def compute_beta(alpha, delta, eps)
      a = Math.sin(delta) * Math.cos(eps) \
        - Math.cos(delta) * Math.sin(alpha) * Math.sin(eps)
      return Math.asin(a)
    rescue => e
      raise
    end
def compute_beta(alpha, delta, eps)
      a = Math.sin(delta) * Math.cos(eps) \
        - Math.cos(delta) * Math.sin(alpha) * Math.sin(eps)
      return Math.asin(a)
    rescue => e
      raise
    end
=========================================================================
 β の計算

 * β = arcsisn((sinδ cosε - cosδ sinα sinε )

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