code: 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
docs: =========================================================================
   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)
  =========================================================================