code: def compute_lambda(alpha, delta, eps)
        a = Math.sin(delta) * Math.sin(eps) \
          + Math.cos(delta) * Math.sin(alpha) * Math.cos(eps)
        b = Math.cos(delta) * Math.cos(alpha)
        lambda = Math.atan2(a, b)
        lambda %= PI2 if lambda < 0
        return lambda
      rescue => e
        raise
      end
docs: =========================================================================
   λ の計算
  
   * λ = arctan((sinδ sinε + cosδ sinα cosε ) / cosδ cosα)
  
   @param:  alpha   (unit: rad)
   @param:  delta   (unit: rad)
   @param:  eps     (unit: rad)
   @return: lambda  (unit: rad)
  =========================================================================