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
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
=========================================================================
 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)
=========================================================================