code: def compute_alpha(lambda, beta, eps)
        a = -Math.sin(beta) * Math.sin(eps) \
          +  Math.cos(beta) * Math.sin(lambda) * Math.cos(eps)
        b =  Math.cos(beta) * Math.cos(lambda)
        alpha = Math.atan2(a, b)
        alpha %= PI2 if a < 0
        return alpha
      rescue => e
        raise
      end
docs: =========================================================================
   α の計算
  
   * α = arctan((-sinβ sinε + cosβ sinλ cosε ) / cosβ cosλ)
  
   @param:  lambda  (unit: rad)
   @param:  beta    (unit: rad)
   @param:  eps     (unit: rad)
   @return: alpha   (unit: rad)
  =========================================================================