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