where δ is the declination. The factor in cos δ accounts for the widening of the lines (hours) of right ascension away from the poles, cos δ, being zero for a hypothetical object fixed at a celestial pole in declination. Thus, a co-efficient is given to negate the misleadingly greater east or west velocity (angular change in α) in hours of Right Ascension the further it is towards the imaginary infinite poles, above and below the earth's axis of rotation, in the sky. The change μ α, which must be multiplied by cos δ to become a component of the proper motion, is sometimes called the "proper motion in right ascension", and μ δ the "proper motion in declination".