 The factor in cos2"δ" accounts for the fact that the radius from the axis of the sphere to its surface varies as cos"δ", becoming, for example, zero at the pole.Thus, the component of velocity parallel to the equator corresponding to a given angular change in "α" is smaller the further north the object's location.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".