自行是恒星相对于太阳系的质量中心，随着时间变化的推移所显示出在位置在角度上的改变，它的测量是以角秒/年为单位（3600角秒才等同于角度的1度）。反之，径向速度是在视线方向上天体接近或远离的速度，随着时间推展的变化率，通常是测量辐射中的都卜勒频移。自行不是恒星的本质（即恒星的内禀性质），因为它包含了太阳系本身运动的元素在内。由于光速是有限的，遥远恒星的真实速度很难观测得到，观测自行反映的是恒星当时辐射光的运动。

自行的测量需要排除下列会影响观测天体位置座标值的因素，这些因素主要有：

Proper motion is the astronomical measure of the observed changes in apparent positions of stars in the sky as seen from the center of mass of the Solar System, as compared to the imaginary fixed background of the more distant stars.

It was first physically measured individually in both seconds of time in right ascension (RA or α) and seconds of arc in declination (Dec. or δ), whose calculated combined value, is the total proper motion (μ) expressed in seconds of arc per year (arcsec/yr) or per century (arcsec/100yr), where 3600 arcseconds equal one degree. Because most proper motions are much less than one arcsec per year, most modern catalogues, like the Hipparcos Index Catalogue (HIP), now express proper motion in terms of milliarcseconds per year (mas/yr), where 1000 mas equals one arcsecond. This measured sky motion is separate from the radial velocity, measured in kilometres per second (km/s), being the velocity moving toward or away from the observer, as usefully obtained by the small Doppler shifts seen in starlight when using astronomical spectroscopy. Knowledge of the proper motion and Doppler shift allow approximate calculations of a star's true motion in space in respect to the Sun.