Proper motion

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.