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Proper motion
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<p>Proper motion is the <a href="/facts/Astrometry/9ycEEAvD">astrometric</a> measure of the observed changes in the <a href="/facts/Apparent_place/VHqbq1Ga">apparent places</a> of <a href="/facts/Star/PFbd6ICx">stars</a> or other <a href="/facts/Celestial_objects/BCHLa0Q2">celestial objects</a> in the sky, as seen from the <a href="/facts/Center_of_mass/n4Lr9GAz">center of mass</a> of the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>, compared to the abstract background of the more <a href="/facts/Fixed_stars/grlDbIPX">distant stars</a>. </p><p>The components for proper motion in the <a href="/facts/Equatorial_coordinate_system/dn5W4Eoo">equatorial coordinate system</a> (of a given <a href="/facts/Epoch_(astronomy)/RDdj6SmA">epoch</a>, often <a href="/facts/J2000.0/RDdj6SmA">J2000.0</a>) are given in the direction of <a href="/facts/Right_ascension/z4CEsTFu">right ascension</a> (<i>μ</i>α) and of <a href="/facts/Declination/5X4S4k9u">declination</a> (<i>μ</i>δ). Their combined value is computed as the <i>total proper motion</i> (<i>μ</i>). It has <a href="/facts/Dimension/0ktmUyac">dimensions</a> of <a href="/facts/Angle/gFUTEQYq">angle</a> per <a href="/facts/Time/HceJKUGB">time</a>, typically <a href="/facts/Arcsecond/tybs6YGC">arcseconds</a> per <a href="/facts/Year/51V242hh">year</a> or milliarcseconds per year. </p><p>Knowledge of the proper motion, distance, and <a href="/facts/Radial_velocity/B3NmbVLP">radial velocity</a> allows calculations of an object's motion from the Solar System's <a href="/facts/Frame_of_reference/15yYfB9M">frame of reference</a> and its motion from the galactic frame of reference – that is motion in respect to the Sun, and by <a href="/facts/Coordinate_transformation/Mtd0q4SP">coordinate transformation</a>, that in respect to the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a>. </p>