Editing Proper motion (section)

== Examples ==

For most stars seen in the sky, the observed proper motions are small and unremarkable. Such stars are often either faint or are significantly distant, have changes of below 0.01″ per year, and do not appear to move appreciably over many millennia. A few do have significant motions, and are usually called ''high-proper motion stars.'' Two or more stars which are moving in similar directions, exhibit so-called shared or ''[[common proper motion]]'' (or cpm.), suggesting they may share similar motion in space (if the distances and radial velocities are also consistent) and thus be gravitationally linked as [[Binary star|binary stars]] or [[Star cluster|star clusters]].

[[Image:Barnard2005.gif|thumb|[[Barnard's Star]], showing position every 5 years 1985–2005.]]

[[Barnard's Star]] has the largest proper motion of all stars, moving at 10.3″&nbsp;yr<sup>&minus;1</sup>. Large proper motion usually strongly indicates an object is close to the Sun. This is so for Barnard's Star, about 6 [[light-year]]s away. After the Sun and the [[Alpha Centauri]] system, it is the [[List of nearest stars|nearest]] known star. Being a [[red dwarf]] with an [[apparent magnitude]] of 9.54, it is too faint to see without a [[telescope]] or powerful binoculars. Of the stars visible to the naked eye (conservatively limiting unaided visual magnitude to 6.0), [[61 Cygni A]] (magnitude [[UBV photometric system|V=]]5.20) has the highest proper motion at 5.281″&nbsp;yr<sup>&minus;1</sup>, discounting [[Groombridge 1830]] (magnitude [[UBV photometric system|V=]]6.42), proper motion: 7.058″&nbsp;yr<sup>&minus;1</sup>.<ref>Hipparcos: Catalogues: The Millennium Star Atlas: [https://www.cosmos.esa.int/web/hipparcos/high-proper-motion The Top 20 High Proper Motion], [[European Space Agency]], retrieved 2019-06-27</ref>

A proper motion of 1&nbsp;arcsec per year 1&nbsp;light-year away corresponds to a relative transverse speed of 1.45&nbsp;km/s. Barnard's Star's transverse speed is 90&nbsp;km/s and its radial velocity is 111&nbsp;km/s (perpendicular (at a right, 90° angle), which gives a true or "space" motion of 142&nbsp;km/s. True or absolute motion is more difficult to measure than the proper motion, because the true transverse velocity involves the product of the proper motion times the distance. As shown by this formula, true velocity measurements depend on distance measurements, which are difficult in general.

In 1992 [[Rho Aquilae]] became the first star to have its [[Bayer designation]] invalidated by moving to a neighbouring constellation &ndash; it is now in [[Delphinus]].<ref name="jrasc92">{{cite journal
 | title = Book-Review – Sky Catalogue 2000.0 – V.1 – Stars to Magnitude 8.0 ED.2
 | date = 1992
 | bibcode = 1992JRASC..86..221L
 | last = Lemay
 | first = Damien<!-- book authors:
 | last1 = Hirshfeld
 | first1 = Alan
 | last2 = Sinnott
 | first2 = Roger W.
 | last3 = Ochsenbein
 | first3 = François -->
 | volume = 86
 | page = 221
 | journal = Journal of the Royal Astronomical Society of Canada
}}</ref>