<P> Stellar proper motions have been used to infer the presence of a super-massive black hole at the center of the Milky Way . This black hole is suspected to be Sgr A *, with a mass of 4.2 × 10 M, where M is the solar mass . </P> <P> Proper motions of the galaxies in the Local Group are discussed in detail in Röser . In 2005, the first measurement was made of the proper motion of the Triangulum Galaxy M33, the third largest and only ordinary spiral galaxy in the Local Group, located 0.860 ± 0.028 Mpc beyond the Milky Way . The motion of the Andromeda Galaxy was measured in 2012, and an Andromeda--Milky Way collision is predicted in about 4 milliard years . Proper motion of the NGC 4258 (M106) galaxy in the M106 group of galaxies was used in 1999 to find an accurate distance to this object . Measurements were made of the radial motion of objects in that galaxy moving directly toward and away from us, and assuming this same motion to apply to objects with only a proper motion, the observed proper motion predicts a distance to the galaxy of 7023222168785865637 ♠ 7.2 ± 0.5 Mpc . </P> <P> Proper motion was suspected by early astronomers (according to Macrobius, AD 400) but a proof was not provided until 1718 by Edmund Halley, who noticed that Sirius, Arcturus and Aldebaran were over half a degree away from the positions charted by the ancient Greek astronomer Hipparchus roughly 1850 years earlier . </P> <P> The term "proper motion" derives from the historical use of "proper" to mean "belonging to" (cf, propre in French and the common English word property). "Improper motion" would refer to "motion" common to all stars, such as due to axial precession . </P>

Who discovered the first known instance of proper motion