Proper motion

As a self-motion is called in astronomy based on the spatial movements of celestial bodies, slow change in position on the imaginary celestial sphere. In Astrometry it is specified ( north and east ) in two spherical components and is usually less than 1 " per year for objects outside the solar system. Together with the radial velocity it gives the space motion of the object.

In contrast to the annual parallax exists a progressive change in the self- movement of the star positions.

Rarely, the term peculiar motion (intrinsically from Latin peculiaris = ) is used for the proper motion, which is misleading, because it can be confused with the peculiar velocity, which has a completely different meaning.

Proper motion and speed

The proper motion indicates an angular velocity. Hence the velocity component calculated perpendicular to the connecting earth and Stern ( tangential ) by multiplying by the distance. For example corresponds to a proper motion of a second of arc per year at a distance of one parsec just a rate of one AE per year or about 4.75 km / s For the speed relative to the sun ( heliocentric ) radial velocity is also observed.


The star with the highest previously measured proper motion is Barnard's Star, the year moves with 10.34 " and is only six light years away from us. At the second fastest moving of the Kapteyn's star on the imaginary celestial sphere, although its actual speed because of its greater distance is higher.

The Triangulum Galaxy, a neighboring galaxy, one of the few extragalactic objects whose Einbewegung could be measured. This amounts to about 50 micro- arcseconds per year.

Indication of direction

To specify the apparent direction of the proper motion on the celestial sphere in addition to the angular difference per year, two systems are used:

  • In addition to the overall proper motion per year in addition a position angle is noted as a deviation from the north. It is north of 0 °, 90 ° east, south and west of 180 ° to 270 °. For example, Barnard's Star, therefore, a still of the position angle is adjacent to μ = 10.34 " / 355.8 ° given by.
  • The total proper motion per year is broken down into two components μ (RA ) ( RA ) and μ (Dec) ( declination ). For Barnard's Star, the values ​​are:

Discovery history

Proper motions were recognized in 1728 by James Bradley, as they take place only very slowly because of the large star distances; until then, was therefore generally spoken of fixed stars. They are unit " / a, measured in seconds of arc per year, and usually have the symbols μ. Christian Mayer suggested in 1777 described a method to investigate the proper motion based on closely spaced stars. During his further observations in 1779, he distinguished between potential physical and only optical double stars.

William Herschel examined in 1783 based on 14 stars the proper motion and found out that eleven stars move at a common point near the star Lambda Herculis. He concluded on an absolute motion of the solar system. The three stars, their movement was not aligned to this point, he wrote to a genuine own movement. This study was repeated 1838-1840 by Argelander based on nearly 600 stars. Argelanders investigation confirmed Herschel's result. Thus, one could attribute the proper motion to an absolute motion of the solar system and a real proper motion of the fixed stars from 1840.

Other movements on the celestial sphere

Must be distinguished from apparent motions of the celestial sphere, which are caused in another way:

  • The annual movement of the earth around the sun causes a parallax, that is, move nearby stars due to the different observation angle slightly against the background of much more distant stars.
  • The fluctuations of the Earth's axis, essentially precession and nutation, lead to a uniform displacement of the entire celestial sphere.
  • The finite speed of light, together with the Earthmoving Equipment for aberration ( aberration ) of the star's light as the planet moves away under the incident light.