出典:Wikipedia
出典:『Wikipedia』 (2011/07/18 11:42 UTC 版)
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in distance toward or away from the viewer, usually measured by Doppler shift of received radiation. The proper motion is not entirely "proper" (that is, intrinsic to the star) because it includes a component due to the motion of the solar system itself. Due to the finite speed of light, the true velocities of distant stars are not observable, the observed proper motion reflects the motion of a star at the time the light was emitted.