시선속도
Stellar drift is measured by two components: proper motion (multiplied by distance) and radial velocity. Proper motion is a star's motion across the sky, slowly changing the shapes of constellations over thousands of years. It can be measured using a telescope to detect small movements over long periods of time. Radial velocity is how fast a star approaches or recedes from us. It is measured using redshift. Both components are complicated by the Earth's orbit around the Sun, so the motions of stars are described relative to the Sun, not the Earth (kinematics of stars).