Here's why:
* Proper motion refers to the actual movement of a star across the celestial sphere as observed from Earth. This is a direct consequence of the star's velocity relative to the Sun.
* Radial velocity describes the motion of a star towards or away from the Earth. It's measured through the Doppler shift of the star's light. While related to the star's intrinsic motion, it doesn't fully describe the complete movement.
* Parallax is the apparent shift in a star's position due to the Earth's orbit around the Sun. It's used to calculate a star's distance but doesn't directly represent its intrinsic motion.
Therefore, proper motion best captures the intrinsic movement of stars across the sky as seen from our perspective. It combines both radial velocity and the transverse component of the star's motion.
