PX158 - C2 - proper motion

in 1718, edmund halley noticed that some 'fixed' stars showed movement
stars move in orbits around the galactic centre with periods in units of $100 s M y r$
proper motion can be observed over years or decades
typical unit: $μ$ : $\frac{m a s}{y e a r}$
proper motion in 2D is given as:
- $μ_{R . A .} :$ right ascension direction
- $μ_{D e c} :$ declination direction
3D space vector for the motion of a star can be obtained using the doppler shift