Looking at the sky night after night, gives the impression that the sun, moon and planets are the only objects that are really moving and that the stars in the background, although they have their daily rotation, remain stationary with respect to each other. Even observations over many years may hardly give evidence of any motion in the Universe at all.
Nevertheless nothing in the Universe is stationary; all celestial objects are moving. Stars move with respect to nearby stars, often in binary or multiple systems, and they all rotate in their galaxy about the galactic centre. The whole galaxy moves with respect to other galaxies, etc.
All these motions are typically in the order of tens to hundreds of km per second.
Because of the vast distances to us, these motions are hardly, if at all, noticeable within a human life span. We need accurate and systematic measurements to find the actual motions of celestial objects.
In this module we will discuss the various techniques
used by astronomers to measure velocity of celestial objects.
It is recommended to first read our Modules "Stellar Radiation" and "Stellar Distance",
before continuing with this Module.