The Doppler effect can measure the component of the motion of a light source along the line of sight between the source and the observer. It arises due to the change in the observed frequency of light waves as the distance between the source and the observer changes over time. This effect allows us to determine whether an object is moving toward us (blueshift) or away from us (redshift).

However, the Doppler effect provides limited information about the motion of a celestial object across the celestial sphere. This is because the Doppler shift depends only on the relative motion along the line of sight, and it cannot distinguish between motion directly toward or away from us and motion perpendicular to the line of sight.

To fully understand the three-dimensional motion of celestial objects, additional observations and techniques are necessary, such as proper motion measurements and astrometry. Proper motion is the angular displacement of an object across the sky over time and can be used to determine the tangential motion of the object across the celestial sphere. Astrometry is the precise measurement of the positions and movements of celestial objects, allowing us to determine their distances, motions, and trajectories in space.

By combining Doppler shift measurements with other observational techniques, astronomers can obtain a more comprehensive understanding of the motion of celestial objects, including their radial velocity (motion along the line of sight) and their proper motion (motion across the celestial sphere).