The radial velocity is easily found by observing the spectrum of a star, but finding the proper motion is tougher and requires careful observation of the movement of the star with respect to others over a number of years.
Measuring distances and motions of stars is fundamental to our understanding of the nature of the Universe. Knowing the distance to a star, we can deduce its true luminosity and size and so we can derive essential information about its nature and age. On the other hand, knowing the motion of stars we can deduce not only where they were millions of years ago but also what their positions will be in the future.
Ancient civilizations already realised that objects in the sky appear to move in a regular manner which can be useful for determining directions and time on the Earth. The need to solve problems originating from early communities (e.g. establishing accurately the optimum moment for planting and harvesting) constituted a starting point for precision astrometry.
Making accurate angular measurements and cataloguing celestial positions has been the fundamental task of astronomy until the 19th century and still constitutes a basic element of astronomical research. The angles involved are extremely small and improving the accuracy of astrometric measurements has been a constant goal of astronomers. Improved accuracy has come from the development of new and more precise observing instruments and has led to a series of very fundamental changes in scientific belief.
Click on the thumbnail below to find out how we can measure the radial velocity of a star using Doppler's effect: