Transcript: The branch of astronomy that deals with the positions and motions of stars is called astrometry. The positions of stars are measured by taking images of the sky. We also need to know a stars distance, typically, which is most directly measured through the parallax technique, the application of geometry. Stars are so far away that their motions are difficult to detect, and in general stellar motion is composed of two components, each of which has to be measured using a separate technique. There is the radial component of the motion, or its component to and away from the observer, called the radial velocity, and then there’s the transverse velocity or its component of motion on the plane of the sky. To get the total space motion these two components are combined in quadrature using Pythagoras’ theorem.