Proper motion

Proper motion is the angular change in a celestial object's position across the sky over time, usually measured relative to distant background objects. It is the component of motion perpendicular to the line of sight.

Radial velocity: The component of an object's velocity along the line of sight, measured through shifts in its spectrum. Together with proper motion and distance, radial velocity specifies a star's three-dimensional motion.
Gaia (spacecraft): The European Space Agency mission that measures positions, distances, and motions of stars across the Milky Way. Gaia's repeated all-sky astrometry has produced proper motions for over a billion stars.
Celestial sphere: An imaginary sphere centered on an observer onto which celestial positions are projected. Proper motion describes changing position on this apparent sky.
Annual parallax: The apparent yearly change in a nearby object's position caused by Earth's orbit around the Sun. Unlike proper motion, annual parallax is a periodic viewpoint effect rather than steady angular drift.
Parallax: The apparent shift in an object's position caused by a change in the observer's viewpoint. Parallax supplies the distance needed to convert angular proper motion into transverse velocity.
Stellar kinematics: The study of the motions of stars and stellar populations. Proper motions, combined with distances and radial velocities, map stellar kinematics.
Tangent plane: A local flat coordinate plane used to represent small angular offsets on the sky. Astrometric proper-motion components are commonly expressed along two tangent-plane axes.
Precession: The slow change in the orientation of Earth's rotational axis and coordinate system. Coordinate precession can alter catalog positions without representing an individual star's motion.
Astrometry: The measurement of celestial positions, distances, and motions with precision. Astrometric observations track the changing positions from which proper motion is calculated.
Solar apex: The direction in the sky toward which the Sun moves relative to nearby stars. The common apparent drift of nearby stars helps determine the Sun's motion.

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