Astrometry is primarily concerned with measuring the positions and motions of celestial objects, which allows astronomers to determine their distances and study their dynamics.

Parallax is a method used to measure the distances to nearby stars by observing their apparent shift in position as the Earth orbits the Sun.

Parsecs (pc) are commonly used to express the distance to stars. One parsec is approximately 3.26 light-years or 206,265 astronomical units.

Barnard's Star has the largest proper motion among stars visible to the naked eye, moving across the sky at a rate of about 10.3 arcseconds per year.

Radial velocity refers to the motion of a star towards or away from the observer, measured along the line of sight.

A spectrograph is an instrument used to measure the radial velocities of stars by analyzing the Doppler shift in their spectral lines.

The Hipparcos mission was a space astrometry mission that accurately measured the positions and distances of over 118,000 stars.

The Gaia spacecraft is a space astrometry mission that aims to create a precise 3D map of our galaxy by measuring the positions, distances, and motions of stars.

Measuring the proper motions of stars helps astronomers study the dynamics of star clusters, understand their evolution, and identify binary or multiple star systems.

Cepheid variables are pulsating stars with a known relationship between their pulsation period and luminosity. By measuring the period of a Cepheid variable, astronomers can estimate its distance.

Aberration of light refers to the apparent change in the direction of light from a star due to the Earth's motion around the Sun.

Proxima Centauri, the closest star to our solar system, exhibits the largest parallax among stars visible to the naked eye.

VLBI is a technique that combines signals from multiple radio telescopes to achieve high angular resolution, allowing astronomers to measure the positions of stars with great precision.

Lunar laser ranging involves bouncing laser pulses off the Moon's surface and measuring the time it takes for the light to travel to and from the Moon. This technique is used to study the Earth-Moon system and test theories of gravity.