What are proper motions of stars, and how did William Herschel study them?

Proper motion is the apparent, slow drift of a star across the celestial sphere at right angles to an observer's line of sight. This motion, which causes the shapes of constellations to change over time, is measured relative to distant background stars or galaxies.12

William Herschel studied these motions to better understand the structure of the universe. By observing that many stars with significant proper motion were bright, he inferred they were likely closer to Earth. In 1783, Herschel published an analysis of 19 stellar proper motions. He hypothesized that any discernible pattern in these movements was not random but resulted from the Sun's own travel through space.2

His analysis led him to conclude that the Sun is moving toward a point in the constellation Hercules, known as the solar apex. Although this conclusion was later challenged by Friedrich Bessel due to the availability of more extensive data, Herschel’s fundamental insight was ultimately proven correct.2