Whose observations first revealed that stars can move measurably across the sky?

Edmond Halley compared his own observations with ancient star catalogs and noticed that certain bright stars, including Arcturus and Sirius, had changed position over time. He published this in 1718, providing one of the first clear demonstrations that stars are not fixed on the celestial sphere. The phenomenon is now called proper motion. Halley’s insight helped astronomers understand the nearby structure and motion of the Milky Way.