After compiling a fundamental catalogue of 3000 stars (1818), he began an extensive programme of measuring star positions and proper motions. By 1833 he had highly accurate positions for 50 000 stars, which he combined with refinements of previous observations into a catalogue of 63 000 stars, marking the beginning of modern astrometry. In 1838 he announced the first reliable determination of a star’s parallax (61 Cygni), just 6% above the present-day value. In 1844 he deduced that oscillations in the proper motions of Sirius and Procyon indicated the existence of unseen companion stars, subsequently detected visually.