What observational geometry technique was historically used to help determine the precise value of the AU?

Tracking transits of Venus across the Sun's face.

Before the advent of modern electronic ranging techniques, determining the Earth-Sun distance required painstaking observational geometry relying on parallax methods applied across the solar system. One of the most significant historical methods employed by astronomers to achieve a relatively accurate measurement of the AU involved timing and observing the transits of Venus. During a transit, Venus passes directly between the Earth and the Sun, allowing observers from different points on Earth to measure slight angular discrepancies in the transit time. These triangulation calculations, based on the known separation between the observers, allowed astronomers to geometrically extrapolate the scale of the entire solar system, thereby determining the value of one AU.