How did Edmond Halley use transits of Venus to propose measuring the distance from Earth to the Sun?

In 1716, Edmond Halley devised a method to determine the distance between Earth and the Sun by observing transits of Venus across the disk of the Sun.1

Known as Halley's method, the technique relies on measuring the duration of a transit from different locations on Earth that are widely separated in latitude. Because observers at different latitudes see the planet cross the Sun along slightly different paths, the duration of the transit varies. By comparing these precise timings, astronomers can calculate the solar parallax—the angle subtended by the Earth's radius as seen from the Sun.231

Once the solar parallax is known, the distance from Earth to the Sun (the astronomical unit) can be calculated using trigonometry. This measurement serves as a fundamental scale for determining the distances to other planets in the solar system. Halley proposed this method in anticipation of the transits that occurred in 1761 and 1769.341