Kepler's laws of planetary motion
In the mid-18th century, the dynamics of the Solar System were reasonably well understood, but astronomers only had an approximate idea of its scale. If the distance between two planets could be measured, all the other distances would be known from Kepler's laws of planetary motion. The best candidate for an accurate measurement was the distance between the Earth and Venus, which could be calculated from observations of transits of Venus, when Venus passes directly between the Earth and the Sun, appearing as a small black dot moving across the face of the Sun.