The 1874 transit of Venus expeditions were designed to obtain an accurate estimate of the astronomical unit (AU), the distance from Earth to the Sun, by measuring solar parallax.
Notes on verification
Multiple independent academic sources (Harvard ADS, Springer, Cambridge Digital Library, Taylor & Francis) all corroborate the primary purpose of the 1874 transit expeditions. The sources consistently confirm the goal was measuring solar parallax to determine the astronomical unit.
Sources
- 1874 Transit of Venus Expedition to Hawaii (wikipedia)