The Transit of Venus Expeditions Collection consists largely of the scientific output (e.g. observation notebooks, photographs, and journals) of USNO expeditions to sites around the world to observe the transits of Venus across the Sun in 1874 and 1882, there are well over 300 items comprising the observations of astronomers from sixteen sites. The information collected from these expeditions supplied the raw data for William Harkness's final calculation of the distance between the Earth and Sun: 92,797,000 miles with a probable error of 59,000 miles. In addition to astronomical observations, the scientists recorded detailed information about their environment and routine activities in journals and ledgers, and used their photographic equipment to capture stunning images of their respective sites. The archival material from the 1874 and 1882 expeditions is complemented by contemporary published materials in the Library's 19th century Collection and related objects in the Museum's Scientific Instrument Collection.