What scientific event did James Cook's expedition observe from Tahiti in 1769?

In 1769, James Cook and his expedition observed the transit of the planet Venus across the face of the Sun from Tahiti.21

This scientific observation was the primary objective of Cook's first voyage, commissioned by the Royal Society and the Admiralty. By carefully timing the transit, 18th-century astronomers were able to accurately measure the distance between Venus and Earth, which in turn allowed them to calculate the astronomical unit—the distance between Earth and the Sun—and determine the distances to other planets in the solar system.12