The transit of Venus is an astronomical event that occurs when the planet Venus passes directly between the Earth and the Sun. During this event, Venus appears as a small black dot moving across the bright surface of the Sun. Transits of Venus happen in pairs, approximately every 105.5 and 121.5 years, with the last pair occurring in 2004 and 2012. Historically, the transit of Venus was significant for astronomers, as it provided a way to measure the distance from the Earth to the Sun, known as the astronomical unit. Observations of these transits were made by various explorers and scientists, including James Cook in 1769, who traveled to the South Pacific to observe the event.