Transit of Venus is an astronomical phenomenon in which the planet Venus passes directly between the Earth and the Sun, appearing as a small black dot moving across the face of the Sun. It is a rare event, occurring in a pattern that repeats every 243 years, with pairs of transits eight
From Wikipedia
A transit of Venus takes place when Venus passes directly between the Sun and the Earth, becoming visible against the solar disk. During a transit, Venus is visible as a small black circle moving across the face of the Sun.