What astronomical event occurs when Venus passes between Earth and the Sun?

A transit of Venus occurs when Venus passes directly between Earth and the Sun. From Earth, Venus appears as a small, dark disk moving across the Sun's bright face.

This alignment is rare because Venus's orbit is tilted about 3.4 degrees relative to Earth's orbital plane. Most times Venus passes between Earth and the Sun, it travels slightly above or below the Sun from our viewpoint. A transit requires Venus to reach that alignment while also crossing the appropriate point in its orbit.

Transits usually arrive in pairs separated by eight years, followed by a gap of more than a century. The most recent pair occurred in 2004 and 2012; the next is scheduled for 2117 and 2125. Historically, astronomers used these events to estimate the distance from Earth to the Sun. A transit must never be viewed without certified solar-filter equipment, because ordinary sunglasses cannot protect the eyes.