Transit is the movement of one body in the sky in front of another body that appears larger. As seen from Earth, the planets Mercury and Venus occasionally transit the sun. During a transit, the planet appears as a dark dot moving across the sun’s disk. Transits of Mercury occur about 13 times in each century. Venus transits occur in pairs in which one transit happens 8 years after the other. These pairs are separated by alternating intervals of 105 1/2 or 121 1/2 years.
In 1639, the English astronomers Jeremiah Horrocks and William Crabtree recorded the first observations of a transit of Venus. Edmond Halley , another English astronomer, later proposed timing transit events from various locations to calculate the distance from Earth to the sun. The average such distance is called the astronomical unit (AU). Astronomers refined their measurements of the AU during the Venus transits of 1761, 1769, 1874, and 1882. In 2004, the first of a pair of Venus transits occurred. The second transit took place in 2012.
In the late 1990’s, astronomers began to use observations of transits to help discover distant planets. Although the planets are too far away to be seen, astronomers can detect the slight dimming they cause when they pass in front of the star they are orbiting. In 2006, a team led by the French national space agency launched a spacecraft called COROT to search for distant transits of planets only slightly larger than Earth. The United States launched the Kepler spacecraft, designed to look for transits of Earth-sized planets, in 2009.
The word transit can also mean the instant when a body passes directly over an observer’s meridian. The meridian is an imaginary north-south line passing through the observer’s position.
