When an inferior planet is in superior conjunction, and “has a latitude or distance from the ecliptic less than the Sun's semi-diameter, it will be less distant from the Sun's centre than such semi-diameter, and will therefore be within the Sun's disc. In this case, the planet being between the Earth and the Sun, its dark hemisphere being turned towards the Earth, it will appear projected upon the Sun's disc as an intensely black round spot. The apparent motion of the planet being retrograde, it will appear to move across the disc of the Sun from east to west, in a line sensibly parallel to the ecliptic.” Such a phenomenon is called a transit, and as it can only occur in the case of the inferior planets, or those which pass between the Earth and the Sun, it is limited to Vulcan, Mercury, and Venus. The observations of these planets are used in a manner which we cannot here explain, for the purpose of ascertaining the distance of the Earth from the Sun.

James Gregory (inventor of the reflecting telescope which bears his name) seems to have been the first to point out this application of transit observations.