The term “transit” is used in several ways in astronomy. A hundred years ago its most common usage referred to the image of a star, planet, or other celestial body traversing the cross-hairs of a telescope that was fixed to move only in a north-south direction, and was essential to the determination of time and celestial coordinates. A second common usage of transit refers to a satellite passing in front of its parent planet as seen from the Earth. It was suggested as early as Galileo that the timing of such transits across the face of Jupiter by its four largest satellites would provide a solution to the age-old problem of navigation at sea—a solution which, however, never proved satisfactory. A third usage of transit refers to the passing of either Mercury or Venus in front of the Sun as seen from Earth. It is this phenomenon that will be examined in this article.