The apparent passage of a celestial body across the apparent disk of a larger body, such as a planet across its parent star or of a satellite across its parent planet; also, the apparent passage of a celestial object or reference point across an adopted line of reference in a celestial coordinate system. Classically, the observed data were instants of internal and external tangency of the disks (contacts) at ingress and egress of the smaller body. In the modern era, data may also include the differential brightness of the two disks and the duration of any change of brightness.