an astrometric instrument used to determine the transit times of heavenly bodies in their apparent diurnal motion through a given vertical. The transit—more precisely, its line of sight—usually has its line of collimation in the plane of the meridian in order to obtain the right ascensions of stars and to determine corrections to the clock. To obtain stellar declinations and local latitude, the line of collimation should be in the plane of the prime vertical.
The transit instrument was invented in 1689 by the Danish astronomer O. Romer. A stationary transit consists of an astronomical telescope having an objective lens with a diameter of about 18 cm and a focal length of about 2 m. The instrument is mounted on a horizontal axis of rotation (about 1 m) and rests on pillar supports. Smaller, portable transits are used for time services. An ocular micrometer having a reticle with vertical and horizontal wires is situated in the focal plane of the transit objective. The times at which the image of a star passes across the vertical wires are registered by a chronograph. In the mid-19th century, keys depressed by the observer at the appropriate time were used to make the recordings. In modern transits a registering micrometer, invented in the late 19th century, is used for visual observations.
Two Soviet astronomers, N. N. Pavlov and V. E. Brandt, developed a method of photoelectrically registering the transits of stars, thereby increasing the precision with which clock corrections could be determined by time services and eliminating human error from transit observations. The accuracy of a single calculation of the right ascension of a star using a stationary transit is about ±0.015 sec, while the precision of a single photoelectrically determined clock correction is about ±0.005 sec.
V. V. PODOBED