TRANSIT TELESCOPES AND MERIDIAN CIRCLES
The transit telescope is used for computing the time by observing the transit of a star across the meridian and subsequently checking that its right ascension corresponds to the time recorded at that moment by a sidereal time clock. It consists of a telescope that is able to revolve around the east-west axis and that can consequently carry out observations exclusively along the local meridian.
The measurements of star transits on the multi-wire reticle present in the focus of the object lens were performed using the so-called "eye and ear" method. This method consisted of noting the time (hour and minute) signaled by the clock when the star approached the wire. The astronomer then counted the seconds struck by the clock observing the transit of the star on the cross wires of the reticle. In this way it was possible to estimate fractions of a second too by computing the moment of transit between two successive cross wires of known distance.
To avoid the obvious "personal errors" associated with this method, a recording mechanism was introduced in the XIXth century consisting of a telegraphic-type system on which the observer punched in the signal every time the star crossed each of the wires. It was Johann Adolf (Hans) Repsold (1838-1919) who in 1890 introduced the "impersonal" micrometer, comprising a vertical wire that could be moved by means of a micrometer screw. Following the motion of the star with a movable wire, the micrometer screw was able to break contact with a telegraphic recorder, called a chronograph since it could record on paper times taken from a clock.
The operation mode of the transit telescope was, consequently, identical in principle to the later Meridian Circle, though simpler, since it did not have the large graduated circle for measuring declinations attached to the axis, and less accurate.
The Meridian Circle also consented the measurement of zenithal distances, making it thus possible to calculate, the local latitude being known, the other coordinate of the star - the declination - as well as the coordinate that could be worked out from determination of the transit time - the right ascension.