Transit Instrument

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Transit Instrument, one of the most important of astronomical instruments, consisting of a telescope fixed to a horizontal axis, so as to revolve in the plane of the meridian. It is employed in the observation of the meridian transits of the heavenly bodies. See Transit.

The meridian is marked by a reticle in the telescope composed of from 5 to 15 vertical wires and 2 horizontal wires. A lamp is used to illuminate the wires so that they can be seen at night through the telescope. The horizontal mounting axis is composed of two metal shafts fixed to the telescope tube and exactly perpendicular to it. The axis shafts rest on two support arms and bearings, which allow the telescope to be pointed from the horizontal through to the vertical along the meridian. These axis shafts must be so precisely machined and positioned in line with each other as to be essentially the same, as if they were portions of a solid shaft resting across the support arms. Four adjustments are necessary before a transit can be observed: The axis must be horizontal; the line of collimation must be at right angles to the axis of motion; the axis of motion must be placed so as to point accurately east and west; and the reticle must be exactly at the center of the line of collimation of the object glass. The accurate notation of the instant of time, by the astronomical clock, at which the object, such as a star, is seen to pass the center of the field of view is the essential part of a transit observation. This is effected by the use of electricity. At a certain point of its swing, a seconds pendulum makes a dot on a uniformly moving slip of paper. The instant of transit is similarly noted when the observer taps an electrical contact key; and the distance of this dot from the previous seconds dot, compared with the distance between two seconds dots, gives the time accurately almost to 0.01 sec.

The transit circle differs mainly from the older transit instrument in the addition, on the axis of rotation, of two large graduated circles that are read off by microscopes fixed on an independent coaxial wheel called an alidade; any variation in the position of the alidade may be detected by a large spirit level attached to it.

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