Transit Instrument
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