Transit Circle

Transit Circle, or MERIDIAN CIRCLE, is an improved form of transit instrument. The latter was invented by Romer at the end of the 17th century for noting the time of a star's passage over the meridian. The time noted on a sidereal clock, and converted into degrees at the rate of 15° per hour, gives the right ascension of the star. A meridian quadrant had been used by Tycho Brahe, but neglected until Romer revived the idea that a fixed meridian instrument would enable observations to be made with far greater convenience than the method generally employed. It consists of a horizontal axis, with a telescope fixed at right angles to it, the whole being adjusted so that the telescope shall revolve in the plane of the meridian. The axis of the instrument consists of a cube of metal, at opposite sides of which are two cones ending in cylindrical pivots, resting in V-shaped bearings. The telescope is fixed in a hole through the centre of the cube, and a certain arrangement of levers and friction wheels is adjusted to relieve the pivots of most of the weight of the instrument. So far this is a transit instrument, and the transit circle differs from it in possessing two large circles - a yard or more in diameter - fixed on the axis at each side of the telescope, the circumference of each being graduated to 5' or less. The readings are made by four or more microscopes (which thus correct each other), often fixed to the stone piers which support the whole. The instrument is adjusted so that its axis is horizontal and points due east and west, while the axis of the telescope is at right angles to it. Although workmanship of the very highest order is expended, absolute accuracy in these adjustments is never obtained, but the errors due to the different deviations can be separately estimated, and the observed result can be corrected for them. It is essential that the telescope shall be quite rigid, for any bending will give rise to errors in the calculated declination of the observed star. The telescope is provided with several vertical and one or two horizontal spider-lines, and the time of passing each vertical line is noted by a clock. This is usually provided with a seconds pendulum which, by passing through a globule of mercury, completes an electric circuit once every swing. A time-recorder or similar contrivance is then brought into play, and a dot is marked on a moving slip of paper every second; but the observer, as he sees the star cross a certain line, can, by pressing a knob, also cause a dot to be made on the slip of paper. The distance of this dot from the previous second's dot enables him to tell to less than one-fifth of a second the exact time of passage of the star. By means of the transit circle not only is the time of transit of a star measured but also its declination - a feat impossible with the older transit instrument. The first transit circle used in Great Britain was set up at Greenwich in 1850, the transit instrument and mural circle having been used till then, although long superseded at Gottingen and Konigsberg by the circle.