MERIDIAN CIRCLE. An instrument used for determining the meridian altitude or zenith distance of a star. It consists of an astronomical telescope firmly fixed to a graduated circle. which moves about a horizontal axis, resting OD a pair of very solid supports. In the common focus of the eye-piece and object-glass of the telescope is a system of fixed eross-wires (spider lines are generally used for the purpose), one being hori zontal, and five or more vertical, with equal spaces between. An imaginary line passing through the optical centre of the object-glass and the intersection of the horizontal and middle ver tical wires is called the line of collimation of the telescope, and, when the instrument is in per fect adjustment, this line moves in the plane of the meridian. Besides the above-mentioned fixed wires there is a movable one, called a micrometer wire, which is moved by means of a screw, re maining always parallel to the fixed horizontal wire. If the instrument is in perfect adjustment, and if the image of a star, while passing across the middle vertical wire in the field of view, is at the same time bisected by the fixed horizontal wire. the star is at that moment in the line of collimation of the telescope. It is therefore at that moment in the meridian. and its meridian zenith distance is the angle through which the circle would have been (turned from the position it had when the line of collimation of the tele scope pointed to the zenith. There is a fixed pointer, for the purpose of approximately read ing the instrument. if the instillment was ad justed so that the pointer was opposite the zero point of the circle. when the line of collimation of the telescope pointed to the zenith. the arc measured on the circle between these two posi tions of the instrument is the meridian zenith distance of the star.
Great nicety is required in reading the instru ment; i.e. in determining exactly the are through which the circle has moved in lninging the tele scope from the vertical to any other position. The rim is usually graduated at intervals of five minutes; and the eye could determine only the division nearest to the fixed index. But by means of a reading microscope or micrometer (q.v.), fixed opposite to the rim, the portion of the in terval to the Dearest division on the rim can be read to seconds. There are sometimes six such microscopes fixed opposite different points of the rim; and the reading of the instrument is the mean of the readings of all the microscopes. 'This tends to eliminate errors arising from imperfect graduation and errors of If the instrument is properly adjusted, the zero point of the circle will be opposite the tixed pointer when the line of collimation of the telescope points to the zenith. In practice, however, this
is not always accurately, or even approximately, the ease, and is really of no consequence, as the thinI result, of every observation is the difference between two readings. It is evident that the difference between any two readings of the instru ment will represent the angle through which the line of collimation of the telescope moves in pass ing from one position to the other. It remains to show how a fixed point. viz. the nadir (q.v.), is observed, and then how an observation is taken of the star itself on its meridian passage. It must be explained here that the fixed horizontal wire in the eye-piece of the telescope, in the instru ment as now used, is only an imaginary line, which determines the line of collimation of the telescope. It coincides with the position of the micrometer wire when the screw-head of the micrometer marks zero.
To observe the nadir, a trough of mercury is placed underneath the instrument, and the tele scope is turned so as to look vertically down ward into it. An image of the system of cross wires which is in the common focus of the object glass and eye-piece will be reflected back again to nearly the same focus. Looking into the tele scope, the ObSerVCr now adjusts it by means of a slow-motion screw till the reflected image of the horizontal wire coincides with the real one. The tinal adjustment is perhaps most delicately ef fected by turning the serew-load of the mierome ter which moves the wire itself. When they coincide, the line joining the centre of the object glass of the telescope with the intersection be tween the middle vertical and horizontal micro metric wire will be vertical. For thht position of the movable wire, the circle now gives the exact nadir reading, which differs 150° from the true zenith reading.
Again, to observe a star in the meridian, the instrument is previously adjusted so that the star, in passing the meridian, shall pass over the field of view of the telescope. As the image of the star approaches the centre of the field. the oh server adjusts the telescope by the slow-motion screw, so as very nearly to bring the image of the star to the horizontal wire. Finally, just as the star passes the middle vertical wire, lie bisects the image of the star with the hori zontal wire ln• a touch of the micrometer screw heml. The circle tieing now clamped (or made fast ), the• reading is determined as before by reading• the pointer and microscopes, and adding or subtracting, as the ease may he, the reading of the micrometer. This reading now subtracted from the zenith reading gives I he meridian zenith distance of the star; and this, igain, subtracted from Ott , gives its meridian altitude above the horizon. See TR.% NSIT IN STRU NI ENT.