ZENITH SECTOR

An astronomical instrument, con trived for the purpose of measuring, with great accuracy, the zenith distances of stars which pa near the zenith. It was by means of a zenith sector that Bradley discovered the existence and magnitude of two most important astronomical elements — the aberration of light, and the nutation of the earth's axis. The instrument has also been generally used (in this country at least) in trigonometrical surveys for determining the difference of latitude of two stations,a purpose for which it is very convenient; for the difference of the zenith distances of the same star, observed at its meridional pa ages at two places, gives the difference of the astronomical latitudes of the places without any regard to the star's declination. The general description of the zenith sector has been given by the present mer Royal as follows (Encyc. Mctrop., art. "Figure of the Earth"): "In the annexed figure A B is a bar of iron with a oi.)=-. piece C D, the whole in one piece. The top A is formed in such a way that the instrument can be turned half around in azimuth when suspended at the top, and that the bottom can be moved freely in the directionsDC or C D. The bracket or other support E, on which it rests, is attached to some firm part of the building. To the bar A B is firmly attached a telescope F G. At a point a, near A, is attached a plumb line a H; sometimes it is fastened at a point of attachment which is moveable, in order that by moving the point of suspension the plumb line may be made to pa over a fine dot at a. The limb C D is graduated, sometimes on a circular arc of which a is the centre, and sometimes on a straight line. L D is a screw pa ing through a block strongly connected with the floor, and acting with its point against the end D of the piece C D; and M N O are a string and weight pre ing the sector against the The point method of the of screw."observing with the zenith sector is this: A clock being regulated by transits or equal altitudes, the direction of the meridian is found, and guides fixed to compel the sector to move in the meridian. When a star is to be observed,the screw L D is turned till the plumb line falls ex actly on some point K of the graduated arc, such that the telescope is very nearly directed to the point through which the star will pa when it comes to the meridian. When the star enters the field of view, the screw is again turned till the star appears to glide exactly along the wire fixed ia the focus of the eye gla ; and the motion of the screw being ascertained, and the value of the division known, the apparent zenith distance of the star is found. But it is evident that this is not the true zenith distance, unle the zero point of the division on the scale be quite accurate; that ii to say, so placed that when the plumb line falls on it, the [s. 1362]