ZENITH TELESCOPE. An instrument espe cially adapted for the measurement of small differences in the zenith distances of celestial objects. It consists of a vertical column re volving about a vertical axis and carrying a horizontal axis, to one end of which the tele scope is attached and to the other a weight to counterpoise the telescope. To this horizontal axis is also attached a striding level. The ver tical column carries also a vernier and a clamp. The distinctive features of the instrument are a spirit level and a micrometer. The spirit level is attached by means of a pivot and a bar to the telescope and at right angles to the horizon tal axis so that it is free to move in the plane of the telescope and consequently may he inclined to the optical axis at any angle, read off by means of a vernier attached to the bar carrying the spirit level. When the bubble of the level is in the middle of the tube, this circle, graduated from 0° to 90° in each direction, will give the zenith distance of the star toward which the telescope is directed. The micrometer consists of one or more movable threads carried by a single micrometer screw with a graduated head reading to the required accuracy.
The zenith telescope was invented by Captain Andrew Talcott, of the 'United States Engineers (1834), to carry out practically the principle that when the meridian zenith distances of two stars one north and the other south of zenith), at their upper culminations, are equal, the colat itude is the mean of their north polar distances.
This method of finding the latitude is known as Talcott's method. Since two stars having exactly the same meridional zenith distance can rarely lie found. two having nearly the same are selected. Refraction is consequently almost elimi nated. The telescope is first pointed to the star which first comes to the meridian, and the spirit level is set horizontal. As the star passes the field of view its distance north and south of the central horizontal wire is measured by the micrometer. The instrument is then revolved I SO' and the second star observed in the same manner. In revolving the instrument care must be taken that the angle between the level and the telescope is not disturbed. The latitude is then given by the formula = , + where and are the declinations of the two stars; and — the difference of their zenith distances, determined by the micrometer meas urement. Consult Chauvenet, Spherical and Practical Astronomy (Philadelphia, 1863).