## Azimuth of Polaris at Greatest Elongation

The1 simplest method of determining the direction of the meridian with accuracy is by means of an observation of the Fig. 65. Constellations near the North Pole. Polaris at Western Elongation Fig. 65. Constellations near the North Pole. Polaris at Western Elongation polestar, or any other close circumpolar, when it is at its greatest elongation. (See Art. 19, p. 36.) The appearance of the constellations at the time of this observation on Polaris may be seen by referring to the star map (Fig....

## Chapter

SYSTEMS OF COORDINATES ON THE SPHERE ii. Spherical Coordinates. The direction of a point in space may be defined by means of two spherical coordinates, that is, by two angular distances measured on a sphere along arcs of two great circles which cut each other at right angles. Suppose that it is desired to locate C (Fig. 13) with reference to the plane OAB and the line OAy 0 being the origin of coordinates. Pass a plane OBC through OC perpendicular to OA B these planes will intersect in the line...

## Elimination of Errors

It is usually more difficult to measure an altitude accurately with the transit than to measure a horizontal angle. While the precision of horizontal angles may be increased by means of repetitions, in measuring altitudes the precision cannot be4 * The axis of a level may be defined as a line tangent to the curve of the glass tube at the point on the scale taken as the zero point, or at the centre of the tube. increased by repeating the angles, owing to the construction of the instrument. The...

## Azimuth by an Altitude of a Star near the Prime Vertical

The method described in the preceding article applies equally well to an observation on a star, except that the star's image is bisected with both cross hairs and the parallax and semi-diameter corrections become zero. The declinatioh of the star changes so little during one day that it may be regarded as constant, and consequently the time of the observation is not required. Errors in the altitude and the latitude may be partially eliminated by combining two observations, one on a star about...