§1. The Various Types
98. The Altitude Azimuth Method of Locating the Geographic Meridian.-From the known position of a celestial body, together with simple astronomical observations, the direction of the true north at the place of observation can be determined. Accurately predicted positions of various celestial bodies as well as the times at which certain celestial phenomena will occur are given in Nautical Almanacs published annually by the governments of our maritime nations. Useful tables and all the standard methods of determining navigational quantities are given in Bowditch's American Practical Navigator published by the Hydrographic Office of the U. S. Navy.
The geographic meridian of a place on the earth is the great circle of the earth that passes through the given place and the poles of the earth. This line is in the true north-south direction at the given place. The line in the direction of the horizontal component of the magnetic field of the earth at any given place is called the magnetic meridian at that place. At very few places is the magnetic meridian in the plane of the geographic meridian. A magnetic needle unaffected by any force except that due to the earth's magnetic field sets itself in the plane of the magnetic meridian at the given place. The compass bearing of any object is the angle at the center of the compass card between the magnetic axis of the compass needles and the straight line from the center of the card to the given object.
A celestial body may be located by two quantities. There are three pairs of such quantities commonly employed to locate an object in the sky. They are called altitude and azimuth, declination and right ascension, celestial latitude and celestial longitude.
The celestial sphere is an imaginary sphere of infinite radius onto which, to an observer on the earth, the celestial bodies appear to be projected. Figure 127 represents the celestial sphere, drawn as though it were of finite radius. The small circle represents the