NAVIGATION AND SURVEYING. CHAPTER I. PLANE SAILING. 1. The daily revolution of the earth is performed around a straight line, passing through its centre, which is called the earth's axis. The extremities of this axis on the surface of the earth are the terrestrial poles, one being the north pole, and the other the south pole. The section of the earth, made by a plane passing through its centre and perpendicular to its axis, is the terrestrial equator. [B. p. 48.] 2. Parallels of latitude are the circumferences of small circles, the planes of which are parallel to the equator. 3. Meridians are the semicircumferences of great circles, which pass from one pole to the other. The first meridian is one arbitrarily assumed, to which all others are referred. In most countries, that Latitude. Longitude. has been taken as the first meridian which passes through the capital of the country. But, in the United States, we have usually adhered to the English custom, and we consider the meridian, which passes through the Observatory of Greenwich, as the first meridian. [B. p. 48.] 4. The latitude of a place is its angular distance from the equator, the vertex of the angle being at the centre of the earth; or, it is the arc of the meridian, passing through the place, which is comprehended between the place and the equator. [B. p. 48.] Latitude is reckoned north and south of the equator from 0° to 90°. 5. The difference of latitude of two places is the angular distance between the parallels of latitude in which they are respectively situated, the vertex of the angle being at the centre of the earth; or it is the arc of a meridian which is comprehended between the parallels of latitude. [B. p. 52.] The difference of latitude of two places is equal to the difference of their latitudes, if they are on the same side of the equator; and to the sum of their latitudes, if they are on opposite sides of the equator. [B. p. 50.] 6. The longitude of a place is the angle made by the plane of the first meridian with the plane of the meridian passing through the place; or it is the arc of the equator comprehended between these two meridians. [B. p. 48.] |