Earth and its coordinates

The Earth is an oblate spheroid shape (a sphere flattened at the poles). The Earth's Coordinate System (Latitude and Longitude) It is covered with a network of lines – both vertical and horizontal from which the location of a place can be described. The system that has been developed over many centuries is called latitude and longitude. The axis on which the earth rotates is called the geographic Polar Axis. The two points where the axis meets the surface of the sphere are called: * the North Geographic Pole - or True North; and * the South Geographic Pole - or True South Equator (Great circle): Half way between the two poles a plane perpendicular to the axis intersects the surface of the earth in a line known as the equator. Great circle is a circle on a sphere whose plane passes through the centre of the sphere. If one cuts a sphere along a great circle the plane will pass through the centre and divide the sphere into two equal halves.

Equator (Great circle): Half way between the two poles a plane perpendicular to the axis intersects the surface of the earth in a line known as the equator. Great circle is a circle on a sphere whose plane passes through the centre of the sphere. If one cuts a sphere along a great circle the plane will pass through the centre and divide the sphere into two equal halves. Poles of the earth: Poles of the earth are the two points 90o away from any point on the equator and are the points through which the axis of the earth is assumed to pass. Great circles: A great circle is a circle formed on the surface of the earth by the intersection of a plane passing through the center of the earth, thereby dividing the earth into two equal parts.

Parallels and meridians: Certain great and small circles have applications in navigation. A parallel is a small circle on the earth’s surface whose plane of inter section is parallel to the plane of the equator. Any circular line which runs around the Earth at its fattest point is called a Great Circle. Small circle  It is a circle on a sphere whose plane does not pass through the centre. Terrestrial co-ordinates The location of any point on the earth may be defined in a system of terrestrial coordinates.

Latitude (L, lat.) It is the angular distance from the equator, measured north-ward or south-ward along a meridian from 0° at the equator to 90° at the poles. It is designated north (N) or south (S) to indicate the direction of measurement Longitude (l, long.) It is the angular distance between the prime meridian and the meridian of a point on the earth, measured eastward or westward from the prime meridian through 180°. It is designated east (E) or west (W) to indicate the direction of measurement. The prime meridian, longitude 000°, is universally taken as the meridian which passes through the original site of the Royal Observatory, Greenwich, England. Degree length : The length of a degree of latitude (measured along a meridian ) is everywhere the same on a sphere, from the equator to the poles. On the earth, for practical navigational purposes, it is equal to 60 nautical miles(111.12 km), and 1 minute of latitude is equal to 1 nautical mile (1.852 km).

Distance in Navigation The shortest distance (abbreviated as Dist., symbol D) between any two points on the surface of the earth is always along the great circle between them. Speed (S) It is rate of motion, or distance per unit of time. A knot (kn.), the unit of speed commonly used in navigation, is a rate of 1 nautical mile per hour. Directions in Navigation Direction is the position of one point relative to another. Navigators express direction as the angular difference in degrees from a reference direction, usually north or the ship’s head. (C, Cn) is the horizontal direction in which a vessel is steered or intended to be steered, expressed as angular distance from north clockwise through 360°. The course is often designated as true, magnetic, compass or grid according to the reference direction. True Direction If direction is expressed in reference to the geographic pole, then it is said to be True Direction

Magnetic Direction The surface of the Earth is covered by a weak magnetic field. The entry and exit points of the field through the earth's surface (which is where the field lines are perpendicular to the surface) are called the Magnetic Poles. Rhumb lines For practical purposes, a rhumb line (also known as loxodromes or loxodromic spirals) can be defined as a line which crosses every meridian of the sphere at the same angle. In other words, a ship may maintain a true heading without change from starting point to destination (if, for the moment, one disregards factors such as currents, wind and changing magnetic variation).