Navigation and Seamanship

1.2.2. Equator

                                                     

         The Earth it is covered with a network of lines – both vertical and horizontal from which the location of a place can be described. Very early maps (which usually showed small local or regional areas) used a grid technique which relied on simply measuring the distance and direction between points of interest and then plotting these onto the ‘piece of paper’. This method assumed that the Earth was flat. With the general agreement that the Earth was, in fact, round; a different methodology needed to be developed. The system that has been developed over many centuries is called latitude and longitude. The location and measurement of latitude and longitude essentially involves complex mathematics (especially geometry) and a series of international agreements/conventions for recording locations on the surface of the Earth.

         The earth can be described more exactly as a “spheroid,” which simply means a less than perfect sphere. The equatorial diameter of the earth is not quite 6,888 nautical miles (12,757 km); the polar diameter is nearly 6,865 nautical miles (12,714 km), or about 23 miles(43 km) less.

         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

           Points on the surface of a sphere at rest are similar because they are equidistant from its center. Lines passing through the center of the sphere, between two points on its surface, are also similar. None of the lines has distinguishing characteristics which render it suitable as a reference for navigational measurements.