Monday, April 14, 2008

Terrestrial Sphere

The earth is called the terrestrial sphere, although it is a little flattened at the poles instead of being perfectly spherical. Reference points for location of objects on earth, with two exceptions, have been established by general agreement among maritime nations. The two exceptions are the North and South Poles, located at the ends of the axis on which the earth rotates. Imaginary lines running through the poles and around the earth are called meridians (these divide the earth into sections).

If you start at the North Pole and travel along a meridian exactly halfway to the South Pole, you will then be on the equator (an imaginary line running around the earth). The equator bisects every meridian and divides the earth in half. The half the North Pole is located is called the Northern Hemisphere and the half the South Pole is located is called the Southern Hemisphere.

CIRCULAR MEASUREMENT You must know something about how distances are measured along the circumference of a circle. Measurement along a meridian, which is a great circle, is expressed in terms of degrees of arc. These degrees of arc may be transformed into linear measurement expressed in nautical miles. The best example of circular measurements in degrees of arc is the compass card. Whatever the size of the card, its circumference is always 360° . Each degree contains 60 minutes, and each minute has 60 seconds.

MERIDIANS AND PARALLELS In developing a system for locating points on the terrestrial sphere, there are a series of meridians running through the poles of the earth and a single line called the equator, running around the earth at right angles to its axis. The equator divides each meridian and the earth itself into two exact halves.

There is a meridian (360 of them) for each degree around the earth’s rim. A starting point for numbering these meridians was required, and most of the maritime countries decided that the starting point should be the meridian passing through the Royal Observatory at Greenwich, England. The Greenwich meridian is number 0, meridians run from that meridian east and west to the 180th, on the opposite side of the earth from Greenwich. The complete circle formed by the 0 and the 180th meridians, the prime meridian, like the equator, divides the earth into two exact halves, the Eastern and the Western Hemispheres. Meridians run true north and south.

Beginning with the equator, you see lines that appear to be parallel to it, one for each of the 90° of arc from the equator to the North Pole. The planes forming these lines on the earth’s surface are actually parallel to each other, and for this reason they are called parallels. If you shift your eye to a point just above the pole, you can see that they are actually circles, growing smaller as they get farther from the equator and nearer the poles.