Figure and size of the earth

As a first approximation the earth may be considered as a sphere, but according to accurate observations, its figure is more closely represented by an ellipse of rotation - that is, an oblate spheroid, the shorter axis being the axis of rotation. The figure of the earth has been defined by various empirical.equations, the constants of which being based on observations and being subject to modification as the number of observations increases and their accuracy is improved. The geometrical figures defined by these equations cannot exactly represent the shape of the earth because of the a symmetrical distribution of the water and land masses.

To define the position of a point on the earth's surface a system of coordinates is needed, and as such the terms latitude, longitude and elevation or depth are used. The first two are expressed by angular coordinates and the third is expressed by the vertical distance, stated in suitable linear units, above or below a reference level that is closely related to mean sea level. The latitude of any point is the angle between the local plumb line and the equatorial plane. Because the earth can be considered as having the form of a spheroid, and as the plumb line, for all practical purposes, is perpendicular to the surface of the spheroid, any plane parallel to the equator cuts the surface of the spheroid in a circle,. all points on this circle having the same latitude. These circles are called parallels of latitude. The latitude is measured in degrees, minutes, and seconds north and south of the equator, the linear distance corresponding to a difference of one degree of latitude would be the same everywhere upon the surface of a sphere, but on the surface of the earth the distance represented by a unit of latitude increases by about 1 per cent between the equator and the poles.

The line in which the earth's surface is intersected by a plane normal to the equatorial plane and passing through the axis of rotation is known as a meridian. The angle between two meridian planes through two points A and B on the surface of the earth is the difference in longitude of the two points. In practice, the longitude is the angular distance measured in degrees, minutes, and seconds east or west from a standard meridian plane, generally that of the royal observatory at Greenwich, England. Thus, the longitude is measured from 0° to 180° east and west of Greenwich. On the surface of the earth the linear distance equivalent to unit difference in longitude is nearly proportional to the cosine of the latitude.

The distance between points on the earth's surface and the area represented by a given zone cannot be correctly represented unless the size of the earth is known.

Дата добавления: 2015-07-19