An understanding of the relationship that exists between the earth and the celestial bodies is essential for establishing the position of the celestial bodies in relation to the observer’s position on earth, and for understanding the phenomenon of time. First of all, to determine the observer’s position on the earth’s surface, we assume that the earth is a sphere and that a co-ordinate system (grid) based on the earth’s axis of rotation, which is perpendicular to the equatorial plane, is established (fig.1). The ends of the axis are designated as the north and south poles of the grid. The great circle (formed by a plane passed through the centre of the sphere) that is half way between the poles is the equator. The great circles that pass through both the poles and intersect the equator at right angles are meridians. The meridian, which passes through the Greenwich Observatory, England, is the reference meridian or zero meridian.
The two co-ordinates, LONGITUDE and LATITUDE, determine the observer’s position on the earth’s surface. Longitude is the angle measured from the reference meridian, east or west, along the equator, to the meridian that passes through the site of interest. It varies from 0° to 180° east, or west. Latitude is the angle measured from the equator, north or south along a meridian, to the latitude circle that passes through the site. It varies from 0° to 90° north, or south. Thus, the position of the boat in fig.1 is 45° E, 20° N.