Although the earth is not actually a true sphere, it is presumed to be such for the purpose of astronomy. Astronomic determinations are based on the relationships that exist among sets of spherical coordinates: the terrestrial system stated in latitude and longitude; the celestial system of right ascension and declination, or its subsidiary system of hour angle and declination; and the horizon system in terms of altitude and azimuth.

The terrestrial system of coordinates refers to the location of points on the terrestrial sphere (the earth). In the terrestrial system, the fundamental reference lines (fig. 15-1) are the axis of the earths rotation and the earths equator. The ends of the axis of rotation are known as the poles, designated as the North and South

A great circle passing through both poles is called a meridian. The equator is a great circle about the earth equidistant from the poles and perpendicular to the axis of rotation. Through any point removed from the equator, a circle whose plane is parallel to that of the equator is called a parallel of latitude. The numerical value of the parallels defines latitude and that of the meridians defines longitude.

As shown in figure 15-2, geographic latitude of a point may be defined as its angular distance above or below the equator. Latitudes are expressed in degrees and are measured from 0 to 90 north or south. The conventional symbol for latitude used in computation is the Greek letter F.

As shown also in figure 15-2, the longitude of a point is the angular distance measured along the equator between the meridian passing through a point and a reference meridian. The chosen reference meridian is the Greenwich meridian that passes through Greenwich, England. That meridian is known as the primary or prime meridian. Longitude is also expressed in degrees but is measured from 0 to 180 west or east from the prime meridian. The conventional symbol for longitude is the Greek letter l (lambda).