As they journey for some distance, travelers observe that the celestial bodies appear to change their paths across the sky and to rise and set at new points along the horizon. Since the apparent positions of celestial bodies thus change with time and with changes in an observer’s position on the nearly spherical earth, the location of a ship or other craft may be determined by careful observations of celestial bodies.
Celestial bodies – such as the stars – are so far from the earth that they appear to be located on the inside surface of an imaginary hollow sphere. This sphere – which has an infinite radius – is called the celestial sphere. Its center coincides with the center of the earth. All points on the earth’s surface are considered to be projected onto the celestial sphere, as are the equator, the parallels of latitude, and the meridians.
For the purpose of navigation, a system of coordinates is required on the celestial sphere in order that the position of a celestial body at any time may be accurately described. One such system is the celestial equator or equinoctial system.
In this system the celestial equator or equinoctial is the base – or primary – circle. It corresponds to the earth’s equator. At right angles to the celestial equator are the hour circles. An hour circle is a great circle on the celestial sphere that passes through the poles and through a celestial body or point. Each meridian of the celestial sphere is identical with an hour circle.
The declination (dec.) of any point on the celestial sphere is its angular distance north or south from the celestial equator, measured along the hour circle that passes through the point. Declination on the celestial sphere corresponds to latitude on the earth’s surface.
The Greenwich hour angle (GHA) of any point or body is the angle – measured at the pole of the celestial sphere – between the celestial meridian of Greenwich and the hour circle of the point. The angle is measured along the celestial equator westward from the Greenwich celestial meridian – from 0? through 360?. The GHA differs from longitude on the earth’s surface in that longitude is measured east or west – from 0? through 180? – and remains constant. The GHA of a body increases through each day as the earth rotates.
The GPS satellites orbit the earth twice a day, 11,000 miles above the earth, transmitting their precise signal position and elevation. The GPS receiver acquires the signal, then measures the interval between transmission and receipt of the to determine the distance between the receiver and the satellite. Once the receiver has calculated this data for at least 3 satellites, its location on the earth’s surface can be determined.