A small circle on a sphere representing the Earth, at every point of which, at the instant of observation, the observed celestial body (Sun, star, planet) has the same angular elevation and therefore the same zenith distance. At the instant of observation, the centre (on the sphere) of the circle of position is that point which has the same longitude and latitude as the celestial body. The radius of the circle is the observed zenith angle of the body. The point representing the observer is therefore somewhere on that circle. A second observation on the same object at a different time (or on a different object at the same time) will determine a second and different circle of position, and if the observer has not moved, his representing point will be at the intersection of these circles. If the observer has moved, as may occur if he is on a ship or aircraft, allowance is made for the direction and amount of movement and a location deter-mined for either point of observation. In navigation, a short portion of the circle of position is plotted as a straight line and termed a line of position or a Sumner line. A Sumner line, however, is merely a particular variety of line of position and the term should not be used as a synonym for line of position.