Circle of Position (Circle of Equal Altitude)

Definition

Observers anywhere on a circle centred on the geographical position see the body at the same altitude. The circle's radius, the zenith distance, equals 90 degrees minus the observed altitude, and one minute of arc equals one nautical mile. A body observed at 40 degrees altitude thus gives a circle of radius 3000 nautical miles. Because such circles are enormous, only a short arc near the assumed position is plotted, and it is treated as a straight line.