A circle on the Earth's surface on which the observer is situated at some point. The centre of the circle is directly under a celestial body, and its radius is equal to the zenith distance of the star. The position of the circle on the Earth is calculated by measuring the altitude of the celestial body at a particular time. In practice it is possible to select a small part of the circle, called the position line, on which the observer is situated. If a second position line is found from another observation (either of the same object at a different time, or of a different object at the same time) the intersection of these lines identifies the observer's location.
Subjects: Astronomy and Astrophysics.