Each position circle found using the Sextant Altitude is of immense radius. Therefore, any short length of the arc of some circle can be considered as an straight line .
Comparing the observed distance to the celestial body the calculated distance using an estimated position provides the distance towards, or away from the body..
The observed distance is known as the True Zenith Distance (TZD.) The value based on the assumed position is the Calculated Zenith Distance (CZD.) The difference between the two is known as the Intercept.
The closest point on this circle is known as the Intercept Terminal Position (ITP) and the line representing the circle at that position is called a Position Line.