year.
The measured angle between the celestial body and the visible horizon is directly related to the distance between the celestial body's GP, and the observer's position. After some computations, referred to as "sight reduction," this measurement is used to plot a line of position (LOP) on a navigational chart or plotting work sheet, the observer's position being somewhere on that line. (The LOP is actually a short segment of a very large circle on the earth which surrounds the GP of the observed celestial body. An observer located anywhere on the circumference of this circle on the earth, measuring the angle of the same celestial body above the horizon at that instant of time, would observe that body to be at the same angle above the horizon.) Sights on two celestial bodies give two such lines on the chart, intersecting at the observer's position. That premise is the basis for the most commonly used method of celestial navigation, and is referred to as the "Altitude-Intercept Method."
There are several other methods of celestial navigation which will also provide position finding using sextant observations, such as the "Noon Sight", and the more archaic "Lunar Distance" method. Joshua Slocum used the Lunar Distance method during the first ever recorded single-handed circumnavigation of the world. Unlike the Altitude-Intercept Method, the noon sight and lunar distance methods do not require accurate knowledge of time. The altitude-intercept method of celestial navigation requires that the observer know exact Greenwich Mean Time (GMT) at the moment of his observation of the celestial body, to the second.