Celestial navigation is a fundamental skill for pilots, particularly in situations where modern electronic navigation systems may fail. It involves using celestial bodies (stars, the Sun, the Moon, and planets) to determine a vessel's or aircraft's position on Earth. This sub-topic will introduce the core principles behind this ancient yet vital navigation technique.

Imagine an imaginary sphere of infinite radius surrounding the Earth. All celestial bodies appear to be projected onto this sphere. This concept, known as the celestial sphere, simplifies understanding the apparent motion of stars and other celestial objects from our perspective on Earth. Key points on this sphere include the celestial poles (extensions of Earth's rotational axis) and the celestial equator (an extension of Earth's equator).

While many celestial bodies can be used, the Sun, Moon, and specific bright stars are most commonly employed in celestial navigation. The Sun's position is vital for determining time and latitude, especially during the day. Stars, due to their relative fixed positions and brightness, are used for more precise fixes, particularly at night. The Moon's position can also be used, though its movement is more complex.

The sextant is the primary instrument for celestial navigation. It measures the angle between a celestial body and the horizon. This measured angle, known as the altitude, is crucial for calculating the observer's position. The sextant works by using mirrors to bring the image of the celestial body down to the horizon line, allowing for precise measurement.

A single measurement of a celestial body's altitude doesn't give a precise position. Instead, it defines a Line of Position (LOP). An LOP is a circle on the Earth's surface where, if you were located anywhere on that circle, the celestial body would appear at the measured altitude. By obtaining two or more LOPs from different celestial bodies (or the same body at different times), navigators can intersect these lines to determine their actual position, known as a 'fix'.

To perform celestial navigation calculations, navigators rely on publications like the Nautical Almanac or Air Almanac. These almanacs contain precise astronomical data for celestial bodies at specific times, including their Greenwich Hour Angle (GHA) and Declination. This information is essential for comparing the observed altitude with the calculated altitude for a given position and time.

The process of calculating a fix involves several steps: observing the altitude of a celestial body, noting the precise time of observation, looking up the body's position in the almanac, and then performing calculations (often using sight reduction tables or trigonometric formulas) to determine the LOP. Intersecting multiple LOPs provides the navigator's position. This process requires accuracy in observation, timing, and calculation.