Many cadets preparing for officer exams feel that celestial navigation is difficult. But in fact, once you understand the core principle, it’s quite logical and elegant. Here's a simplified explanation to help make this topic more accessible to everyone.
1. Basic Principle
- The Earth rotates on its axis and revolves around the Sun. Meanwhile, celestial bodies (e.g., the Sun, stars, planets) appear to have specific positions in the sky at any given time.
- The Nautical Almanac provides the Geographical Position (GP) of a celestial body — the point on Earth directly underneath it (where the body would be seen at the zenith).
- GP Coordinates:
- Latitude = Declination (Dec) of the celestial body.
- Longitude = GHA (Greenwich Hour Angle) – measured westward from Greenwich.
2. Observing the Celestial Body
- From your location at sea (Point A), use a sextant to measure the angle between the celestial body and the horizon — this is the observed altitude (Ho).
- Because celestial bodies are very far away, their light rays are nearly parallel as they reach Earth.
- By measuring Ho, we can calculate the angle between the observer, the Earth's center, and the GP (angle AOC).
- The smaller this angle, the higher the celestial body appears → meaning the ship is closer to its GP.
3. Sight Reduction: Intercept Method
Since the distance between the ship and the GP is usually large (hundreds or thousands of nautical miles), we can't use a compass and circles like we do in terrestrial navigation. Instead, we use the intercept method
- Choose an assumed position (AP) or estimated position (Point M) near the expected ship's location.
- Use a sight reduction table, calculator, or ECDIS to compute:
- Hc (Computed Altitude) – the altitude the celestial body would have if you were at point M.
- Zn (Azimuth) – the direction from M to the GP.
- Compare Ho and Hc:
- If Ho > Hc → the ship is closer to the GP than point M.
- If Ho < Hc → the ship is farther from the GP than point M.
- From point M, draw a line toward the GP (along azimuth Zn), then draw a line perpendicular to this azimuth, shifted by the intercept (Ho - Hc) → this is a Line of Position (LOP).
4. Fixing the Position
- Repeat the process with a second celestial body to get another LOP.
- The intersection point of these two lines is your true position.
- (If the two sights are taken at different times, use Running Fix, as in terrestrial navigation.)
5. Corrections to be applied
- Parallax: is the apparent shift in the position of a celestial body when viewed from Earth compared to when viewed from the center of the Earth. This correction is typically needed when observing celestial bodies that are close, such as the Sun, Moon, or planets. Parallax corrections are usually provided in the Nautical Almanac.
- Semi-Diameter : since we measure the altitude of a celestial body from its edge, not from the center of the body, this requires a semi-diameter correction. This is especially important when observing the Sun or Moon. Semi-diameter corrections are also provided in the Nautical Almanac.
- Solar Parallax Correction: The Sun is one of the most frequently observed celestial bodies. However, since the Sun is not a point but a large body, a correction for light scattering when observed from its edge is necessary. This correction adjusts the distance between the Sun and Earth.
- Lunar Parallax Correction: Similar to the Sun, the Moon also requires a parallax correction when observed at the edges, since we measure from the edge and not from the Moon's center.
- Refraction Correction (Atmospheric Distortion): When observing celestial bodies near the horizon, Earth's atmosphere distorts the line of sight, making the celestial body appear higher than it actually is. This is called refraction, and it needs to be corrected to improve accuracy, especially when celestial bodies are close to the horizon.
- Time Error Correction: Time measurement is a crucial factor in determining position. If the ship’s chronometer is inaccurate (not synchronized with UTC - Universal Time Coordinated), it will introduce errors in calculating GHA and LST (Local Sidereal Time). Corrections for time errors are necessary to ensure accurate positioning.
- Dip of the Horizon Correction: When observing from a ship or elevated position (e.g., on a ship's deck), the horizon is not perfectly flat but is slightly lowered. This effect, called the dip of the horizon, needs to be corrected for more precise measurements, especially when the observer is at height. The correction for the dip is usually available in Nautical Tables or can be calculated using a formula based on the height of the observer.
6. In Summary, You Need to Know:
- How to properly use a sextant.
- How to use the Nautical Almanac to obtain Dec and GHA (coordinates of the GP).
- How to use a sight reduction table, calculator, or ECDIS to find Hc and Zn.
- How to plot the LOP using the intercept method, and intersect multiple LOPs to fix the ship’s position.
BBC – SMR Marine
Crewing Department
Capt. Tong Tran Trung
