A Mariner’s Guide to Principles, Usage, and Maintenance
The sextant, an essential instrument for celestial navigation, allows mariners to measure the angle between the horizon and celestial bodies like the Sun, Moon, or stars, determining latitude and longitude at sea.
Deriving its name from the Latin “sextus” (one-sixth), the sextant’s arc spans 60 degrees, based on the principle of double reflection, enabling measurements up to 120 degrees.
How it Works
To use a sextant, hold it vertically, align the horizon through the unsilvered part of the horizon mirror, and adjust the index arm until the celestial body’s reflected image appears to rest on the horizon.
The altitude is then read from the scale on the sextant’s arc. This versatile tool can measure Vertical Sextant Angles (VSA), Horizontal Sextant Angles (HSA), and altitudes, crucial for determining a ship’s position.
A Brief History
Early navigation tools like the kamal, used by Arab navigators from the 6th century, evolved into more precise instruments such as the astrolabe and quadrant. By the 15th century, Portuguese explorers used quadrants to measure the sun’s altitude, determining latitude based on its declination. Christopher Columbus extensively used this method on his voyages, marking latitudes of visited locations.
Key Components and Principles
The sextant includes an arc-shaped frame, a horizontal mirror, an index mirror, shade glasses (sunshades), a telescope, a graduated scale, and a micrometer drum gauge. A sextant operates on two basic principles: the angle of incidence equals the angle of reflection, and the angle between the incident and reflected rays is twice the angle between the mirrors after two successive reflections.
Navigating with Precision: Readings and Errors
The sextant’s arc is graduated “ON” (0 to 130 degrees) and “OFF” (a few degrees to the right of zero). Accurate readings are crucial for celestial navigation. Sextant errors fall into two categories: adjustable and non-adjustable. Adjustable errors, such as perpendicularity, side error, and index error, can be corrected onboard using specific procedures involving adjustments to the mirrors. Non-adjustable errors, including graduation, centering, shade, and optical errors, are typically due to manufacturing defects or wear and tear, requiring professional attention. It’s also essential to account for dip, the angle between the sensible and visible horizons, which varies with the observer’s height.
Mastering the Craft: Pointers for Accurate Sextant Use
- Always check for errors before use.
- Focus the telescope on the horizon and mark the stem’s circumference.
- Maintain a steady stance with feet slightly apart.
- Swing the sextant and measure altitude at the celestial body’s lowest point.
- Use appropriate dark shades when observing the Sun.
- Avoid nighttime observations due to potentially false horizons.
Care and Maintenance: Preserving Precision
- Avoid stressing the index bar and touching the arc.
- Keep the worm and rack clean and lubricated with Vaseline.
- Clean mirrors, lenses, and shades with a soft cloth.
- Store the sextant in its box, away from sunlight, dampness, and heat sources.
The sextant is a precision instrument requiring careful handling and maintenance. By understanding its principles, mastering its usage, and diligently addressing errors, mariners can ensure accurate navigation and preserve this timeless tool’s functionality.