A sextant is a navigational instrument designed to measure the angular distance between two visible objects. Its primary application is in celestial navigation, determining the altitude of celestial bodies like the Sun, Moon, or stars, above the observer’s horizon. This measurement, known as a “sight,” was historically crucial for sailors to calculate their position at sea. The sextant’s name originates from “sextans,” meaning “one-sixth,” as its graduated arc spans approximately 60 degrees.
The Optical Principles Guiding the Sextant
The sextant operates on the principle of double reflection. When a light ray reflects successively off two mirrors, the angle between the initial incident ray and the final reflected ray is twice the angle between the mirrors. The sextant utilizes a movable index mirror and a fixed horizon mirror. Light from a celestial body first strikes the index mirror, then reflects onto the horizon mirror, and finally enters the observer’s eye through a telescope.
The sextant’s design leverages this double reflection by marking its arc to directly display the true angle of the celestial body. For instance, a 60-degree arc allows for the measurement of angles up to 120 degrees. The horizon mirror is often partially silvered, allowing the observer to view the actual horizon directly while seeing the reflected image of the celestial body. This dual view allows for the superimposition of the celestial body onto the horizon, providing a stable reference point for measurement.
Anatomy of a Sextant: Essential Parts
A sextant is composed of several key components. The frame forms the instrument’s structural base. Attached to the frame is the arc, also known as the limb, a curved scale graduated in degrees, used for reading the measured angle. A movable index arm pivots at the center of the arc, carrying the index mirror and the reading mechanism.
The index mirror is mounted on the index arm and reflects light from the celestial body. The horizon mirror is fixed to the frame and is often half-silvered, allowing the observer to view the horizon directly through one part and the reflected celestial body through the other. A telescope is affixed to the frame, providing magnification to aid in aligning the celestial body with the horizon. Additionally, shades or filters are positioned in front of both mirrors to reduce glare, especially when observing bright objects like the Sun, protecting the observer’s eyes.
Step-by-Step: Taking a Measurement with a Sextant
Taking a measurement with a sextant involves a sequence of actions to determine a celestial body’s altitude. Hold the sextant vertically and look through the telescope, aligning the clear portion of the horizon mirror with the actual horizon. The index arm, initially set near zero, is then slowly moved while observing the horizon. As the index arm moves, the image of the celestial body appears in the mirrored section of the horizon glass, descending towards the horizon.
Adjust the index arm until the celestial body’s image, such as the lower edge of the Sun, appears to rest precisely on the horizon line. For accuracy, the sextant is gently swung from side to side in a small arc. This “swinging” technique reveals the lowest point of the celestial body’s arc, which represents its true altitude. Once the alignment is perfected, the index arm is locked, and the angle is read from the graduated arc and the finer scale on the micrometer drum or vernier.
The Sextant in Modern Navigation
While electronic navigation systems like GPS have become the standard for position determination, the sextant retains an important place in contemporary navigation. Modern vessels are typically equipped with advanced digital tools, yet the sextant remains on board many ships as a reliable backup instrument. Its independence from electrical power or satellite signals makes it a valuable tool in situations where electronic systems might fail or be unavailable.
The sextant also serves educational purposes, connecting modern navigators to historical maritime traditions and fostering a deeper understanding of celestial mechanics. For offshore yachtsmen and in scenarios where self-reliance is paramount, the ability to navigate using a sextant provides a robust alternative. Its enduring presence in navigation, despite technological advancements, highlights its utility and the ingenuity of its design.