A sextant is an optical instrument used in navigation to measure the angular distance between two visible objects, most commonly a celestial body and the horizon. This measurement, known as a “sight,” allows navigators to determine their position. Historically, sextants played a significant role in precise oceanic exploration and trade for centuries. Even with modern satellite navigation, the sextant remains relevant as a reliable backup if electronic equipment fails. Many mariners also use them for education or to maintain traditional sailing skills.
Understanding the Sextant’s Components
A sextant features several components for angular measurement. The frame forms the instrument’s structural base, holding all other parts. The arc, a graduated scale spanning 60 degrees (one-sixth of a circle, hence “sextant”), is attached to the frame. Due to double reflection, it can measure angles up to 120 or 130 degrees.
The index arm pivots on the frame, carrying the index mirror and reading mechanism. The index mirror reflects the celestial body’s image towards the observer. The horizon mirror is often half-silvered, allowing a direct view of the horizon while reflecting the celestial body’s image from the index mirror. This superimposes the body’s image with the horizon. A micrometer drum, sometimes with a vernier scale, is attached to the index arm for fine-tuning and fractional degree readings. A telescope magnifies the view, aiding alignment and observation. Colored glass filters, or shades, protect the observer’s eyes, especially when observing the sun.
Measuring Celestial Altitude
Taking a celestial observation with a sextant requires a precise sequence of actions for accuracy. Before any measurement, check for index error, which occurs if the index and horizon mirrors are not parallel when the sextant is set to zero. Determine this error by observing the horizon or a star with the sextant at zero; if images don’t align, adjust the micrometer drum until they do. The drum reading indicates the index error.
To take a sight, hold the sextant vertically by its handle, avoiding contact with the arc. Point the sextant towards the celestial body and look through the telescope, swinging the index arm to bring the body’s reflected image down to the horizon. For precision, gently “rock” the sextant side to side. This motion ensures the celestial body’s image, particularly the Sun or Moon’s lower limb, appears to just touch the horizon at its lowest arc point. Once the body’s image touches the horizon, use the micrometer drum for fine adjustments to achieve tangency. Read the final angle from the sextant’s arc and micrometer drum.
Plotting Your Position
After obtaining a raw altitude reading (Hs) from the sextant, several corrections must be applied to derive the true observed altitude (Ho). First, add or subtract the previously determined index error from Hs. Next, apply a “dip” correction, accounting for the observer’s height above the sea surface, which makes the visible horizon appear lower than the true horizon. This correction is always subtracted. Atmospheric refraction also requires a correction, as light bending through the atmosphere makes celestial bodies appear higher than their actual position. This negative correction is subtracted from the observed altitude. For Sun and Moon observations, apply a semi-diameter correction because the measurement is taken to the body’s limb (edge) rather than its center. This accounts for the body’s apparent radius and ensures the altitude refers to its true center. Finally, for bodies close to Earth (like the Sun and Moon), a parallax correction may be needed to adjust for the observer’s position relative to Earth’s center.
With the corrected observed altitude (Ho), the next step involves using a nautical almanac and sight reduction tables. A nautical almanac provides the Greenwich Hour Angle (GHA) and Declination (Dec) for celestial bodies at specific times, which are their celestial coordinates. Navigators input the observed altitude, time, and an assumed position (based on dead reckoning) into sight reduction tables (e.g., HO-249 or Pub. 229). These tables calculate the celestial body’s expected altitude (Hc) and azimuth (Zn) from the assumed position.
The “altitude intercept method” compares the observed altitude (Ho) with the calculated altitude (Hc). The difference, known as the intercept, indicates the distance in nautical miles between the assumed position and the actual line of position (LOP). If Ho is greater than Hc, the observer is closer to the celestial body than the assumed position, and the intercept is plotted towards the body along its azimuth line. If Ho is less than Hc, the observer is farther away, and the intercept is plotted away. A single LOP indicates the vessel’s true position lies somewhere along that line. By taking multiple observations of different celestial bodies or the same body at different times, multiple LOPs can be generated. The intersection of these LOPs on a nautical chart provides a precise “fix” of the vessel’s current location.