Sextant

A sextant is a highly precise navigation instrument used to measure the angular distance between two visible objects.

Its most critical role is in celestial navigation, where it measures the altitude of an astronomical object relative to the horizon. This process of measurement is often called "sighting" or "shooting" an object. By combining this measured angle with an exact time, a navigator can calculate a position line on a nautical or aeronautical chart. For example, sighting the Sun at noon or the star Polaris at night helps estimate latitude.

The instrument operates through a principle known as double reflection. The frame is shaped like a sector, or a slice, of a circle. The name comes from the Latin word "sextans," which means one sixth. Inside the frame, there is an index arm attached to an index mirror. There is also a fixed horizon mirror. When the navigator moves the index arm, the index mirror reflects light from a celestial object toward the horizon mirror. This allows the user to see both the object and the horizon in a single field of view. Because the light reflects twice, the scale on the arc must be graduated to show twice the angle of the index arm's movement. For instance, if the arm moves 20 degrees, the measured angle between the rays increases by 40 degrees.

Different versions of these instruments exist based on the size of the circular sector they use. An octant uses a 45-degree sector, while a quintant uses a 72-degree sector. A quadrant uses a 90-degree sector.

All of these can be categorized as sextants. Modern professional sextants often include a micrometer drum gauge or a vernier scale for extreme precision. These allow navigators to read measurements down to 0.1 minutes, which is 1/600 of a degree. Because an error of just one minute is roughly equal to one nautical mile, this level of accuracy is vital for safe travel.

The history of the sextant involves several important figures and discoveries. The core principle was implemented around 1731 by John Hadley and Thomas Godfrey. Interestingly, similar ideas were later found in the unpublished writings of Isaac Newton. As technology progressed, the tool was adapted for new environments. In 1922, a Portuguese naval officer modified the instrument for aeronautical navigation. This allowed pilots to use celestial bodies to find their way through the sky. Unlike modern GPS, a sextant does not rely on electricity or human-controlled signals, making it an essential backup tool for ships.

Precision is maintained through careful design and material choice. Temperature changes can warp the metal arc, which causes inaccuracies. To prevent this, many navigators use weatherproof cases so the tool can reach equilibrium with the outside air. Some high-precision models use invar, which is a special type of low-expansion steel. Other scientific versions are made of quartz or ceramics to minimize expansion. While brass frames are heavy and steady in high winds, aluminum frames are lighter and easier to hold without trembling.

Navigators often choose their specific model based on these physical properties.

Taking a sight is a disciplined procedure. For solar observations, the navigator must use filters to protect their eyes from the Sun's brightness. These filters may be a series of dark glasses or adjustable polarizing filters. The navigator holds the sextant by the handle, carefully avoiding touching the arc with their fingers to prevent heat transfer. They move the index bar until the lower limb, or the bottom edge, of the celestial object appears to touch the horizon. The navigator then "swings" the sextant to ensure it is held vertically. Finally, they record the angle and the exact time of the measurement.

Beyond simple latitude, the sextant has many specialized applications. It can be used to measure the lunar distance between the Moon and a star or planet to determine Greenwich Mean Time. This measurement is a key step in calculating longitude. It can also be held horizontally to measure the angles between landmarks on a coast. If the horizon is invisible due to fog or darkness, navigators use an artificial horizon. This might be a pool of water or a fluid-filled tube with a bubble. These various uses demonstrate how a single optical principle can solve complex problems in global navigation.