Sextant

Sextant

Marine sextant

Using sextant swing

Käpt'n Jonny Arndt bei einer Horizontalwinkelmessung.

Sextant is a navigational instrument used primarily for the purpose of measuring the angular distances between objects. Its primary use is to determine the observer's position by sighting the horizon and a celestial body such as the Sun, Moon, planets, or stars. This measurement, known as a sight, can then be used with a Nautical almanac and Sight reduction tables to calculate a line of position on a nautical chart. The sextant is an essential tool for Celestial navigation and has been in use since its invention in the 18th century.

History

The development of the sextant is a significant advancement in navigational technology. It evolved from earlier instruments such as the Astrolabe and the Cross-staff, with significant improvements made by John Hadley and Thomas Godfrey in the early 1730s. The sextant's name derives from its arc, which spans 60 degrees, or one-sixth of a circle. This design allows for a wide range of angular measurements, making it more versatile and accurate than its predecessors.

Design and Operation

A sextant consists of a frame, an index arm that moves over a graduated scale, mirrors, and a telescope. The frame is typically shaped like a sector of a circle, and the scale is calibrated in degrees and minutes. One of the mirrors, the index mirror, is mounted on the movable index arm, while the other, the horizon mirror, is fixed. The observer looks through the telescope and moves the index arm until the celestial object is superimposed over the horizon in the field of view. This angle is then read from the scale, providing the data needed for navigation.

Applications

While traditionally used at sea, sextants can also be used for land surveying and in other forms of navigation. With the advent of GPS technology, the use of sextants has declined. However, they are still valued for their reliability and independence from electronic systems, serving as a critical backup navigation tool on ships and small vessels.

Modern Sextants

Modern sextants are precision instruments made from corrosion-resistant materials such as aluminum and stainless steel. They often include features such as micrometer adjustments for fine-tuning the angle measurement and coated optics to reduce glare and improve visibility. Despite technological advancements, the basic principles of sextant operation have remained unchanged since its invention.

See Also

• Celestial Navigation
• Nautical Almanac
• Nautical Chart
• Astrolabe
• Cross-staff