Navigation Instruments

Chronometer, Octant and Sextant: Navigation History

Today a ship’s captain uses gps (global positioning system or satellite navigation system), to determine location while at sea. But until the 20th century, ships used a variety of instruments to help them navigate unknown waters. Some of the instruments were perfected over hundreds of years, and all were important in determining distances, angles, and other measurements to pinpoint a physical location using a navigational chart.



Celestial navigation is a centuries-old method of determining your position at sea. The method used a combination of citing physical objects, such as planets, stars, the sun and moon, with mathematical calculations. Distances and angles for the calculations were obtained with special hand-held instruments that could read an object on the horizon and give it a geometric angle: this helped to calculate their distance from a known point. Three historical instruments for taking measurements on ocean waters were the octant, sextant and chronometer.

This octant in the Museum’s collections is made of ebony (a type of wood), ivory (probably from an elephant’s tusk or whale’s tooth), and brass. Its measuring device is a scale in increments of 5 degrees, from -2 to 95. An index mirror, three index shades, and horizon glass complete the moving parts. This octant was made in Boston by Browning, probably in the early 1800s.


Octants and sextants measured angles; they were held up and pointed at the horizon, and using the citing glass, their arcs were positioned to record a physical object near the horizon. The name octant comes from the shape and size of its arc which can measure 1/8th of 360°, a circle.

A brass Sextant in the Museum's collections was made in Germany in the 1920s.

A brass Sextant in the Museum’s collections was made in Germany in the 1920s.

Similarly, the name sextant comes from the size of its arc which can measure 1/6th of a circle. Sextants were used by seafarers when exploring the North and South Poles in early 1900 and looked very similar to this one. Sextants aided sailors and explorers with navigation. They help calculate the distance between the horizon and celestial bodies, like stars and planets. The calculations can then be used to establish how far north or south a person is standing. Sextants like this one can be used without a stand so that on even or uneven ground or at sea an accurate calculation can be made.


Sextant on view in Navy Hall in the Library's Exhibit "To the Poles!"

Sextant on view in Navy Hall in the Library’s Exhibit “To the Poles!”

This sextant was made in Germany and includes a telescope and four eyepieces. It dates from the early 1900s. The frame is made of brass, the arc is metal, and its scale is also in increments of 5 degrees from 0 to 150.



This Chronometer in the Museum's collections was used by Dwight Long on his ketch in the 1930s.

This Chronometer in the Museum’s collections was used by Dwight Long on his ketch in the 1930s.

Octants and sextants were invented to measure angles, but a third measurement was needed for an accurate position. This third measurement was needed because the earth rotates 15 degrees each hour and thus increases or decreases its distance from the world time standard by just a little bit each minute. So, a chronometer, or time meter, was invented in the 1730s and was indispensable for precision in celestial navigation; it was in use until long after World War II. It showed the time no matter where on the ocean it was positioned and thus helped in establishing longitude.

The clock face of the Chronometer.

The clock face of the Chronometer.

This chronometer from the Museum’s collections lies in its original case, and was used by Dwight Long aboard his ketch IDLE HOUR during his circumnavigation about 1930. The clock is set in gimbals and fixed in a wood case with brass handles and inlaid decorative brass pieces. It was made in Seattle, Washington by Max Kuner Nautical Instruments Chronometer and Watch Makers Agents U.S. Government Charts.