Using Lunar Distances to Find Longitude
Using Lunar Distances to Find Longitude
Astronomers had long known that the Moon changes its position against the background of the sky and stars fairly quickly. This steady motion could be used to measure time.
Credit: Smithsonian Institution Libraries

Observing the Skies

Navigation improved with the help of new astronomical information and new instruments.

Leading English astronomers were certain the solution to finding longitude lay in better celestial observations. They convinced King Charles to establish an observatory at Greenwich in 1675. Two centuries later, a line through his observatory would become Earth’s “Prime Meridian.”

The octant and sextant were invented to make use of the observatory’s new star catalogs, published beginning in 1725. These portable angle-measuring tools served two purposes: improving latitude observations and finding longitude.

Celestial Navigation at Sea
To locate themselves on the open ocean, navigators can determine their position by observing the Sun, Moon, stars, or planets. Some of these techniques involved using the North Star, the Lunar Distance Method, and finding local noon with a sextant.
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Using Lunar Distances to Find Longitude
Using Lunar Distances to Find Longitude, from Peter Apian, Cosmographia Petri Apiani, 1524. Astronomers had long known that the Moon changes its position against the background of the sky and stars fairly quickly. It moves the distance of its own diameter in about hour, a distance called a “lunar.” This steady motion could be used to measure time. To figure out longitude, a navigator measured the separation between the Moon and a particular star. Based on that separation, he used a book of tables to look up the predicted time at a reference point such as Greenwich. The difference between that reference time and the local time aboard ship was used to determine longitude.
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Octant
Octant marked: "Andrew Newell / Maker / Boston," about 1800. Independently invented in England and the United States in 1730s, the octant is a portable instrument for measuring the angle of the Sun, the Moon, or a star above the horizon. The instrument’s name comes from its scale, which is 45 degrees or 1/8th of a circle.
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Sextant
Sextant, made by Jesse Ramsden, last quarter of 18th century. The sextant became the symbol of navigation. The instrument is named for its scale—60 degrees or 1/6th of a circle—and can measure even greater angles than the octant.
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Dividing Engine
Dividing engine, made by Jesse Ramsden, London, 1775. This machine permitted the automatic and highly accurate division of a circle into degrees and fractions of degrees of arc. Invented by Englishman Jesse Ramsden in the 1770s, the machine ultimately led to mass production of precision octants and sextants and gave British manufacturers dominance in the field of marine instruments for decades. Ramsden’s invention was so valuable to the nation’s maritime interests that he received a share of the Longitude Prize.
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The Nautical Almanac
The Nautical Almanac, first published in 1766, contained new lunar tables from German mathematician Tobias Mayer. Mayer’s tables enabled navigators to determine longitude with an accuracy within a few nautical miles. After Mayer's death, the English Board of Longitude awarded his widow a longitude prize.
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